Behavioral Ecology Vol. 13 No. 3: 328-336
© 2002 International Society for Behavioral Ecology
Dispersal strategies in Tasmanian native hens (Gallinula mortierii)
Department of Zoology and Entomology, The University of Queensland, Brisbane QLD 4072, Australia
Address correspondence to A.W. Goldizen. E-mail: agoldizen{at}zoology.uq.edu.au .
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Individuals in cooperatively breeding species face a complex set of decisions when they reach reproductive maturity. During an 8-year study, we examined the histories of 214 Tasmanian native hens (Gallinula mortierii) from hatching to examine the strategies they used to acquire breeding positions and the reproductive success they experienced in those breeding positions. Two-thirds of young delayed dispersal from their natal groups for at least a year. Ecological constraints were a partial cause of delayed dispersal; high-quality territories were rare and remained occupied due to high adult survivorship. There were also clear benefits of philopatry. Individuals that inherited breeding positions on their natal territories gained better quality positions and experienced higher reproductive success in their first breeding attempts than did individuals who dispersed to other groups. Multivariate analyses showed that the method of acquisition of breeding positions was the only factor significantly related to the quality of the breeding positions attained. Males were more likely to inherit breeding positions in their natal groups than were females. The compositions of individuals' natal groups had no effect on whether they inherited breeding positions or dispersed. In contrast, the compositions of groups did appear to affect whether other birds dispersed into them, with birds rarely moving into groups that contained breeders or nonbreeders of the same sex as the potential dispersers. Short-term removals of breeders confirmed this finding. These results suggest that both ecological constraints and benefits of philopatry explain delayed dispersal in this species.
Key words: cooperative breeding, dispersal, ecological constraints, Gallinula mortierii, philopatry, territoriality.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Cooperatively breeding groups of birds and mammals are characterized by having more than two individuals rearing a single set of young (Skutch, 1961
). These extra
individuals may be either mate-sharing cobreeders or nonbreeding auxiliaries.
In most species, nonbreeding auxiliaries are usually offspring that have
delayed dispersal from their natal group for one or more breeding seasons
(reviewed in Brown, 1987;
Cockburn, 1998;
Stacey and Koenig, 1990). In
many cooperatively breeding bird species, individuals often delay both natal
dispersal and first breeding as a result of ecological constraints on
independent reproduction and/or benefits to remaining on the natal territory.
Much debate in the literature has focused on the relative importance of
ecological constraints versus benefits of philopatry in explaining delayed
dispersal in cooperatively breeding species.
Ecological constraints models suggest that young are prevented from
dispersing out of their natal groups by some extrinsic constraint such as a
shortage of suitable habitat or mates or a very low probability of successful
independent breeding by young individuals
(Brown, 1969;
Emlen, 1982;
Koenig and Pitelka, 1981;
Selander, 1964;
Stacey, 1979).
Benefits-of-philopatry models suggest that the intrinsic benefits that young
gain from remaining in their natal groups make delaying dispersal a better
option than dispersing in some species (e.g., Stacey and Ligon,
1987,
1991;
Zack, 1990;
Zack and Stutchbury, 1992).
The possible fitness benefits of natal philopatry include higher survival
rates, obtaining a breeding position on a better quality territory (perhaps
through inheritance of the natal territory), indirect fitness benefits if the
individual increases its relatives' breeding success in some way, and
increased future reproductive success as a result of experience gained.
More recently, these two categories of explanations for delayed dispersal
have been regarded as complementary parts of a unified explanation (e.g.,
Emlen, 1994;
Koenig et al., 1992;
Ligon, 1999;
Stacey and Ligon, 1991). Any
model explaining natal philopatry will necessarily reduce to the same fitness
inequality: delayed dispersal is likely when the benefits of philopatry minus
costs of philopatry are greater than the benefits of dispersal minus costs of
dispersal (Stacey and Ligon,
1991). Ecological constraints may not provide a complete
explanation for natal philopatry, but ecological parameters must inevitably
have an important effect on the benefits and costs of the two opposing
strategies in the above inequality.
Decisions about dispersal are only a part of the complex set of decisions
that individuals in cooperatively breeding species must make. In such species,
strategies for obtaining breeding positions and associated decisions are
likely to be more complex than for many other species because of the varying
sizes and compositions of groups and the occurrence of nonbreeding helpers
and/or mate sharing. A nonbreeding individual in its natal group (an
"auxiliary") may have a number of options: remaining in its natal
group until it can inherit a breeding position in that group; remaining in its
natal group until it can disperse to an existing group or form a new group;
making forays away from its natal territory to look for breeding vacancies
elsewhere; or leaving its natal group to become a floater while it searches
for a breeding position. These options are not mutually exclusive; an
individual could exercise combinations of options. Individuals may also have
to choose between having their own mate(s) or sharing mates, dispersing alone
or with siblings, and/or joining groups with or without helpers. It is clear
that the best strategies for becoming breeders should differ for different
individuals, depending on such factors as the quality of their natal
territories, the sizes and compositions of their natal groups, and their own
age, sex, condition, aggressiveness, and experience (e.g.,
Koenig et al., 1992).
The strategies that individuals use to obtain breeding positions, and the
factors affecting them, have been studied in some cooperatively breeding bird
species. These species include the splendid fairy-wren (Malurus
splendens; Russell and Rowley,
1993), Seychelles warbler (Acrocephalus sechellensis;
Komdeur, 1992;
Komdeur et al., 1995), acorn
woodpecker (Melanerpes formicivorus;
Stacey and Ligon, 1987), green
woodhoopoe (Phoeniculus purpureus;
Ligon and Ligon, 1988),
red-cockaded woodpecker (Picoides borealis;
Walters et al., 1992), and
stripe-backed wren (Campylorhynchus nuchalis;
Zack and Rabenold, 1989).
