Behavioral Ecology Vol. 12 No. 3: 325-329
© 2001 International Society for Behavioral Ecology
Fitness costs and benefits of cowbird egg ejection by gray catbirds
Department of Zoology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Address correspondence to J.C. Lorenzana.
Received 30 August 1999; revised 4 September 2000; accepted 12 September 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Gray catbirds (Dumetella carolinensis) eject over 95% of brown-headed cowbird (Molothrus ater) eggs placed into their nests. Ejection behavior could be maintained by selection from either: (1) cowbird parasitism, if the costs of accepting a cowbird egg outweigh the costs of ejecting it, or (2) conspecific parasitism, if such parasitism occurs naturally and results in ejection. This study tested the above hypotheses by measuring the cost of acceptance of cowbird parasitism (n= 38 experimentally introduced cowbird chicks) and of cowbird egg ejection (n = 94 experiments), as well as the frequency of natural conspecific parasitism among 229 catbird nests observed and the frequency of conspecific egg ejection (n = 27 experiments). The conspecific parasitism hypothesis was not supported because catbirds accepted all foreign conspecific eggs placed into their nests, and no natural conspecific brood parasitism was detected at any nests. The cowbird parasitism hypothesis was strongly supported because the cost of accepting a cowbird chick (0.79 catbird fledglings) is much greater than the cost of ejecting a cowbird egg (0.0022 catbird fledglings per ejection).
Key words: brood parasitism, brown-headed cowbird, cost of parasitism, Dumetella carolinensis, egg ejection, gray catbird, Molothrus ater.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Brown-headed cowbirds (Molothrus ater) are obligate brood parasites that lay their eggs in nests of over 220 species. Hosts that raise parasitic young often raise fewer of their own young (e.g., Lorenzana and Sealy, 1999
;
Payne, 1977) due to host egg
removal by the female parasite, reduced hatchability of host eggs, and
nestling competition (Rothstein,
1990). In response to the cost of parasitism, species may evolve
behaviors that thwart a parasitism attempt, such as egg ejection, nest
desertion, or egg burial. Nonetheless, acceptance of cowbird eggs is the most
common response of cowbird hosts
(Rothstein, 1990). Only about
20 species parasitized by brown-headed cowbirds eject foreign eggs
(Ortega, 1998).
Only Røskaft et al.
(1993) experimentally
quantified the costs of ejection and acceptance of cowbird eggs. They tested
Bullock's orioles (Icterus bullockii), which puncture cowbird eggs
before removing them from their nests (their bills are too narrow to grasp an
intact cowbird egg). Interestingly, Bullock's orioles did not incur a
statistically significant cost of parasitism. Røskaft et al.
(1993) nevertheless concluded
that ejection behavior in Bullock's orioles was selected for because the cost
of parasitism for a brood of four chicks (0.40 oriole chicks) was greater than
the cost of ejection (0.26 oriole eggs).
In this study, we compare the cost of egg ejection and the cost of
parasitism for a grasp-ejecter, the gray catbird (Dumetella
carolinensis). Catbirds eject over 95% of cowbird eggs placed
experimentally in their nests (Rothstein,
1975). Although catbirds (36.9 g) are slightly larger than
cowbirds and are large relative to most hosts of the cowbird, they are one of
the smallest ejecter species; only the warbling vireo (Vireo gilvus)
is much smaller, at 15 g (Sealy,
1996). The cost of ejection for catbirds was expected to be low
because grasp-ejecters have a lower probability of damaging their own eggs
than puncture-ejecters (Rohwer et al.,
1989; Table 1).
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The hypothesis that parasitism by brown-headed cowbirds selects for egg ejection in gray catbirds requires that acceptance is more costly than ejection of cowbird eggs. An alternative hypothesis is that egg ejection evolved in catbirds in response to conspecific brood parasitism. This hypothesis predicts that catbirds regularly lay their eggs in other catbirds' nests, and that the victims eject the foreign eggs. Catbirds were not expected to eject other catbird eggs because both intra- and interclutch variations in the appearance of their immaculate eggs are small. If ejection of cowbird eggs is a secondary result of ejection of conspecific eggs, acceptance of cowbird parasitism would not necessarily be costly.
