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Behavioral Ecology Vol. 14 No. 3: 360-366
© 2003 International Society for Behavioral Ecology
Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds
Museum of Vertebrate Zoology, University of California, Berkeley, Hastings Natural History Reservation, 38601 E. Carmel Valley Road, Carmel Valley, CA 93924, USA
Address correspondence to J.L. Dickinson. E-mail: sialia{at}uclink4.berkeley.edu.
Received 21 November 2001; revised 16 August 2002; accepted 26 August 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Approximately 45% of western bluebird (Sialia mexicana) females have some chicks in the nest that are not sired by their social mates. Extrapair fertilizations account for 42% of offspring in these nests and 19% of nestlings overall. I tested the hypothesis that males reduce nestling provisioning when their certainty of paternity or share of paternity is reduced. Capture and detention of socially monogamous males for 1 h or 24 h during the laying period reduced males' copulatory access and their ability to mate guard, increasing the frequency with which extrapair males intruded and attempted to copulate with resident females. Males detained during laying did not reduce their share of feeding trips compared to control males detained during incubation, compared to unmanipulated males, or compared to males that were captured but not detained. Males detained on territory for 1 h during the laying period did not reduce their share of feeding trips when they observed male intrusion, nor when they observed their mates accepting extrapair copulations. Males that witnessed their mates accepting extrapair copulations did not reduce their share of risk in provisioning. Genetic fingerprinting at nonexperimental nests indicated that males also failed to reduce their feeding contributions when their estimated share of paternity was reduced, even when a helper male was present to reduce the impact on nestlings. These results suggest that male western bluebirds do not make significant adjustments in their share of provisioning when they have evidence of partial paternity loss. Together with prior results, this study suggests that western bluebird males use an all-or-none rule, contributing approximately half of the parental provisioning at nests, as long they have some copulatory access to the female during egg laying.
Key words: cuckoldry, extrapair paternity, parental care, parental investment, paternal care, Sialia mexicana, western bluebirds.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The role of certainty of paternity in determining the degree and nature of male investment in offspring is equivocal. Although there are sound reasons that males should reduce their contribution to chick feeding when their share of paternity is reduced, there are also sound arguments for why they should not. The propensity to reduce parental effort with loss of paternity should depend on the costs and benefits of care, measured as the impact of chick feeding on the survival and reproductive success of a male's genetic offspring and any impact of chick feeding on the male's current and future survival or mating opportunities (Owens, 1993
; Werren et al., 1980; Westneat and Sherman, 1993; Whittingham et al., 1992). As these parameters are rarely measurable, most studies of paternity and parental care begin without species-specific, a priori predictions, and the job of discerning general patterns is of necessity left to comparative biology (Møller and Birkhead, 1992; Schwagmeyer et al., 1999).
The paradox of males feeding chicks that are not their own can be explained in terms of both parental effort and mating effort (Meek and Robertson, 1991; Trivers, 1972). In most species of passerines, parents recognize nests, not individual nestlings; a lack of offspring recognition may preclude discrimination of own versus extrapair (EP) chicks (Davies et al., 1992; Leonard et al., 1995). Without recognition, a reduction in chick feeding will affect both the EP chicks and the male's own young. Misdirected parental effort may thus be an unfortunate side effect of provisioning rules that increase the survival and reproductive success of a male's offspring.
If females are more likely to divorce males that reduce their care, then a male's feeding contribution to EP young may be viewed as mating effort because it increases the likelihood that the male will have a partner the next time around (Meek and Robertson, 1991). Although in some species males reduce their care when their share of paternity is reduced (Dixon et al., 1994; Freeman-Gallant, 1996; Møller, 1988), mounting evidence suggests that we cannot generalize from these results (Kempenaers et al., 1998; MacDougall-Shackleton and Robertson, 1998; Wagner et al., 1996; Whittingham et al., 1993). Information is needed from a variety of systems if we are to reach general conclusions about male investment strategies in broods of mixed paternity.
Variation in male response to cuckoldry may result from interspecific differences in the costs and benefits of reduction in male care, or it may be a function of variation in the ability of males to witness events leading to loss of paternity (Kempenaers and Sheldon, 1997). In the absence of reliable cues, withdrawal of care would be costly for males. Without behavioral data, genetic paternity estimates are a blunt instrument with respect to divining an effect of extrapair paternity (EPP) on male care. Failure to find an effect may be a consequence of heterogeneity among nests, which vary in the degree to which the male has detected his mate's participation in extrapair copulation (EPC).