However, the detailed and longterm data required to understand dispersal
decisions are not available for many species, particularly in cases where the
fates of dispersers are often unknown.
The Tasmanian native hen (Gallinula mortierii) is a flightless
rail endemic to Tasmania, which exhibits frequent mate sharing by males and
occasional joint nesting by females. Some groups also contain nonreproductive
individuals that provide infrequent helping behavior (Goldizen et al.,
1998a,b,
2000;
Maynard Smith and Ridpath,
1972; Ridpath,
1972a). In the population of Tasmanian native hens that we
studied, monogamy was the most frequent mating pattern, followed by polyandry,
with polygyny and polygynandry the least frequent. Cobreeders of the same sex
were usually close relatives (Goldizen et
al., 2000). Tasmanian native hens are an ideal species for
studying the acquisition of breeding positions because dispersal distances
tend to be short (Ridpath,
1972b,c),
and individuals' histories are thus easy to document.
In this article, we examine the strategies used by individual Tasmanian native hens to obtain their first breeding positions and the relationships between these strategies and characteristics of individuals and of their natal groups. Finally, we discuss the relative importance of ecological constraints and benefits of philopatry in explaining delayed dispersal in this species.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study site and study population
Our study population of Tasmanian native hens was located at Maria Island (42°30' S, 148°00' E), Tasmania, Australia. During the September through December breeding seasons from 1989 through 1997, the population contained between 33 and 46 groups, in which all members were individually color-banded.
Methods for trapping, banding, aging, and sexing birds
Over the 8 years of the study, we trapped and banded 373 individuals, of
which 214 were juveniles of known age and history when first banded. Trapping
and handling methods are described in Gibbs et al.
(1994) and Goldizen et al.
(1998b). To assign ages to
individuals for the analyses reported here, all individuals were treated as
though their birthdays were at the start of the breeding season (i.e., all
were considered to become 1 year old at the start of the first breeding season
after they hatched). Laparotomies and genetic techniques were used to
determine sexes (Goldizen et al.,
1998b).
Definition of likely breeding positions
Tasmanian native hens are sexually mature in the breeding season following
the one when they hatch (Ridpath,
1972b); however, in our study population, many did not begin
breeding until 1 or 2 years later. An individual was considered to have been a
likely breeder during a particular year if it satisfied at least one of the
following criteria: (1) it hatched at least three breeding seasons previously,
(2) it immigrated into its group, (3) it was the only member of its sex in the
group, or (4) it was observed engaging in mounts or copulations
(Goldizen et al., 2000).
Observational data on sexual behavior have provided solid support for this
definition (Goldizen et al.,
2000). Individuals fitting at least one of these criteria
participated in sexual behavior even when the only opposite-sex breeders were
close relatives. Inbreeding was common in this population, including several
cases where groups consisting entirely of siblings and/or half-siblings bred
successfully (Putland and Goldizen,
2001). The mating patterns of groups (monogamous, polyandrous,
polygynous, or polygynandrous) were assigned on the basis of the numbers and
sexes of likely breeders. Before 1991, we did not have enough information on
the ages of birds to accurately assign breeding status; therefore, most
analyses presented here used data from 1991 onward.
Monitoring of group compositions, groups' breeding success, and
annual survivorship
We recorded most groups' compositions daily during each September through
February period, so during those months we knew to within a few days when
individuals joined or left groups. All groups' breeding attempts were
monitored, and the number of young surviving to the end of each breeding
season (mid-February) was used as a measure of a group's annual breeding
success (Goldizen et al.
1998a). During September through December 1997, we manipulated
group compositions through a series of removal experiments (see below).
Therefore, we do not present any observational data from after the start of
the 1997 breeding season.
We calculated annual survivorship as the percentage of individuals present
in the population at the start of one breeding season that were still alive at
the start of the next breeding season. Areas containing short pasture with
cover and water and thus suitable for breeding
(Goldizen et al., 1998a) were
scarce on the island. The four suitable areas (each less than 10 ha in size)
were surveyed regularly either on foot or from a vehicle, so we believe that
most of the birds that disappeared died rather than dispersed.
Measures of territory quality
In this population, the characteristics of groups and territories that most
affected the breeding success of individuals and groups were the availability
of drinking water and the length of edge between short pasture and tall, dense
vegetative cover (pasture/cover edge; PCE) present in a group's territory
(Goldizen et al., 1998a). We
assessed the presence of water and the total amount of PCE in the middle of
each breeding season (November 1) in all territories with known boundaries.
The quality of territories was classified as low, medium, or high based on
water and PCE levels. Using a logistic regression procedure, we determined
that a group had a better than 50% chance of successful reproduction if its
territory contained water and at least 132.5 m of PCE. Using this information,
we assigned territory quality grades as follows. Territories with no water
were considered low quality; chances of reproductive success were nearly zero
on these territories. Medium-quality territories were those that contained
water but had less than 132.5 m of PCE; groups occupying these territories had
less than a 50% likelihood of successful reproduction on an annual basis.
High-quality territories contained water and more than 132.5 m of PCE.
Histories of individuals
In our population, 188 banded individuals hatched between 1989 and 1994.
These birds would have been at least 3 years old by 1997 and thus would have
acquired likely breeding positions if they survived. We knew the following
information about each of these individuals: sex; natal group; year of
hatching; whether it eventually became a likely breeder or died or disappeared
before doing so; group in which it acquired its first likely breeder position
(FLBP); and at what age it acquired its FLBP. For a subset of these
individuals (n = 67), we also knew the quality of their natal
territories and the territories in which they obtained their FLBPs. For some
analyses, such as for patterns of dispersal, we included individuals that
hatched after 1994.
Birds were considered to have dispersed from one group to another if they moved to a new group and stayed there for at least a week. In a few cases, subsets of groups fissioned off from groups to form new groups and eventually defended territories separate from the parent groups. In these cases, individuals were also considered to have dispersed from one group to another. Dispersal distances were measured by counting the number of territory boundaries between an individual's old and new territories.
