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Behavioral Ecology Vol. 10 No. 6: 675-687
© 1999 International Society for Behavioral Ecology
The evolution of communal roosting in birds: origin and secondary losses
Faculty of Veterinary Medicine, University of Montréal, CP 5000, St-Hyacinthe, Québec, J2S 7C6, Canada
Address correspondence to G. Beauchamp. E-mail: beauchgu{at}medvet.umontreal.ca .
Received 14 December 1998; accepted 22 April 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Three main benefits are thought to underlie communal roosting in birds: a reduction in thermoregulation demands, a decrease in predation risk, and an increase in foraging efficiency. I investigated interspecific variation in communal roosting tendencies across categories of several ecological factors to examine the relevance of each functional hypothesis in the evolutionary transition to communal roosting and the secondary reversal to solitary roosting habits. The study phylogenetic tree included 30 families and 437 species. Evolutionary transitions to communal roosting occurred more often on branches with flocking species and with larger species but were not associated with diet, territoriality, geographical area, or time of day. The association with flocking activities suggests that increased foraging efficiency, a factor thought to operate through the formation of flocks, may have been a key factor in the origin of avian communal roosting. However, several transitions to communal roosting occurred on branches with nonflocking species, indicating that foraging efficiency may not be the only factor involved in the evolution of communal roosting. Secondary losses of communal roosting habits occurred on several branches, with a concomitant loss of flocking behavior and a tendency to exhibit territorial behavior and nocturnal foraging. Secondary losses suggest that communal roosting is costly to perform and maintain and may be lost when an asocial selection regime operates. The large number of exceptions to the above patterns may force a reevaluation of current functional hypotheses about communal roosting in birds.
Key words: Accipitridae, Anatidae, birds, communal roosting, evolutionary reversals, flocking, phylogeny.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Aggregations of roosting individuals are common in primates (Anderson, 1998
), bats
(Lewis, 1995;
Wilkinson, 1995), and birds
(Eiserer, 1984). In birds, and
in other animals as well, the adaptive value of communal roosting is not
clearly understood. Avian communal roosting is thought to confer benefits in
terms of reduced thermoregulation costs, decreased predation risk, and
increased foraging efficiency (Eiserer,
1984; Ydenberg and Prins,
1984) but the results are still controversial
(Richner and Hebb, 1996). A
greater knowledge of the ecological correlates of communal roosting could help
unravel issues about the origin and maintenance of the trait in birds.
Studies of ecological correlates have focused thus far on simple
interspecific comparisons. Ydenberg and Prins
(1984) showed that duck
species foraging primarily on vegetable matter tend to form communal roosts to
a greater extent than carnivorous species. In general, factors that promote
flock feeding, such as diet type, appear to favor the evolution of communal
roosting in ducks and in other species as well
(Coombs, 1978;
Newton, 1972). However, it is
conceivable that information from taxa including many species may have
inflated observed relationships. In this study, I relied on phylogenetic
hypotheses to examine the ecological correlates of avian communal roosting. I
also used a large number of families to broaden the comparative analysis and
included taxa, such as raptors and owls, that are often ignored in
conventional studies of communal roosting
(Holt and Leasure, 1993).
Comparative analyses based on phylogenetic hypotheses have been useful in
shedding light on several types of social behavior in birds, including
colonial nesting (Beauchamp,
1999; Rolland et al.,
1998) and cooperative breeding
(Edwards and Naeem, 1993;
Peterson and Burt, 1992). In
the context of communal roosting in birds, comparative analyses can be useful
for two purposes. First, use of phylogenetic information can pinpoint the
ecological factors that are associated with the origin of communal roosting in
several taxa. Second, phylogenetic information can also be used to identify
under which circumstances communal roosting secondarily reverts to the
solitary state. If roosting communally is costly to maintain and perform,
changes in the environment leading to reduced benefits from aggregations could
induce the disappearance of the trait in favor of solitary habits
(Wcislo and Danforth, 1997).
In this paper, I first review functional hypotheses for communal roosting in
birds. I then use functional hypotheses to predict under which ecological
circumstances communal roosting is expected to arise and be lost
secondarily.
Functional hypotheses and predictions
Several features of communal roosting in birds may provide adaptive
benefits (Table 1). Advantages
of communal roosting that are commonly mentioned include thermoregulation
benefits, protection from predation, and foraging efficiency
(Eiserer, 1984;
Ydenberg and Prins,
1984).