Manipulating the male's access to behavioral evidence of female participation in EPC allows a more exact analysis. Behavioral experiments that manipulate the information males have about their potential loss of paternity are not foolproof, however, because it is difficult to know whether EPCs occurring during manipulations provide the same cues as those occurring naturally. Likelihood of detecting an effect can be increased by including a range of manipulations and analyses to tease apart male response to EPP and to examine a variety of possible cues.
Western bluebirds, Sialia mexicana, breed primarily as socially monogamous pairs but occasionally have adult male helpers at the nest (Dickinson et al., 1996). Male care is important for nestling growth and survival (Dickinson and Weathers, 1999). Even though females appear to compensate for lack of male care by nearly doubling their feeding rates, unaided females increase provisioning at the expense of brooding, which has severe consequences for nesting success. EPC attempts occur primarily when neighboring males intrude onto the territories of pairs (Dickinson and Leonard, 1996; Dickinson, unpublished radiotracking data). Although such attempts are usually thwarted by mate guarding (Dickinson and Leonard, 1996), the overall level of EPP in the population is 19%, and nearly half of females have at least one extrapair chick in their nests (Dickinson and Akre, 1998).
When male western bluebirds are detained, their mates experience a dramatic increase in the frequency of EP male intrusion and EPC attempts (Dickinson, 1997). Females express patterns of receptivity that favor EP males that are older than the female's social mate (Dickinson, 2001). By manipulating a male's ability to witness the outcome of his mate's EP encounters, it is possible to determine what sort of information, if any, will cause a male to reduce his share of feeding nestlings.
Provisioning rates are widely used to quantify parental effort, not only because they are relatively easy to measure, but because provisioning parents likely incur both energetic costs and increased risk of predation. In this study I conducted male detention experiments, manipulating the duration of detention, the resident male's ability to witness EPCs, and the timing of detention to determine if males respond to evidence of potential paternity loss by reducing their share of provisioning. As females vary in their receptivity to EPCs, I also analyzed male provisioning as a function of EPC acceptance and share of paternity.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Study population
These experiments were conducted in 1992–1998 on a population of western bluebirds nesting in boxes placed on Hastings Reserve and the adjacent Oak Ridge Ranch in 1983–1985. Blood samples analyzed for paternity were from 1984–1996. Since the onset of the study, adults and chicks have been color banded. Further descriptions of the study site and long-term monitoring procedures are reported in Dickinson et al. (1996)
.
Male removal experiments
The first experimental treatment, conducted in 1992–1993, involved removal of the breeder male to an indoor cage for 24 h on the second day of laying. This removal (
nests) was designed to reduce the male's copulatory access, ability to mate guard, and certainty of paternity. The control for capture involved removal of the resident breeder male for 24 h during incubation (
nests). These 12 nests included nests allocated to replace 3 nests where removed males were attacked and evicted upon returning to their territories. I designated a third group of 15 nests as an unmanipulated control group.
Nests were assigned to one of three treatment/control groups at random, except that 1.5 times as many nests were allocated to the unmanipulated as the experimental categories. Excluding three nests where the resident male was evicted, the percentage of nests surviving long enough to obtain provisioning data was 60% (laying removal; LR), 55% (incubation removal; IR) and 80% (unmanipulated control; UC). Nests failed due to death of one or both breeders or to predation on eggs and nestlings; none was abandoned.
Male detention experiments
In 1993–1998 I assigned pairs to one of the following treatments: unmanipulated control (
), capture control (), incubation control (), and laying detention (
). Assignment was at random except that four times as many nests were assigned to the laying detention treatment as to the controls. Additional nests were required in the laying detention treatment to determine whether male provisioning varied as a function of EP male intrusion, visibility of the EP male–female interaction, and EPC acceptance by the female.
I lured resident males into mist-nets using a decoy male (Dickinson, 2001). The capture control involved capturing the resident male and releasing him immediately. When detained, males were placed in cages within 3 m of their nest-box. Incubation control males were captured during the incubation period and detained for 1 h. Laying detention males were captured after the female finished laying the second egg and were detained for 1 h in an uncovered cage (
) or in a cage shielded with a cloth bag (visually occluded, ). At all but two nests, two to four observers watched the female continuously during detention to record EPC attempts and to score refusal/acceptance of EPCs by the female (Dickinson, 2001).