Multivariate analyses of the factors affecting the quality of birds'
first breeding territories and their breeding success in their first likely
breeder positions
We used multivariate logistic modeling to test whether the quality of
birds' first breeding territories (low, medium, or high) depended on the sexes
or ages of the birds, the methods by which they acquired their FLBPs
(inheritance, dispersal to a new group, or dispersal to an existing group),
and whether they acquired their FLBPs alone or jointly with a sibling. We also
included birds' natal group numbers to find out whether clutch mates could be
considered statistically independent and included first-order interactions
between method of acquisition of FLBPs and the other variables. We employed
backward elimination from the model at a significance level of.05. The data
set included all birds that acquired an FLBP on a territory of known quality
(n = 69).
Similar logistic modeling was carried out to investigate factors that might
have affected the breeding success of individuals in their FLBPs. Data from
all 125 individuals that acquired breeding positions between 1991 and 1996 and
had known levels of success in their first breeding seasons were included in
the analysis. We tested whether breeding success depended on any of the same
five main effects as in the previous analyses. Breeding success (number of
young surviving in mid-February/likely breeder) was categorized into three
levels (none, 
1, > 1). In the absence of information on genetic
parentage, which is impossible to determine for this species due to low levels
of genetic variation (Buchan,
2000; Gibbs et al.,
1994), we assumed that parentage was shared equally among
cobreeders of the same sex within groups. Extrapair copulations were never
seen, nor was there any evidence of intraspecific brood parasitism;
preliminary genetic work also found no evidence of either of these behaviors
(Buchan, 2000). We used
backward elimination at a significance level of.05.
Experimental removals of breeders
We temporarily removed 15 likely breeders (7 females and 8 males) from
monogamous breeding groups in separate removal experiments performed during
the 1997 breeding season to investigate the factors that affect whether
breeding vacancies are filled and which individuals fill them. Before
removals, all potential replacements (same-sex nonbreeders in groups within
two boundary crossings away from removal groups) were identified, and
territory quality was measured for the removal group and for all groups
containing potential replacements. Removed birds were held in a
wooden/steel-mesh enclosure away from their territories and were supplied with
food (panicum seed, layer pellets, and grass). Only one bird was held in
captivity at any given time.
After removal, we observed the removal group continuously for 12 h per day during daylight. All intra- and intergroup interactions involving the remaining group members were recorded on an ad libitum basis. All birds from neighboring territories that appeared to show interest in the newly vacant breeding position were identified. If an individual moved into the removal group's territory and was involved in either sexual activity or joint territorial defense with the remaining birds, this individual was assumed to be the replacement breeder.
We re-released removed birds into their territories 24 h after a replacement had become established, or after a total of 48 h of captivity, whichever came first. All interactions involving the removed, remaining, and replacement (if present) birds were recorded after re-release. Intensive observations ceased when it became obvious that the composition of the group had stabilized, but the group composition was monitored several times daily for at least 7 days after re-release and daily for the remainder of the breeding season.
Statistical analyses were performed using SAS software version 6.12.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Patterns of dispersal and acquisition of breeding positions
Ages at acquisition of first breeding positions
We banded 172 birds (89 males, 80 females, 3 unsexed) as juveniles during the 1989 through 1993 breeding seasons. Of these, 135 were successful in acquiring FLBPs, and there was no difference between the sexes in the likelihood of acquiring FLBPs (
12 = 0.536,
n = 169, p =.464; 73 males, 82.0%, and 62 females, 77.5%,
obtained such positions). The others were either known to have died or
disappeared from the study site before obtaining breeding positions. Of the
135 individuals that obtained FLBPs, 49 (36%) obtained their first breeding
positions in the breeding season after the one in which they hatched, 46 (34%)
did so in the next year, and 40 (30%) waited 2 years for their first breeding
positions. The proportions of birds that postponed their first breeding
attempts by at least one breeding season did not differ between the sexes
(Table 1;
12 = 0.03, n = 135, p =.86).
However, females seemed to gain their breeding positions marginally earlier
than did males; females were more likely to gain breeding positions at 2 years
of age rather than at 3, and the opposite trend was observed for males
(Table 1; 22 = 5.945, n = 135, p
=.051).
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Locations of males' and females' first likely breeder positions
Males were significantly more likely than females to first become breeders
in their natal groups (inheritance). Of the 135 young that hatched before the
1994 breeding season and became likely breeders, 28 of the 73 males (38.4%)
inherited their FLBPs in their natal groups, compared with 12 of the 62
females (19.4%; 12 = 5.82, n = 135,
p =.016). The remainder of the individuals that obtained FLBPs
dispersed from their natal groups and acquired FLBPs either in previously
existing groups or by forming new groups.
There was also a suggestion that males were more likely than females to
acquire FLBPs in high-quality territories. Between the 1991 and 1997 seasons,
79 birds acquired FLBPs on territories of known quality. Twelve of the 46
males (26.1%) did so in high-quality territories, compared with only 3 of the
33 females (9.1%). However, this difference was not quite statistically
significant (12 = 3.61, n = 79, p
=.058).
Dispersal patterns
Among those birds that did disperse from their natal groups, there was a
trend for females to disperse longer distances than males (measured as the
number of territories moved). Males appeared more likely than females to move
to a territory adjacent to their natal territory, whereas females were more
likely than males to move at least three territories away from their natal
territories (Table 2;
22 = 5.567, n = 103, p
=.062).
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Most birds that dispersed from their natal groups formed new groups with
other individuals (43 males, 39 females; 67.2% of all 122 birds that dispersed
by the 1997 season), rather than joining existing groups (6 males, 9 females;
12.3% of the 122 birds) or dispersing to become solitary floaters for a period
of time (7 males, 11 females, 1 unsexed; 15.6% of the 122 birds). For the
other six dispersers, we did not know whether the groups to which they
dispersed were new or had existed previously. The sexes did not differ in the
frequencies of the three different types of dispersal
(22 = 1.607, p =.448).