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Thermoregulation benefits
The presence of nearby companions in communal roosts is thought to reduce
the energetic demands for thermoregulation through mechanisms such as huddling
and wind reduction. Although the physical structure of the roost can provide
protection from weather, evidence suggests that thermoregulation benefits also
accrue from the presence of companions
(Chaplin, 1982;
Du Plessis and Williams, 1994;
Du Plessis et al., 1994;
Francis, 1976;
Gyllin et al., 1977;
Putaala et al., 1995;
Walsberg and King, 1980;
Wiersma and Piersma, 1994;
Yom-Tov et al., 1977).
Thermoregulation benefits have been argued to be a significant factor in the
origin and maintenance of communal roosting in some species
(Du Plessis and Williams,
1994; Du Plessis et al.,
1994). Decreases in the cost of thermoregulation are probably more
important for species that regularly face detrimental weather conditions
during the nonbreeding season. In addition, smaller species may benefit to a
greater extent from savings in energetic demands due to their high body
surface-to-body mass ratio
(Merola-Zwartjes, 1998). If
the reduced cost of thermoregulation is a driving force in the origin and
maintenance of communal roosting in birds, one might expect the behavior to
arise more often in species that spend a significant part of the nonbreeding
season in cold, northerly areas and that have a small body mass.
Predation avoidance
Roosting communally may serve to protect birds from predation through some
of the same mechanisms thought to operate in bird flocks (see
Elgar, 1989, for a review).
For instance, the presence of more eyes in a communal roost may increase
predator detection. In addition, the sheer number of individuals using a
communal roost may decrease the individual risk of predation through the
dilution effect. The greater conspicuousness of communal roosts to predators
may, however, mitigate against the dilution effect
(Eiserer, 1984).
The geometric structure of the communal roost is also thought to provide
increased predation avoidance. For instance, individuals that occupy central
positions in a communal roost may be buffered from predators to a greater
extent than birds sleeping on the edge
(Weatherhead, 1983).
Weatherhead (1983) proposed
that increased protection from predation was the main factor that attracts
successful foragers to communal roosts. If the reduced risk of predation is a
driving force in the origin and maintenance of communal roosting in birds, one
would expect the behavior to arise more often in species that are more
susceptible to predation. Larger species, because of their size, may be less
exposed to predation threats (e.g.,
Sparling and Krapu, 1994;
Thiollay and Jullien, 1998).
In addition, predatory species such as raptors may face little predation risk
from other species because of their size and weaponry (e.g.,
Coleman and Fraser, 1986;
Parmelee, 1992). Hence,
communal roosting should be less likely to evolve among these species.
Foraging efficiency
Increased foraging efficiency is often considered the main advantage of
communal roosting in birds (Mock et al.,
1988; Richner and Hebb,
1995). Ward and Zahavi
(1973) first proposed that
communal roosts act as information centers whereby unsuccessful foragers can
follow more successful companions to good foraging areas. The information
center hypothesis is a plausible mechanism in a breeding colony because
successful and less successful foragers must return to the colony to attend
growing chicks (Barta and Szep,
1995; Beauchamp and Lefebvre,
1988). As communal roosts need not be visited on a consistent
basis, it is not clear from the hypothesis why a successful forager should
return to the roost unless group benefits are envisaged
(Richner and Hebb, 1995). The
following mechanisms illustrate how roosting communally can increase foraging
efficiency without invoking group selection arguments.
Weatherhead (1983) argued
that successful foragers return to the communal roost to gain protection from
predation by establishing a more central roost location buffered from
predators by surrounding individuals. In this scenario, inferior competitors
increase their foraging success by following successful companions at the
expense of an increased risk of predation at the communal roost.
Alternatively, successful foragers may return to the communal roost to recruit
additional companions to good foraging areas
(Evans, 1982;
Marzluff et al., 1996;
Richner and Hebb, 1995,
1996). The presence of more
companions at the foraging patch can then provide benefits in terms of
predation avoidance and increased feeding rates
(Beauchamp, 1998;
Elgar, 1989). The recruitment
hypothesis is probably more relevant when passive recruitment at the foraging
patch is not expected to attract companions very rapidly.
Another mechanism proposes that the daily return of foragers to the
communal roost induces a local concentration of foragers in space that allows
individuals to more efficiently exploit the food discoveries of nearby
companions (Buckley, 1996).