Sample sizes for analysis of paternal care were reduced due to nest abandonment 1 day after detention (
), nest predation (
), nest failure during cold weather (
), and death of one parent (
). The proportion of nests failing before I could obtain feeding data did not differ among laying detention and control treatments (laying detention: 30% failed; control: 18% failed, 
, 
, Fisher's Exact 
, power = 0.49).
Male's share of provisioning trips
I calculated the male's share of provisioning trips as the proportion of feeding trips to the nest by the male divided by the total number of trips for the male and female combined. Proportion of feedings was selected because prior studies of this population indicated that females compensate for loss of male provisioning (Dickinson and Weathers, 1999). When females compensate, proportion of feedings contributed by the male is an extremely sensitive measure for detecting reduction in male provisioning.
I based feeding data on up to three 90-min nest watches conducted in early (age 4–6 days), middle (age 9–11 days), and late (age 14–16 days) stages of nestling growth. Watches were timed from the first feed to control for differences among pairs in feeding delay after the observer arrived. Nest entrances were watched through a spotting scope from a blind or hiding place, approximately 30 m from the nest-box. Sex and identity (color bands) of the feeding birds were recorded during each feeding trip.
I only analyzed data from nests with at least 10 observed feeding trips (
: 
; range: 10–135). The total number of feedings per nest did not differ between control and laying detention treatments (two-sample t test: 
feeds, 
feeds, 
, 
, , 
). Each nest was treated as an independent event and given equal weight in the analyses regardless of the total number of feeds.
Provisioning risk
I compared risky feeding behavior of males whose mates accepted at least one EPC with that of males whose mates were completely unreceptive. Western bluebird pairs often feed in tandem, where one bird arrives at the box with food and waits to feed until the other has arrived. When birds fed in tandem, entering the box 
1 min apart, I assumed that entering the box first was a more risky behavior. I calculated the proportion of tandem feeds where the male fed first to obtain a proportional index of risk. I counted new tandem bouts only if the interval since the last feed was more than 1 min.
Paternity measures
Blood samples were taken from nestlings twice, when they were 6 and 14 days old. Chicks that died in the nest were collected and muscle tissue frozen at -70°C for later analysis. Unbanded adults were captured on day 9 of the nestling stage for banding and blood samples. I generated multilocus fingerprints using the Jeffreys probes (Jeffreys et al., 1985). Exclusion was based on the number of unattributable minisatellite bands in the offspring and parent-offspring band-sharing coefficients (Dickinson and Akre, 1998).
Statistics
I restricted analysis of the male's share of feeding trips to nests that survived long enough to obtain feeding watches with a total of at least 10 feedings per nest. This requirement is unlikely to bias the data, as unassisted females compensate for nonfeeding males and average 15 feedings per 90 min on their own (Dickinson and Weathers, 1999). However, in the analysis of the relationship between male paternity and male share of provisioning trips, some nests with high EPP could have been abandoned early, during egg laying. In years when I collected blood from 6-day-old nestlings, only 2 of 198 nests with brood reduction failed before the first blood sample was taken, suggesting that bias due to failure to collect blood is unlikely. Because this study was designed to determine whether males reduce (rather than eliminate) their provisioning, biases before hatching would not affect the interpretation.
Means are presented ± SEMs. I used parametric tests on angularly transformed data. In cases where there was no expected effect size, I used G*power (Buchner et al., 1997) and Cohen's (1992) medium effect size to calculate post-hoc power (two tailed). Otherwise, I used G*power and the expected effect size to calculate post-hoc power. In calculating the expected effect size, I assumed that females compensate fully for reduction in male provisioning. This assumption is supported by prior evidence of compensation by unassisted females (Dickinson and Weathers, 1999).