The cost of ejection was measured by adding model cowbird or catbird eggs to catbird nests. The cost of ejection was the frequency with which catbirds mistakenly ejected one of their own eggs instead of the parasite egg, or accidentally lost one or more of their own eggs when ejecting the parasite egg. Traditionally, the cost of ejection is calculated in terms of the number of host eggs lost, and the cost of rearing a cowbird chick is calculated in terms of the number of host chicks fledged. To determine whether the cost of rearing a cowbird chick is greater than the cost of ejection, the two costs must be converted to the same units. Eggs are less valuable than fledglings because the embryos have a lower probability of fledging due to non-viability, inadequate brooding, and depredation.
Egg ejection would be selected only if recoverable costs of
parasitism were greater than the cost of ejection. Costs are recoverable if
they are eliminated when a cowbird egg is ejected, whereas they are
non-recoverable if they are incurred regardless of whether a cowbird egg is
ejected (Røskaft et al.,
1990). Non-recoverable costs include the removal or destruction of
host eggs by the parasite (Røskaft
et al., 1990). Because host egg removal occurs in naturally
parasitized catbird nests (Scott,
1977), host egg removal was simulated in this study.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The study was conducted from mid-May to early July, 1996 to 1998, at Delta Marsh, Manitoba (50°11' N, 98°19' W), along a 10-km strip of a dune ridge forest that runs along the southern shore of Lake Manitoba, Canada. MacKenzie (1982
) and
Neudorf (1991) provide
detailed descriptions of the study area.
Conspecific parasitism experiment
The conspecific brood parasitism experiment was conducted in 1996. Real
catbird eggs were added to catbird nests during the egg-laying stage and the
first 5 days of incubation. Single catbird eggs were switched between pairs of
catbird nests that were close to the same stage of development (see
Lanier, 1982), or a real
catbird egg was added to a catbird clutch without removing any eggs from the
original clutch (see Bischoff and Murphy,
1993). All catbird eggs added to nests were marked with a
non-toxic marker. Nests were checked at least every second day for 6 days
after the introduction of the foreign egg. An egg was considered ejected if it
disappeared from an otherwise active clutch
(Rothstein, 1975). It was
considered accepted if it remained in the active nest for at least 6 days.
The frequency of natural conspecific brood parasitism was determined by recording the appearance in a nest of more than one new catbird egg per day during the laying stage or the appearance of an egg during the incubation or nestling period. All eggs were numbered with a non-toxic marker on the day they were laid to detect the possible replacement of a catbird egg with a foreign catbird egg.
Cost of cowbird egg ejection
The cost of ejection experiment was conducted in 1997. Model eggs were made
of plaster-of-Paris from casts of real cowbird eggs and painted with acrylic
paints and polyurethane to mimic real cowbird eggs
(Rothstein, 1975). All model
cowbird eggs were added before noon. Catbird nests were parasitized with one
model cowbird egg during the egg-laying or early incubation stage. A catbird
egg was not removed when a model cowbird egg was added to the nest because the
probability of ejection is not affected by the removal of a host egg
(Rothstein, 1975). Catbird
eggs were removed during the cost of acceptance experiment (see below) because
an increase in the number of chicks present could affect the survivorship of
the remaining catbird chicks.
Nests were checked 5 h after parasitism and every morning thereafter until the cowbird egg was ejected. Ejection was recorded when the cowbird egg was gone, and acceptance was recorded when it remained in an active nest for at least 6 days. Ejection with an associated cost was recorded when the model cowbird egg, along with one or more host eggs, were missing, but the nest was still active.
The cost of ejection was calculated as the number of catbird eggs that disappeared during the same time period that the cowbird egg disappeared minus the rate of partial clutch reduction at unparasitized nests. To transform the value of lost eggs to chicks, each lost egg was multiplied by the probability that it would have survived to fledging.
Cost of cowbird egg acceptance
The cost of acceptance experiment was conducted in 1997 and 1998.
Estimating the cost of accepting a cowbird egg was done by experimentally
placing newly hatched cowbird chicks into catbird nests. Nests of accepter
species on the study area were monitored for cowbird eggs. All cowbird eggs
found were incubated artificially to eliminate the risk of loss to predators.
One newly hatched cowbird chick was placed into each experimental nest,
whereas no chicks were added to control nests. A catbird egg was randomly
removed from half of the experimental nests to simulate host egg removal by
the female cowbird (e.g., Sealy,
1992). We attempted to place cowbird chicks into nests on the day
before catbird chicks were predicted to hatch because cowbirds usually hatch 1
day earlier than catbird chicks.