Conspecific attraction, in general, is thought to be more efficient when
larger groups of foragers search for food in the same area
(Beauchamp et al., 1997;
Buckley, 1997;
Clark and Mangel, 1986;
Giraldeau and Beauchamp,
1999).
Finally, communal roosts have been hypothesized to be aggregations of
foragers spending the night as close as possible to good foraging areas. Large
roosts are expected to be joined to reduce commuting costs from the daily
center of activity to supplemental feeding areas
(Caccamise and Morrison, 1988;
Caccamise et al., 1997).
Unless roosting space is limiting, it is not clear from the hypothesis why
birds visiting supplemental feeding areas need spend the night together, as
individuals could instead roost solitarily in areas surrounding the rich
feeding patch. The patch-sitting hypothesis can be useful to predict the
location of communal roosts, but additional factors must be invoked to address
issues surrounding the origin and maintenance of communal roosting.
The above hypotheses about foraging efficiency are based on a major,
yet-untested assumption—namely, that birds roosting communally must also
feed in flocks. The daily commute to a communal roost makes little sense in
terms of foraging benefits without a foraging flock system. Generally,
foraging efficiency can only increase when birds use companions to locate rich
food patches and when resources are shared by all flock members. Communal
roosting is not expected to provide foraging benefits to species that exploit
nonshareable resources (Richner and Heeb,
1995). Nonshareable resources may be too small or actively
defended, and in either case this type of resource will necessarily restrict
the opportunity for social foraging. If increased foraging efficiency is a
driving force in the origin and maintenance of communal roosting in birds, one
would expect the behavior to arise more often among flocking species.
Moreover, communal roosting should be least likely to evolve in territorial
species or in species that forage solitarily. Finally, if flock cohesion is
more difficult to maintain at low light levels and companion recruitment is
less efficient due to poor visibility, nocturnal species may be less likely to
use communal roosts to increase foraging efficiency than their diurnal
counterparts.
Secondary losses of communal roosting
If roosting communally entails some evolutionary costs, a reduction in the
extent to which individuals benefit from aggregations could lead to the
disappearance of the trait from a population. For instance, if communal
roosting serves primarily to reduce thermoregulation demands, then
aggregations could eventually disappear from a population newly exposed to
warmer winter conditions. The secondary loss of communal roosting should be
associated with a reversal in the conditions that favored the origin and
maintenance of the trait in the first place.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Phylogenetic information
I used two criteria to choose taxa. First, I relied on taxa with extensive phylogenetic information based on traits not likely involved in the evolution of communal roosting in birds. Phylogenetic hypotheses included in the analysis were thus mostly based on morphological or molecular traits (Table 2). I used a selected set of detailed phylogenies, as opposed to a larger number of only partially resolved hypotheses, to allow more precise mapping of changes in ecological traits along the bird phylogeny. Precise mapping can then be used to establish with more certainty the relationship between the evolution of communal roosting and ecological variables. Second, I sampled diverse taxa along the bird phylogeny in an effort to include clades that vary with respect to their ecological characteristics. The purpose of the two criteria for data inclusion, therefore, is not so much to document all known cases of communal roosting in birds, but to identify more precisely the set of ecological variables that shaped the evolution of the behavior.
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Ecological information
I collected data on communal roosting tendencies and ecological traits from
several sources, including general handbooks on bird ecology and behavior and
ornithological journals (Table
2). Independent ecological variables included female body mass,
diet, feeding territoriality, flocking behavior, roosting area, and roosting
period.
I define a communal roost as an aggregation of more than two birds that sleep together. This definition forces the expression of sociality beyond the bond between two birds of opposite sex. I assessed communal roosting tendencies during the nonbreeding season when parental duties probably interfere less with the expression of sociality. With respect to roosting behavior, I categorized species into one of three possible states: (0) strictly solitary, individual birds always sleep in different places; (1) occasionally solitary, roosting aggregations are known to occur under peculiar circumstances; and (2) strictly communal, individual birds always sleep with companions. For the purpose of the analysis, I created a dichotomous variable by pooling categories 1 and 2 where roosting is known to occur at least occasionally.
I assigned species to one of five categories of body mass: (0) 1-499 g; (1)
500-999 g; (2) 1000-1499 g; (3) 1500-1999 g; and (4) 
2000 g. I only
recorded female body mass to provide a more uniform assessment of body mass
across species. The choice of five body mass categories was made to facilitate
comparisons across taxa that often diverged widely in terms of mass.