Of the 15 birds that dispersed to existing groups when acquiring FLBPs, only one (a male) moved to a group that already contained a likely breeder of the same sex. Another (a female) joined a group that contained auxiliaries of the same sex. In six cases, there were definitely no birds already present in the group that were of the same sex as the immigrant. In the remaining seven cases, we did not know whether the group contained birds of the same sex as the immigrant at the time of dispersal either because the group contained unbanded and therefore unsexed birds or because the dispersal event occurred between March and September when we were not present at the study site.
The majority of individuals dispersed alone, but joint dispersal was also
common for both sexes. Sixty percent of birds (31 males, 42 females, 1 unsexed
bird) dispersed from their natal groups alone, and 40% (26 males, 22 females)
dispersed jointly with other members of their natal groups. Males and females
did not differ in the frequency of joint dispersal
(12 = 1.59, n = 121, p =.207).
Seven cases of joint dispersal involved only females, eight involved only
males, and three involved both sexes. The sexes did not differ in the number
of individuals involved in each joint dispersal event (2.50 ± 0.27
males, 2.33 ± 0.33 females; Wilcoxon rank sum test, U = 52.00,
n = 10, 9, p =.423), nor in the average ages of joint
dispersers (Wilcoxon rank sum test, U = 255.50, n = 25, 21,
p =.859). Most joint dispersal events included only first-time
dispersers, but a few cases included both first-time dispersers and
individuals moving from one breeding position to another. All three cases of
joint dispersal that involved both sexes were cases where subsets of
individuals fissioned off from their parent groups to form new groups. These
subgroups contained varying ratios of males and females (2 M:5 F, 1 M:1 F, 6
M:3 F).
Effects of the compositions of individuals' natal groups on their
strategies for obtaining breeding positions
The compositions of individuals' natal groups did not appear to affect
whether they remained in their natal groups or dispersed. We investigated the
effects of stepparents on individuals' dispersal decisions. The presence of
same-sex stepparents might increase the chances that individuals would
disperse due to reproductive competition; in contrast, opposite-sex
stepparents might make individuals less likely to disperse because such
individuals might be attractive breeding partners
(Emlen, 1996). Stepparents
were rare. Of the 123 birds that were still in their natal groups at 1 year of
age, only seven (5.6%) had opposite-sex stepparents, and five (4.1%) had
same-sex stepparents. All birds with same-sex stepparents remained in their
groups as auxiliaries for the next year, and of the seven birds with
opposite-sex stepparents, six remained as auxiliaries for a further year, and
one inherited a breeding position that year.
The presence in birds' natal groups of same-sex siblings, either of the
same age or older, also did not increase the chance that birds would disperse
rather than remain in their natal groups. Whether 12-month-old birds were more
likely to disperse or to remain in their natal groups over the following 12
months was not affected by the presence in their groups of 2-year-old siblings
of their sex (12 = 0.045, n = 27,
p =.831 for males; 12 = 0.05, n =
29, p =.823 for females) or of 1-year-old siblings of the same sex
(12 = 1.556, n = 27, p =.212 for
males; 12 = 0.042, n = 29, p
=.837 for females).
Evidence for ecological constraints on breeding due to habitat
saturation
Only a small proportion of the territories that were occupied in each
breeding season allowed a good chance of successful breeding. On average, only
20.5 ± 2.4 % (n = 6 years) of the territories were of high
quality. Another 21.3 ± 3.8 % of territories were of low quality, and
58.7 ± 2.5 % were of medium quality. The distribution of the numbers of
territories in the three quality grades did not vary significantly between
years (102 = 11.3, n = 190, p
=.332). Therefore, data from different years are combined in subsequent
analyses.
Similarly, less than 20% of birds acquired their FLBPs in territories that allowed a good chance of successful breeding. Of the 135 birds that obtained their first breeding positions during the 1991-1996 breeding seasons, 79 did so in territories of known quality. Of these, 14 (18%) obtained breeding positions on low-quality territories and 50 (63%) on medium quality territories, whereas only 15 (19%) obtained their first breeding positions on high-quality territories.
The shortage of available territories arose, at least in part, because of
high annual survivorship of breeding adults. Annual survivorship for breeding
adults averaged 84.0 ± 1.7 % (n = 6 years) during the
1991-1996 breeding seasons. There was no significant difference in
survivorship between males and females in any year (0.02
12 2.05, 102 n 132,.15
p .90).
To test whether groups of Tasmanian native hens recognized whether particular territories offered reasonable chances of successful breeding, we compared the proportions of groups that attempted breeding on low-versus medium- and high-quality territories. In 3 out of 5 years, significantly higher proportions of groups on medium- and high-quality territories laid eggs than of those on low-quality territories (Table 3).
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Evidence for benefits of philopatry
The territory quality (low, medium, or high) of birds' FLBPs was
significantly related to the way that they acquired those FLBPs
(42 = 23.934, n = 79, p =.001;
Figure 1). Inheritors gained
the best quality territories, and birds that dispersed to existing groups
acquired the worst territories, whereas those that moved to new groups
obtained territories of intermediate quality. Birds that inherited FLBPs on
their natal territories did not, of course, suffer a decrease in territory
quality by doing so. In contrast, individuals that dispersed to their FLBPs
often moved to a territory of lower quality than their natal territory.
Indeed, no birds ever obtained FLBPs on high-quality territories by dispersal.
Of the 37 birds that dispersed to FLBPs, and for whom the qualities of both
their natal territories and their first breeding territories were known, 18
moved to poorer quality territories, 18 moved to territories of the same
quality category as their natal territories, and only 1 moved to a better
quality territory. Losses in territory quality were thus a significantly more
frequent result of dispersal than were gains (sign test, Z = 3.671,
p <.001). The quality of the breeding territories to which birds
dispersed appeared to vary with the quality of the birds' natal territories,
although this trend was not quite statistically significant
(Table 4;
22 = 0.805, n = 37, p =.067).