A Mann-Whitney Test was used to test whether experimental parasitism with a
cowbird chick affected catbird fledging success. Only successful nests (i.e.,
nests that fledged at least one catbird or cowbird chick) were included in the
analysis. Fledging success (F) was calculated as a proportion of
catbird offspring present on the first day of hatch:
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: 621) was
performed. |
RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Conspecific parasitism experiment
All experimentally introduced catbird eggs were accepted regardless of the stage that the nest was parasitized or whether the egg was switched (n = 17) or added (n = 10). No more than one egg was laid in the same nest on the same day in 1996 and 1997 in 95 catbird nests that were found on or before the first catbird egg was laid, and survived to at least 5 days after the first catbird egg was laid. No catbird eggs appeared after completion of the normal laying sequence.
Cost of cowbird egg ejection
Ninety-six percent of the cowbird eggs (n = 94 experiments)
experimentally added to catbird nests were ejected. The absence of peck marks
on accepted eggs suggests no puncture-ejection attempts that failed. Fifty-six
percent of the model eggs were ejected within 5 h (n = 90 ejections),
with the longest ejection time being 2 to 3 days.
The cost of ejection, measured as the number of catbird eggs that disappeared per ejected cowbird egg, was 0.02 (n=90 ejections). We corrected for the background rate of partial nest predation, which was 0.0094 (2/213) eggs lost per day. (One catbird egg disappeared from each of two nests out of 213 days on which 33 nests were checked during the egg stage.) The corrected cost of ejection was only 0.01 eggs per ejection. The cost of ejection was converted to a cost in terms of host fledglings to make it comparable to the cost of acceptance. Because catbird eggs have a mean probability of fledging from unparasitized nests of 0.22 (n = 204 nests), the cost of ejection of 0.01 eggs is equal to a cost of 0.0022 catbird fledglings.
Cost of cowbird egg acceptance
The presence of a cowbird chick significantly decreased the proportion of
catbird chicks that fledged (Mann-Whitney U = 439.5, p =.016); 0.73
± 0.08 of the catbird brood fledged from 20 parasitized nests compared
to 0.91 ± 0.02 from 62 unparasitized nests. Therefore, the presence of
the cowbird chick decreased the proportion of catbird young that fledged by an
average of 18%. Because the mean clutch size was 4.4 ± 0.6 eggs
(n = 229 nests), the cowbird chick resulted in a mean of 0.79 fewer
catbird chicks fledging per nest.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Gray catbirds accepted experimental foreign conspecific eggs, and did not lay eggs in other individuals' nests, thus lending no support for conspecific brood parasitism as a selective pressure in the evolution of egg ejection. Our results strongly support the hypothesis that interspecific brood parasitism selected for egg ejection in gray catbirds. The cost of ejection by gray catbirds was 0.02 catbird eggs (or 0.0044 catbird fledglings) lost per ejection, a figure identical to that measured by Rothstein (1976
). When the background
rate of partial clutch reduction was taken into account, the cost of ejection
was only 0.01 catbird eggs per ejection (or 0.0022 catbird fledglings). The
cost of parasitism (0.79 catbird fledglings per nest) was 360 times greater
than the cost of ejection (0.0022 catbird fledglings per nest). Therefore, the
hypothesis that cowbird parasitism is a current selective pressure maintaining
egg ejection in catbirds was strongly supported.
Our experiment measured two of three ejection costs: egg damage and
recognition error (Davies and Brooke,
1988). Because nests were not naturally parasitized, retribution
from adult parasites (Soler et al.,
1995; Zahavi,
1979) was not measured. Egg damage, when the host egg is cracked
or broken and/or ejected, was the only type of ejection cost that catbirds
experienced when a model egg was introduced into their nests. Twice, a catbird
egg went missing during the same period that the cowbird egg was ejected.
Catbirds did not accidentally eject one of their own eggs instead of a cowbird
egg. Recognition errors are normally associated with parasitic eggs that are
similar in appearance to host eggs. Catbird eggs, however, are immaculate,
turquoise-blue, whereas cowbird eggs are smaller and are white with brown and
gray spots.
Our study strongly supports the hypothesis that cowbird parasitism provides
the current selective pressure maintaining ejection behavior in gray catbirds
and, indeed, catbirds sometimes are parasitized frequently
(Scott, 1977). Our study may
not, however, accurately predict the selective pressure that existed during
the evolution of egg ejection behavior within the mimid lineage
(Endler, 1986). Ten of 34 mimid
species (as recognized by Monroe and
Sibley, 1993) have been tested for ejection behavior through
experimental parasitism. Seven are ejecters, two are accepters, and one shows
intermediate responses (Carter,
1986; Cruz et al.,
1989; Finch, 1982;
Fraga, 1985;
Friedmann and Kiff, 1985;
Haas and Haas, 1998;
Rich and Rothstein, 1985;
Rothstein, 1975). A
phylogenetic analysis for the Mimidae has not been done; therefore, it is
impossible to discern the pattern of evolution of egg ejection within this
lineage. In addition to differences in size and egg characteristics of the
species, ejection costs were probably higher originally due to more
recognition errors.