Nevertheless, the division into five categories was fine enough to allow
variation within large taxa.
Diet was classified into one of three categories: (0) mostly vegetarian,
diet includes 75% or more plant matter; (1) omnivorous, diet includes 25-75%
plant matter; and (2) mostly carnivorous, diet includes 
25% plant matter.
Quantitative analyses of diet composition were not available for every
species. In this case, diets that included a wide spectrum of animal and plant
matter were categorized as omnivorous.
While feeding or searching for food, birds often tolerate nearby
conspecifics to different extents. I define a feeding territory as a space
where foraging resources are defended from other conspecifics. Most cases of
territorial behavior involve single or paired birds. However, flocks of birds
sometimes defend a feeding territory from neighboring groups (e.g.,
Goodwin, 1986;
Harrap and Quinn, 1995). In
this case, territorial behavior is not expected to affect the expression of
social behavior, as several birds are available to flock and roost within the
shared territory. Therefore, group defense of resources was not considered a
form of feeding territoriality for the purpose of the analysis. I classified
species into one of three categories of feeding territoriality: (0) strictly
nonterritorial, birds show no aggression related to food and often forage in
close proximity; (1) occasionally territorial, territorial feeding occurs at
times under peculiar circumstances; and (2) strictly territorial, individual
birds or pairs defend a territory during the length of the nonbreeding
season
The tendency to search for food or feed in groups was assessed for each species. I define flocking behavior as the aggregation of more than two individuals during foraging activities. I classified species into one of three categories of flocking behavior: (0) strictly solitary, birds forage singly and only aggregate by chance; (1) occasionally social, feeding aggregations of more than two birds occur under peculiar circumstances; and (2) strictly social, individuals search for food and feed in flocks.
I recorded the geographical area used by birds during the nonbreeding season and classified species into one of two possible states: (0) northern, birds spend the nonbreeding season above the 30th parallel; and (1) southern, birds spend the nonbreeding season below the 30th parallel. Common southern areas included Central and South America, Africa, Australia, and India. Although some southern species probably experience climatic conditions that are similar to that of northern species (e.g., species living at the southern tip of South America), it is likely that weather during the nonbreeding season is milder for the southern birds as a whole than for their northern counterparts. Species that occur during the nonbreeding season in northern as well as southern areas were classified in the northern state for the purpose of the analysis.
I recorded the period during the day where roosting activities occurred. I classified species into one of three states: (0) strictly nocturnal; (1) occasionally nocturnal or diurnal; and (2) strictly diurnal. The second category includes, for instance, species that adjust foraging and roosting activities as a function of tide levels regardless of time of day.
Comparative analyses
I performed two types of analysis. The first analysis focuses on the origin
of communal roosting and the second on secondary losses of communal roosting
habits. With respect to the origin of behavior, I used the contingent states
test to examine whether a transition from solitary to communal roosting was
more likely to occur under the different states of each independent variable
(Sillen-Tullberg, 1993). The
contingent states test was developed to investigate the relationship between
discrete characters in an evolutionary context and has been used in several
recent studies (e.g., Beauchamp,
1997; Temrin and
Sillen-Tullberg, 1995). I considered the occurrence or
nonoccurrence of communal roosting as the dependent variable and all the
aforementioned discrete ecological traits as independent variables. The test
requires that all traits under scrutiny have been reconstructed over the
phylogenetic tree. I used the MacClade program (version 3.5) to reconstruct
the occurrence of each variable over the phylogenetic tree
(Maddison and Maddison,
1992). Because branch lengths are usually unknown, another
assumption is that state transitions are equally likely for all branches.
All branches that have maintained the solitary roosting state are regarded as having the potential for a transition to communal roosting. Branches that already carry the communal roosting state as well as those where a reversal of state occurs from the communal roosting state to solitary roosting were left out because a transition to communal roosting cannot take place on these branches. Branches where the roosting state was unknown were also left out. The unit of analysis was thus the set of independent variables on each selected branch.