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Multivariate logistic modeling showed that the method of acquiring an FLBP
(inheritance, dispersal to new groups, dispersal to existing groups) was the
only significant predictor of the quality of the territories on which
individuals obtained their FLBPs (22 = 11.64,
n = 69, p =.020). Group membership was not significant; thus
clutch mates could be considered statistically independent. Further
multivariate modeling investigated the factors that affected the breeding
success of individuals in their first year as likely breeders. In this
analysis, we did not include aspects of territory quality as variables;
rather, we included characteristics of individuals (sex, age) and of the ways
in which they obtained their breeding positions (inheritance, dispersal to
existing groups, dispersal to new groups; lone versus joint acquisition).
Method of acquisition was the only variable that remained in the model after
backward elimination (22 = 9.2796, n =
125, p =.010). Inheritors had the highest reproductive success in
their first year as likely breeders, followed by dispersers that formed new
groups, whereas dispersers to existing groups had the lowest reproductive
success (Figure 2).
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Results of experimental removals of breeders
Six of the 15 breeding vacancies (40%) created by temporary removals of
monogamous breeders were filled by dispersing individuals. Four of seven
females (57%) and two of eight males (25%) were replaced. This difference was
not statistically significant (Fisher's Exact test, p =.23).
Replacement only occurred in groups that did not contain auxiliaries
(non-breeding natal individuals) of the same sex as the removed bird. Four of
the vacancies created by removals of females occurred in groups without female
auxiliaries; all four of these vacancies were filled. Males also only moved
into groups without male auxiliaries; however, only two of six such vacancies
were filled.
All replacements came from adjacent territories. Four (two of each sex) came from lower quality territories, while two (both females) dispersed to territories of equal quality rank. Only one bird (a male) dispersed to a high-quality territory; he eventually shared this territory and the female with the original male, after the original male's release back into the territory. None of the six dispersers moved from high-quality territories. In each case in which a vacancy was filled, the replacement came from the poorest quality territory category that contained potential replacements (nonbreeding birds of the same sex that lived within two territories away from the territory with the vacancy; Table 5). Of the four cases where male vacancies that were created in groups without male auxiliaries were not filled, three were on medium-quality territories and one was on a low-quality territory. In none of these four cases were there any male nonbreeders in low-quality territories that fit our criteria for potential replacements.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Strategies for the acquisition of breeding positions in Tasmanian native hens
More than 75% of the young that hatched in our population over a 4-year period and survived to the age of banding subsequently went on to acquire breeding positions. However, delayed dispersal was common; two-thirds of these successful birds spent one or two breeding seasons as nonbreeding auxiliaries before acquiring their first breeding positions (Table 1). Females appeared to gain their FLBPs marginally earlier than males; however, there was a trend for females' FLBPs to be in poorer quality territories than was the case for males' FLBPs (Figure 1). These differences between the sexes are linked to significant differences in the ways in which the two sexes acquired their FLBPs. Males were significantly more likely to inherit breeding positions in their natal groups than were females, and FLBPs acquired by inheritance were, on average, in significantly higher quality territories than those acquired by dispersal.
Dispersal patterns did not differ significantly between the sexes, but there was a trend for female dispersers to disperse farther from their natal territories than male dispersers (Table 2). Most dispersers moved to new groups formed with dispersers from other groups, rather than moving into already established groups. Individuals were most likely to disperse alone, but joint dispersal by two or more members from the same group was not uncommon. The quality of breeding positions acquired by dispersal, and the reproductive success of individuals in their first year in such positions, was higher when dispersers formed new groups with other individuals than when they joined already existing groups (Figure 1).
Data from this study suggest that the quality of territories on which dispersers acquired their first breeding positions was somewhat related to the quality of their natal territories. The reasons for this are not yet clear, but there are two obvious possible explanations. The first is that birds reared on poorer quality territories may be less fit than others in the population and thus either less able to disperse into good territories or less often chosen as mates by other dispersers. The second possibility is that due to the distribution of water and cover at the study site, poorer territories are more likely to be surrounded by poor territories than are good territories, and thus individuals reared in poorer territories have fewer opportunities for dispersal to high-quality territories.
The dispersal options of individual Tasmanian native hens are clearly limited by the saturation of high-quality habitat. Territories that provided a good chance of successful breeding were rare, and breeding adults displayed high annual survivorship, with the result that 80% of birds acquired their first breeding positions on territories that allowed only a low or moderate chance (< 50%) of reproductive success in any one season. Only 20% of individuals acquired their first breeding positions in high-quality territories (Figure 1). In fact, because our study population was probably centered around the best habitat for Tasmanian native hens on the island, over the whole island it is likely that considerably fewer than 20% of all territories were of high quality.
Given that the chances of acquiring a breeding position on a high-quality
territory are low for any given individual, the ecological constraints model
would predict that young individuals should remain in their natal groups as
nonbreeders. This prediction is supported by the fact that most Tasmanian
native hens spent at least one breeding season as nonreproductive auxiliaries
in their natal groups. In some species, floating is a common alternative to
natal philopatry for nonbreeding individuals (e.g.,
Arcese, 1987;
Hunter, 1987;
Smith, 1978;
Stutchbury, 1991). Floaters
have been present in this population of Tasmanian native hens, but only in low
numbers. Up to four solitary individuals have been recorded in the population
at any one time (Goldizen, unpublished data). However, because an average of
24.5 ± 5.6 (range = 10-51) nonbreeding adults resided in their natal
groups in the population at the midpoints of the 1991-1996 breeding seasons
(Goldizen et al., 1998b),
delayed dispersal is clearly a much more common strategy than is floating in
this population.