Our study provides the first comparison of costs for a grasp-ejecter. The
ejection costs incurred by puncture-ejection are four times greater than that
of grasp-ejection (Table 1).
Even without transforming egg losses to fledgling units and correcting for the
background rate of partial nest predation, the cost of acceptance (0.79
fledgings) was still 40 times greater than the cost of ejection (0.02 eggs) in
catbirds. By contrast, the cost of acceptance (0.40 fledglings) was only 1.5
times greater than the cost of ejection (0.26 eggs) in nests of Bullock's
orioles, puncture-ejecters (Røskaft
et al., 1993). Furthermore, the acceptance cost was not greater
than the ejection cost when the brood size was less than four. Transforming
egg losses into fledgling units is much more important when the costs of
ejection and of rearing a cowbird are nearly equal, as in Bullock's
orioles.
The comparison of acceptance and ejection costs demonstrates that the
advantage of grasp-ejection may be much higher than expected had puncture- and
grasp-ejection been compared on the basis of ejection cost alone. The results
of this study are in accordance with the evolutionary equilibrium hypothesis,
which attempts to explain why so many species accept cowbird eggs
(Rohwer and Spaw, 1988). The
evolutionary equilibrium hypothesis proposes that species accept cowbird eggs
when the cost of rejection behavior exceeds the cost of accepting the parasite
egg. An alternative hypothesis is evolutionary lag, which proposes that given
enough time on an evolutionary scale, all hosts that are under selective
pressure will evolve egg rejection behavior
(Rothstein, 1982).
The cost of ejection is generally greater for puncture-ejecters than
grasp-ejecters because puncture-ejecters may accidentally puncture one of
their own eggs while attempting to eject the cowbird egg
(Rohwer et al., 1989). Small
hosts in particular must puncture-eject cowbird eggs because their bills are
too small to grasp them, and they likely suffer from the highest ejection
costs (Rohwer and Spaw, 1988).
Cowbird eggs have thicker eggshells than expected for their size
(Picman, 1989;
Rahn et al., 1988;
Spaw and Rohwer, 1987), which
increases the probability of damage to host eggs. The evolutionary equilibrium
hypothesis requires that species that incur a high enough cost from
puncture-ejection should accept parasite eggs or reject them by other means.
Nest desertion and egg burial have different costs associated with them,
including the loss of the current reproductive attempt and the time involved
in renesting (Hill and Sealy,
1994; Mico, 1998;
Sealy, 1995). Despite a number of options open to hosts, most hosts tend to
accept cowbird eggs.
In summary, the costs associated with cowbird parasitism account for the nearly complete rejection of cowbird eggs observed in catbird nests. The recoverable cost of parasitism was 360 times greater than the cost of ejection, which provides strong evidence that cowbird parasitism selects for egg ejection in this species. This study is particularly useful in comparing the current costs of ejection and parasitism for grasp- and puncture-ejecters. The cost of puncture-ejection may be high enough in some species to make acceptance less costly and a more adaptive strategy than egg ejection.
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ACKNOWLEDGEMENTS |
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The staff of the University of Manitoba Field Station (Delta Marsh) provided excellent living and working conditions during the field seasons. The officers of the Portage Country Club and Delta Waterfowl and Wetlands Research Station permitted us to conduct some of the work on their properties. Cottage and homeowners in the town of Delta kindly allowed us to search for nests on their properties. Celia McLaren, Heather Hinam, Monika Tan, and Cara Gillard helped find and monitor catbird nests, and measured some catbird chicks. Many others too numerous to mention also found nests. Cori Schuster and Celia McLaren collected some cowbird eggs and monitored their progress in the incubator. Mark Abrahams helped with the statistical analyses. Roger Evans and Terry Galloway made helpful comments throughout the development of the study. Susan Cosens, Eivin Røskaft, Stephen Rothstein, and two anonymous reviewers greatly improved the manuscript with their comments. Funding for this project was provided by a Natural Sciences and Engineering Research Council (NSERC) research grant awarded to S.G.S. and an NSERC postgraduate scholarship to J.C.L.
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