I used logistic regression to explore the relationship between the two
roosting states and the independent variables. The logistic regression relates
the proportion, p, of branches where the communal roosting state
occurs to the states of the independent variables according to the following
equation:
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). The significance level was evaluated with a chi-square
statistic with 1 df. The final model was found when the omission of any
remaining variable caused a significant increase in the deviance
D. The second analysis included the set of branches where a reversal occurred from communal to solitary roosting. If a given ecological factor is involved in the origin of communal roosting, one would expect that a reversal to solitary roosting will be associated with a directional change in the value of this factor. To examine this hypothesis, I calculated for each ecological variable the difference between the value of the trait on the branch where the reversal occurred and that on the branch immediately preceding the change. Under the null hypothesis that the reversal of roosting states is independent of changes in the state of a given ecological variable, mean differences should be equal to zero. I tested the hypothesis separately for each ecological variable with a bilateral paired t test. I used the numerical value of each state, as reconstructed over the phylogenetic tree, to calculate differences. However, I needed a different approach for body mass because the grain of the classification was often too coarse to provide an estimate of change. In this case, I retraced the evolution of body mass over the phylogenetic tree by averaging body mass over the two branches of each tree node. The difference between current and ancestral body mass was calculated as above using averaged values. In the calculation, I ignored branches where the value of body mass was lacking.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The study phylogenetic tree included 30 families and 437 species (Figure 1). Among families and tribes with more than two species, the mean percentage of species with known roosting habits was 85.3% (SD = 14.7%, n = 23).
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Origin of communal roosting
Analysis of the phylogenetic tree for the origin of communal roosting
yielded a total of 201 branches. Communal roosting arose in 21 lineages, or
nearly 10% of the available branches. The reconstruction showed that communal
roosting arose once at the base of the phylogenetic tree and was maintained in
several families (Figure 1).
The largest number of transitions from solitary to communal roosting occurred
in raptors (Accipitridae), with nearly 62% of all transitions.
One portion of the tree was unresolved with respect to roosting behavior. Among passerines, communal roosting behavior is absent in nuthatches (Sittidae) and in the closely related tits (Paridae). One possible scenario involves two successive disappearances of the trait. An alternative involves one disappearance of communal roosting before the two branches and the subsequent reversal to communal roosting in the remaining lineages (i.e., Aegithalidae, Hirundinidae; Figure 1). I favored the former scenario to reduce the risk of inflating the number of evolutionary transitions to communal roosting.
The logistic regression revealed that evolutionary transitions from solitary to communal roosting occurred more frequently among branches with flocking species (D = 16.5, df = 2, p <.005) and with larger species (D = 9.5, df = 4, p <.05; Table 3).
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Nine of the 21 transitions to communal roosting (43%) occurred among solitary clades distributed in several families (Table 4). These nine transitions were restricted to carnivorous clades. No other pattern emerged in this small data set with respect to territoriality, roosting area, or roosting period.
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Secondary losses of communal roosting
Secondary losses of communal roosting tendencies occurred in 32 clades,
primarily in ducks (Anatidae) and to a lesser extent in waders
(Charadriiformes) and herons (Ardeidae)
(Table 5). The transition from
communal to solitary roosting was accompanied by three significant shifts in
ecological traits: (1) a shift from flocking to solitary foraging (t
= -8.20, p <.001, n = 31), (2) a shift from
nonterritorial to territorial behavior (t = 3.13, p
<.005, n = 25), and (3) a shift from nocturnal to diurnal roosting
(t = 2.66, p <.01, n = 28). The remaining
ecological traits failed to show consistent shifts alongside the transition to
solitary roosting (p >.20). With one exception, shifts in
territorial behavior and roosting period occurred concurrently with changes in
flocking activities (Table 5).
In the 31 reversals to solitary roosting where the flocking state could be
assessed, eight (26%) occurred with no changes in flocking behavior
(Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Origin of communal roosting in birds
Results of the comparative analysis show that evolutionary transitions to communal roosting occurred more often among branches with flocking species and with larger species. Transitions to communal roosting were not influenced by diet, territoriality, roosting area, or time of day once flocking tendencies and body mass were taken into account. Communal roosting appears to be an ancestral state that has persisted in several families. Nevertheless, the trait has also emerged more recently in families with no ancestral tendencies to roost communally, especially in raptors (Accipitridae).