The benefits of philopatry to young, nonbreeding Tasmanian native hens are also clear. Birds obtained better quality FLBPs through inheritance than through dispersal. The reasons for this are simple. The majority of young are produced on high-quality territories; if these birds are able to inherit breeding positions in those territories, they can do very well. In contrast, during all of the years of this study, no bird ever dispersed into a first breeding position on a high-quality territory. Groups on such territories almost always contain auxiliaries of both sexes, and the presence of auxiliaries appears to prevent dispersal of same-sex individuals into groups, even after the death or disappearance of a breeder. The temporary removal experiments that we performed in 1997 confirmed this; the only vacancies that were filled were those in groups that did not contain auxiliaries of the same sex as the breeder that was removed. Thus, in this population, ecological constraints in the form of saturation of good quality habitat and benefits of philopatry due primarily to the benefits of territory inheritance both strongly affected dispersal patterns and the strategies that individuals used to obtain breeding positions.
These findings suggest some simple rules that individual Tasmanian native
hens may use to guide their decisions about how to gain breeding positions.
Individuals that hatched in high-quality territories may do best to remain in
their groups in the hopes of inheriting breeding positions there. They would
be likely to have to share those breeding positions with others of their own
sex, and perhaps even breed with related birds
(Putland and Goldizen, 2001),
but our data suggest that they would tend to have high reproductive success
nonetheless (Figure 2).
Individuals that were unlikely to be able to inherit good breeding positions
would usually do best to disperse and form new groups with other birds.
Dispersal into existing groups would generally be the worst choice, perhaps
exercised by the birds with the poorest options. The results of the removal
experiments that we carried out in 1997 confirm this. Only 40% of the
vacancies that we created were filled; note, however, that captured birds were
released back into their groups after 48 h, so that vacancies did not last
very long. More interesting, the birds that did move into experimentally
vacated breeding positions always came from poorer quality territories than
did any of the other birds that we had identified as potential
replacements.
The idea that individual animals might follow the simple decision rules just described assumes that they are capable of assessing the quality of territories and the suitability of those territories for breeding. At our study site, groups residing on low-quality territories were significantly less likely to attempt breeding than were groups on higher quality territories during 3 of 5 years. This finding, in combination with the results of the removal experiments, suggests that Tasmanian native hens may indeed be capable of assessing territory quality.
The decision rules followed by females are likely to differ somewhat from those of males. Females were significantly less likely to inherit breeding positions than males. If inheritance is not likely, then young females would be expected to look for dispersal opportunities and take these when they arise; this may explain why more females than males gained breeding positions at 2 years of age, with the reverse occurring at 3 years of age. The greater frequency of dispersal for females than males may also explain the trend for females to disperse farther than males and the trend for females' FLBPs to be of poorer quality than those of males.
In our study, the compositions of birds' natal groups did not clearly
affect their strategies for obtaining breeding positions. It has been
predicted that young individuals should interact competitively with same-sex
stepparents, with the young individuals usually leaving their natal
territories (Emlen, 1996).
Data from Florida scrub jays (Aphelocoma coerulescens) support this
prediction (Goldstein et al.,
1998). However, in our population, stepparents were rare and
appeared to have no effect on individuals' dispersal patterns, except in one
group (group 16 in 1990). In this case, identified using molecular techniques
as a group with four juveniles, their father and a stepmother
(Gibbs et al., 1994), frequent
aggression was seen between the breeding female and the four sons, leading to
their dispersal from the natal group at less than a year of age. This was the
only case of dispersal during this study that appeared to be triggered by
intense aggression. The presence in a young bird's natal group of nonbreeding,
same-sex siblings either the same age or older also did not affect those
birds' dispersal decisions.
Comparisons with other cooperatively breeding species
As found in this study, both ecological constraints and benefits of
philopatry affect dispersal distances in splendid fairy-wrens
(Russell and Rowley, 1993).
The general patterns of territory acquisition in this species also appear
similar to those in Tasmanian native hens. Both sexes frequently delay
dispersal and those that gain a breeding position in their natal territory or
close to it are most successful. Males are more likely to inherit breeding
positions than are females, and breeding vacancies are only filled by a bird
from outside the group if no group members are available to fill them. Thus,
individuals' dispersal decisions in this species are affected by both
environmental and demographic factors.
Both ecological constraints and benefits of philopatry also affect
dispersal decisions in the cooperatively breeding Seychelles warbler
(Komdeur, 1992;
Komdeur et al., 1995).
However, the sex differences between strategies for acquiring breeding
positions are different from those that we found in Tasmanian native hens and
that Russell and Rowley (1993)
found in splendid fairy-wrens. Female Seychelles warblers are more likely to
serve as helpers than males, and females also remain for longer on their natal
territories (Komdeur, 1996).
In this species, benefits of philopatry appear to be stronger for females than
for males, as helpers gain inclusive fitness benefits
(Komdeur, 1994).
Territory quality has also been shown to have a strong effect on strategies
for obtaining breeding positions in acorn woodpeckers
(Koenig and Stacey, 1990;
Stacey and Ligon, 1987). In a
population at Water Canyon, in New Mexico, individuals in high-quality
territories were more likely than those in low-quality territories to delay
breeding and serve as helpers on their natal territories. Those individuals
that remained in high-quality natal territories and eventually bred there had
higher lifetime reproductive success than did those who dispersed from lower
quality territories at younger ages.
Benefits of philopatry and an age-related queuing system affect the ages at
which young stripe-backed wrens gain breeding positions
(Rabenold, 1990;
Wiley and Rabenold, 1984).
Unlike Tasmanian native hens, there is a strict age-related queue for breeding
positions, with older males obtaining positions before younger males in the
same groups, and older females winning competitions for vacancies before
younger ones. Benefits of philopatry are probably stronger for males, due to
the potential for inheritance, but females also delay breeding and serve as
helpers, presumably gaining inclusive fitness benefits from doing so.