Results provide little support for the hypothesis that thermoregulation
benefits played a leading role in the origin of communal roosting in birds
because the behavior was expected to arise more often in northern areas and in
smaller species. In some species, substantial thermoregulation benefits are
probably associated with the maintenance, rather than the origin, of communal
roosting (Du Plessis and Williams,
1994; Du Plessis et al.,
1994). In other species, the role of thermoregulation benefits has
been questioned, as gains often appear to be insufficient to compensate for
the extra energy needed to travel to distant communal roosts
(Gyllin et al., 1977;
Yom-Tov et al., 1977).
Instead of roosting communally, small species can develop other physiological
or behavioral adaptations to cope with harsh weather conditions. Features such
as nocturnal hypothermia and cavity roosting can compensate for the increased
demands of thermoregulation in solitarily roosting birds
(Cooper and Swanson, 1994;
Du Plessis et al., 1994;
Kendeigh, 1961;
Merola-Zwartjes, 1998)
Predation avoidance also appears unlikely to be a major force in the origin
of communal roosting in birds because the behavior was expected to be least
likely to evolve in large species, especially in raptors. As with
thermoregulation benefits, predation avoidance may best be viewed as a factor
that potentially contributes to the maintenance of communal roosting. Evidence
to support the hypothesis that communal roosting can reduce predation risk is
scant (Eiserer, 1984). Studies
suggest that birds often compete for inner and higher positions in roosts,
presumably as a means to reduce predation risk
(Buckley, 1998;
Feare et al., 1995;
Jenni, 1993;
Moynihan and Hall, 1955;
Swingland, 1977;
Weatherhead and Hoysak,
1984). Differential survival as a function of roost position,
however, has not been documented yet and could be related to other factors
such as thermoregulation demands or plumage protection from droppings
(Yom-Tov, 1979). Other
aspects of communal roosts may be involved in predation avoidance and require
further attention. For instance, certain roosting habitats may provide better
protection against predators, and communal roosting could arise more often in
species that regularly roost in more exposed habitats.
The association between flocking activities and communal roosting suggests
that foraging efficiency may have been a driving force in the evolution of
communal roosting in several species of birds. However, precise mechanisms
that allow birds to benefit from communal roosting cannot be identified by the
comparative analysis. Daily returns to a communal roost could increase the
density of foragers in space and increase the chances of locating companions
through local enhancement (Buckley,
1996). Recruitment of companions at the communal roost to increase
the size of feeding groups has also been proposed
(Marzluff et al., 1996;
Richner and Hebb, 1996). On
the other hand, the hypothesis that birds follow successful companions to rich
feeding areas has received little support, and simpler mechanisms, such as
local enhancement, can often produce similar aggregations of foragers at rich
food patches (Richner and Hebb,
1995). The challenge for future studies will be first to quantify
the impact of communal roosting on foraging efficiency and then to assess
which mechanisms enhances success.
The positive effect of body mass on the occurrence of communal roosting was not expected under any of the above functional hypotheses. The finding probably reflects the preponderance of communal roosting habits in large species such as raptors. Nevertheless, it appears that the association with body mass often weakens once communal roosting has evolved. Several small species show the trait today, such as swallows (Hirundinidae), swifts (Apodidae), and shorebirds (Charadriiformes) (see Figure 1). In addition, reversals to solitary roosting habits occurred without a concomitant reduction in body mass. Body mass probably played a lesser role in the evolution of communal roosting than flocking behavior, but the reasons for the original association are unclear.
Secondary losses of communal roosting
Results reveal that reversals to solitary roosting habits in clades that
roost communally can occur and are, in fact, quite common in several families.
When compared with their immediate ancestors, species that reverted to
solitary roosting also became solitary foragers and to a lesser extent
territorial and nocturnal dwellers. Results also confirm the association
between flock feeding and communal roosting: communal roosting in birds
appears on branches with flocking species and disappears as flocking is
replaced by solitary foraging.
Although thought to be rare in animals
(van Rhijn, 1990), secondary
losses of social behavior have been noted in bacteria, insects, and birds
(Peterson and Burt, 1992;
Velicer et al., 1998;
Wcislo and Danforth, 1997).
Two main hypotheses can be invoked to explain the loss of social behavior in
general (Velicer et al.,
1998). The two hypotheses are not necessarily mutually exclusive
and can act together to produce losses in social behavior. The first
hypothesis proposes that in the absence of positive selection for social
behavior, random drift mutations lead to trait decay over evolutionary time.
Individuals gain no advantage from changes in a trait that occur through
random drift mutations. An alternative hypothesis argues that social behavior
is costly to maintain and perform and that under conditions that favor asocial
activities, individuals gain an advantage when some social functions are
lost.