Experimental removals showed that potential replacements could gauge the
quality of vacancies; vacancies in large groups were contested by more females
than those in small groups (Zack and
Rabenold, 1989). These experiments also showed that females in
adjacent territories were more likely to win contests over vacancies,
presumably due to advantages of familiarity.
The greater frequency of inheritance of breeding positions among male than
female Tasmanian native hens is quite common among birds. Female-biased natal
dispersal occurs in the majority of bird species
(Greenwood, 1980), including
cooperatively breeding species (e.g., Florida scrub jay:
Woolfenden and Fitzpatrick,
1990; groove-billed ani, Crotophaga sulcirostris:
Bowen et al., 1989; Arabian
babbler, Turdoides squamiceps:
Zahavi, 1990). Greenwood
(1980) proposed that
female-biased dispersal in birds results from a combination of inbreeding
avoidance and advantages to males of philopatry for resource competition. In
green woodhoopoes, in contrast, inheritance of breeding positions in their
natal territories is common for both sexes, but more frequent for females than
males. This is presumably due to the importance of tree cavities for roosting
in this species and the benefits of philopatry for birds inhabiting
territories with such cavities (Ligon and Ligon,
1988,
1990).
Most cooperatively breeding birds exhibit strong inbreeding avoidance
(reviewed by Cockburn, 1998).
It is interesting that the only cooperative breeders reported to inbreed
frequently are rails (pukeko, Porphyrio porphyrio:
Craig and Jamieson, 1988;
moorhen Gallinula chloropus:
McRae, 1996). Tasmanian native
hens fit this rallid pattern; inbreeding occurred relatively frequently in our
population, both when birds became breeders in their natal groups and in
several cases where groups of siblings fissioned off from their natal groups
and formed new groups without any birds from other groups
(Putland and Goldizen, 2001).
It is possible that suitable habitat is so restricted for these aquatic and
semiaquatic rails that it is worthwhile for individuals to breed with close
relatives if by doing so they are able to breed on a good territory.
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ACKNOWLEDGEMENTS |
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We thank the following organizations for financial support of this project: the Australian Research Council, the National Geographic Society, the U.S. National Science Foundation, Earthwatch, the Chapman Fund of the American Museum of Natural History, and the M.A. Ingram Trust of Victoria, Australia. We are also grateful to the University of Tasmania and the rangers at Maria Island for their generous support; to Jason Buchan, Ian Jamieson, Darryl Jones, Ian Owens, and two anonymous reviewers for comments on earlier drafts of the manuscript; and to Alan Goldizen, Elsie Krebs, and numerous other volunteer field assistants who made this long-term study possible.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
|---|
Arcese P, 1987. Age, intrusion pressure and territory defence against floaters by male song sparrows. Anim Behav 35: 773-784.
Bowen BS, Koford RR, Vehrencamp SL, 1989. Dispersal in the communally breeding groove-billed ani (Crotophaga sulcirostris). Condor 91: 52-64.[Web of Science]
Brown JL, 1969. Territorial behavior and population regulation in birds. Wilson Bull 81: 293-329.
Brown JL, 1987. Helping and communal breeding in birds: ecology and evolution. Princeton, New Jersey: Princeton University Press.
Buchan JC, 2000. Behavioural and genetic aspects of mate-sharing in the Tasmanian native hen (Gallinula mortierii) and dusky moorhen (Gallinula tenebrosa) (PhD thesis). University of Queensland, Brisbane.
Cockburn A, 1998. Evolution of helping behavior in cooperatively breeding birds. Annu Rev Ecol Syst 29: 141-177.[Web of Science]
Craig JL, Jamieson IG, 1988. Incestuous mating in a communal bird: a family affair. Am Nat 131: 58-70.[Web of Science]
Emlen ST, 1982. The evolution of helping. I. An ecological constraints model. Am Nat 119: 29-39.[Web of Science]
Emlen ST, 1994. Benefits, constraints and the evolution of the family. Trends Ecol Evol 9: 282-285.
Emlen ST, 1996. Living with relatives: lessons from avian family systems. Ibis 138: 87-100.[Web of Science]
Gibbs HL, Goldizen AW, Bullough C, Goldizen AR, 1994. Parentage analysis of multi-male social groups of Tasmanian native hens (Tribonyx mortierii): genetic evidence for monogamy and polyandry. Behav Ecol Sociobiol 35: 363-371.[Web of Science]
Goldizen AW, Buchan JC, Putland DA, Goldizen AR, Krebs EA, 2000. Patterns of mate-sharing in a population of Tasmanian native hens Gallinula mortierii. Ibis 142: 40-47.[Web of Science]
Goldizen AW, Goldizen AR, Putland DA, Lambert DM, Millar CD, Buchan JC, 1998a. "Wife-sharing" in the Tasmanian native hen (Gallinula mortierii): is it caused by a male-biased sex ratio? Auk 115: 528-532.[Web of Science]
Goldizen AW, Putland DA, Goldizen AR, 1998b. Variable mating patterns in Tasmanian native hens (Gallinula mortierii): correlates of reproductive success. J Anim Ecol 67: 307-317.
Goldstein JM, Woolfenden GE, Hailman JP, 1998. A same-sex step-parent shortens a prebreeder's duration on the natal territory: test of two hypotheses in Florida scrub jays. Behav Ecol Sociobiol 44: 15-22.
Greenwood PJ, 1980. Mating systems, philopatry and dispersal in birds and mammals. Anim Behav 28: 1140-1162.
Hunter LA, 1987. Acquisition of territories by floaters in cooperatively breeding purple gallinules. Anim Behav 35: 402-410.
Koenig WD, Pitelka FA, 1981. Ecological factors and kin selection in the evolution of cooperative breeding in birds. In: Natural selection and social behavior (Alexander RD, Tinkle DW, eds). New York: Chiron Press; 261-280.