Two lines of evidence suggest that communal roosting was costly to maintain
and that under asocial conditions the behavior was selected against in favor
of solitary roosting. First, most reversals occurred at the species level,
which suggests that an asocial selection regime operated only recently,
leaving little time for random drift mutations to act so consistently across
so many species. Second, roosting behavior is often polymorphic in birds. In
polymorphic species, some members of the population roost solitarily, while
other members roost communally. For instance, in steamer ducks
(Tachyeres sp.; Weller,
1972, 1976), red
kites (Milvus milvus; Hiraldo et
al., 1993), crested caracaras (Polyborus plansus;
Morrison, 1996), and ravens
(Corvus corax; Marzluff et al.,
1996), adult birds maintain foraging territories year-round, while
immature birds forage in flocks and attend communal roosts. Random drift
mutations would not be expected to consistently affect the roosting
preferences of birds belonging to different age groups within a species.
Territoriality and nocturnal foraging could promote an asocial selection
regime and be involved in the transition to solitary roosting. Several species
that reverted to solitary roosting are specialized foragers holding year-round
territories (Bartmann, 1988;
Callaghan, 1997;
Eldridge, 1986;
Pierce, 1986). Joining a
distant communal roost to increase foraging efficiency makes little sense in a
territorial species and increases the risk that competitors take over the
abandoned territory. Similarly, foraging at night can reduce the efficiency of
visual recruitment and render group cohesion more difficult to maintain.
Joining distant communal roosts can increase commuting costs, and, in the
absence of benefits, communal roosting may make little sense for nocturnal
foragers. In general, conditions that favor solitary foraging can create an
asocial selection regime that could lead to the disappearance of costly social
functions.
Implications for the evolution of communal roosting
Communal roosts have been considered supra-optimal groups where fitness
increases monotonously as a function of roost size
(Kramer, 1985). The finding
that reversals to solitary roosting occur in so many species indicates that
functional analyses ought to consider not only the benefits of communal
roosting but also the costs. Few studies have focused on the potential costs
of communal roosting in birds. In territorial birds, joining distant communal
roosts could increase the risk that competitors take over the territory during
long absences (Caccamise et al.,
1997). Eiserer
(1984) mentions the
possibility that communal roosts could be more conspicuous to predators. In
support of this hypothesis, gibbons (Hylobates lar), a primate
species that forages in groups, avoid forming sleeping groups at night because
large aggregations are more conspicuous to predators than single individuals
(Reichard, 1998). Other
potential disadvantages include commuting costs, increased competition for
food around the roost, deterioration of plumage quality due to droppings
(Yom-Tov, 1979), and the risk
of getting stuck as individuals leave roosting cavities
(Stanback, 1998).
Null hypothesis
Despite a significant association with flocking activities, communal
roosting originated on several occasions in branches with nonflocking species.
In addition, reversals to solitary roosting also occurred occasionally without
concomitant changes in flocking tendencies. Foraging efficiency appears an
unlikely benefit of communal roosting in solitary species, as group formation
and recruitment are necessarily restricted. Therefore, foraging efficiency
cannot be viewed as the only factor involved in the origin of avian communal
roosting, and other aspects of communal roosting beyond thermoregulation
demands and predation avoidance must be examined. The possibility also remains
that communal roosting has no adaptive value in some species.
As a null hypothesis, I propose that communal roosting is a trait not
currently under positive selection. In this scenario, communal roosts
represent chance aggregations of individuals that are searching independently
for roosting sites with similar characteristics. For instance, sites that
offer protection against predators and harsh weather conditions and that are
located close to feeding areas may be actively selected as roosting locations
by solitary individuals (Eiserer,
1984). Roosting aggregations would only occur when the number of
optimal sites is limiting, in which case some individuals are forced to share
the same site. Because communal roosts may form in sites that offer
exceptional qualities, functional considerations must necessarily untangle
effects that arise from the roost location itself from those provided by
companions. Only benefits that arise from the presence of companions can be
invoked as ultimate determinants of communal roosting.
In conclusion, results of the comparative analyses suggest that increased foraging efficiency may have been a key factor in the origin of communal roosting in birds. Nevertheless, several exceptions have been noted, and a research program based on rejection of a null hypothesis could prove a valuable tool in understanding the costs and benefits of communal roosting in birds.
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