Koenig WD, Pitelka FA, Carmen WJ, Mumme RL, Stanback MT, 1992. The evolution of delayed dispersal in cooperative breeders. Q Rev Biol 67: 111-150.[Medline]
Koenig WD, Stacey PB, 1990. Acorn woodpeckers: group-living and food storage under contrasting ecological conditions. In: Cooperative breeding in birds: long-term studies of ecology and behavior (Stacey PB, Koenig WD, eds). Cambridge: Cambridge University Press; 415-453.
Komdeur J, 1992. Importance of habitat saturation and territory quality for evolution of cooperative breeding in the Seychelles warbler. Nature 358: 493-495.
Komdeur J, 1994. The effect of kinship on helping in
the cooperative breeding Seychelles warbler (Acrocephalus
sechellensis). Proc R Soc Lond B
256: 47-52.
Komdeur J, 1996. Facultative sex ratio bias in the
offspring of Seychelles warblers. Proc R Soc Lond B
263: 661-666.
Komdeur J, Huffstadt A, Prast W, Castle G, Mileto R, Wattel J, 1995. Transfer experiments of Seychelles warblers to new islands: changes in dispersal and helping behaviour. Anim Behav 49: 695-708.
Ligon JD, 1999. The evolution of avian breeding systems. Oxford: Oxford University Press.
Ligon JD, Ligon SH, 1988. Territory quality: key determinant of fitness in the group-living green woodhoopoe. In: The ecology of social behaviour (Slobodchikoff CN, ed). San Diego, California: Academic Press; 229-253.
Ligon JD, Ligon SH, 1990. Green woodhoopoes: life history traits and sociality. In: Cooperative breeding in birds: long-term studies of ecology and behaviour (Stacey PB, Koenig WK, eds). Cambridge: Cambridge University Press; 33-65.
Maynard Smith J, Ridpath MG, 1972. Wife sharing in the Tasmanian native hen, Tribonyx mortierii: a case of kin selection? Am Nat 106: 447-452.[Web of Science]
McRae SB, 1996. Family values: costs and benefits of communal nesting in the moorhen. Anim Behav 52: 225-245.
Putland DA, Goldizen AW, 2001. Family dynasties in the Tasmanian native hen (Gallinula mortierii). Behav Ecol Sociobiol 51: 26-32.
Rabenold KN, 1990. Campylorhynchus wrens: the ecology of delayed dispersal and cooperation in Venezuelan savanna. In: Cooperative breeding in birds: long-term studies of ecology and behaviour (Stacey PB, Koenig WK, eds). Cambridge: Cambridge University Press; 159-196.
Ridpath MG, 1972a. The Tasmanian native hen, Tribonyx mortierii. I. Patterns of behaviour. CSIRO Wildl Res 17: 1-51.
Ridpath MG, 1972b. The Tasmanian native hen, Tribonyx mortierii. II. The individual, the group, and the population. CSIRO Wildl Res 17: 53-90.
Ridpath MG, 1972c. The Tasmanian native hen, Tribonyx mortierii. III. Ecology. CSIRO Wildl Res 17: 91-118.
Russell EM, Rowley I, 1993. Philopatry or dispersal: competition for territory vacancies in the splendid fairy-wren, Malurus splendens. Anim Behav 45: 519-539.[Web of Science]
Selander RK, 1964. Speciation in wrens of the genus Campylorhynchus. Univ Calif Publ Zool 74: 1-305.
Skutch AF, 1961. Helpers among birds. Condor 63: 198-226.
Smith SM, 1978. The "underworld" in a territorial sparrow: adaptive strategy for floaters. Am Nat 112: 571-582.[Web of Science]
Stacey PB, 1979. Habitat saturation and communal breeding in the Acorn Woodpecker. Anim Behav 27: 1153-1166.
Stacey PB, Koenig WD, 1990. Cooperative breeding in birds: longterm studies of ecology and behavior. Cambridge: Cambridge University Press.
Stacey PB, Ligon JD, 1987. Territory quality and dispersal options in the acorn woodpecker, and a challenge to the habitat-saturation model of cooperative breeding. Am Nat 130: 654-676.[Web of Science]
Stacey PB, Ligon JD, 1991. The benefits-of-philopatry hypothesis for the evolution of cooperative breeding: variation in territory quality and group size effects. Am Nat 137: 831-846.[Web of Science]
Stutchbury BJ, 1991. Floater behaviour and territory acquisition in male purple martins. Anim Behav 42: 435-443.
Walters JR, Doerr PD, Carter JH III, 1992. Delayed dispersal and reproduction as a life-history tactic in cooperative breeders: fitness calculations from red-cockaded woodpeckers. Am Nat 139: 623-643.[Web of Science]
Wiley RH, Rabenold KN, 1984. The evolution of cooperative breeding by delayed reciprocity and queuing for favorable social positions. Evolution 38: 609-621.[Web of Science]
Woolfenden GE, Fitzpatrick JW, 1990. Florida scrub jays: a synopsis after 18 years of study. In: Cooperative breeding in birds: long-term studies of ecology and behaviour (Stacey PB, Koenig WD, eds). Cambridge: Cambridge University Press; 241-266.
Zack S, 1990. Coupling delayed breeding with delayed dispersal in cooperatively breeding birds. Ethology 86: 265-286.[Web of Science]
Zack S, Rabenold KN, 1989. Assessment, age and proximity in dispersal contests among cooperative wrens: field experiments. Anim Behav 38: 235-247.
Zack S, Stutchbury BJ, 1992. Delayed breeding in avian social systems: the role of territory quality and "floater" tactics. Behaviour 123: 194-219.[Web of Science]
Zahavi A, 1990. Arabian babblers: the quest for social status in a cooperative breeder. In: Cooperative breeding in birds: long-term studies of ecology and behaviour (Stacey PB, Koenig WD, eds). Cambridge: Cambridge University Press; 105-130.
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