I calculated expected effect sizes for analyses in Tables 1–3 as follows. The expected share of feeding trips for males at unmanipulated nests was 0.5. When males were detained for 24 h during the laying period, I expected males to reduce their share of provisioning in proportion to loss of paternity of one egg. This yields an expected reduction of 23.5% from 
to 
of feeding trips (Table 1). When females accepted at least one EPC, I expected males to reduce their share of provisioning from to 
, based on prior work showing that in nests with EP chicks 42% of offspring were sired by EP males (Dickinson and Akre, 1998) (Table 2). Similarly, when comparing nests with and without EP chicks, I expected males to reduce their proportion of feedings from to , in line with proportional loss of paternity in the larger sample of nests with EP chicks (Table 3). Values for µ1, µ2, and 
(the standard deviation of proportion of feeds from a larger data set) were used in G*power to calculate power based on the expected effect size, 
. In these three cases, I report the power to detect an effect as large or greater than the expected effect size, d, while in all other cases I report power to detect an effect as large or greater than the medium effect size of Cohen (1992).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Removal from the territory
Males removed to indoor cages for 24 h and released back onto their territories afterward usually regained their territories and mates (86% of 22 detentions). I excluded from analysis nests of three males that were evicted from their territories by a new male that came in while they were detained, as well as any nests for which I did not have sufficient feeding data (i.e., <10 feedings per nest). In the laying removal treatment, males were denied both copulatory access and the ability to guard their mates during a period when two to three eggs remained to be fertilized. Males removed during laying did not reduce their share of provisioning trips compared to males removed during incubation, unmanipulated males, or a combined control (Table 1).
Detention on the territory
Effect of capture and detention
In four cases, females responded to male detention by laying the third egg the next morning, burying over the three eggs, and renesting in the same box with the same male. This behavioral response never occurred in the control nests, and egg burying rarely occurs in the population, but with just four such abandonments, there was no statistical difference between nests where males were detained for 1 h and control nests (including capture controls, incubation controls, and unmanipulated nests; Fisher's Exact 
, , , 
.
The 1-h detention experiments yielded results similar to the 24-h removal experiments. Males in the capture control groups did not make proportionally fewer feeding trips than did unmanipulated males (Table 2). Feeding rates of control males detained for 1 h during incubation were not distinguishable from those of control males captured and immediately released on the second day of laying (Table 2). Even though 1-h detention during the laying period increased the EPC attempt rate (Dickinson, 1997), it did not result in a partial reduction in the male's share of feeding trips (two-sample t test, , 
; Table 2).
Effect of intrusion and visual access
I observed the female continuously during 30 of 32 1-h detention manipulations for which I had provisioning data. In testing for an effect of EP intrusion, I assumed that even males who were visually obstructed could detect the vocalizations of intruding EP males. I separated the 30 1-h laying detention nests into cases where a male intruded and interacted with the female and cases where no male came in. In the 24 cases where EP males intruded, they also attempted copulations. When EP males intruded, resident males did not contribute proportionally fewer feeding trips than did resident males whose mates were not visited by an intruder (Table 2). The resident male's share of feeding trips was not influenced by whether he could observe the intrusion (Table 2).
Effect of female acceptance of EPCs
Females varied in whether or not they accepted EPCs during the 1-h detention experiments. At nests of resident males that had visual access from an open cage during detention, the male's share of feeding trips was not statistically lower when females accepted at least one EPC than when females were completely unreceptive (Table 2). A potential confounding factor is that females preferentially accept EPCs from males older than their mates (Dickinson, 2001). Where age was known, receptive females in this sample had younger mates than did unreceptive females (Mann-Whitney 
, 
, 
).
I examined the relationship between female acceptance and the index of risk males are willing to assume in biparental feeding. Risk was scored as the proportion of tandem feedings where the male fed first. I found no difference in tendency to assume risk between males whose mates accepted at least one EPC (
) compared with males whose mates refused all EPCs (
). The tendency was opposite to the predicted direction (two-sample t test: 
for an expected reduction from 0.50 to 0.29).
Do males reduce their share of provisioning with extrapair paternity?
EP chicks were present in 20 of 42 nests for which I had both feeding and paternity data. Two of these nests were sired exclusively by extrapair males. However, because the males identified as the social father sired offspring in other nests with the same female, nonpaternity was not the result of mistaken identity or of a blood sampling mix-up. Males with complete paternity did not make a greater share of feeding trips than did males that had extrapair chicks in their nests (Table 3).
I analyzed the data to determine whether males reduced their share of provisioning commensurate with the proportional reduction in their share of paternity. As Figure 1 shows, the male's share of feeding trips did not decline abruptly as expected if there were a threshold level of EPP above which males gain by reducing their provisioning (Whittingham et al., 1992). In a multiple regression, the male's share of feeding trips increased significantly with age but did not decline significantly with the proportion of extrapair chicks in their nests (Figure 1). The age of the breeding male was not a good predictor of extrapair paternity (regression: 
).
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; proportional extrapair paternity: 
The impact of reduced male provisioning on nestling survival should be lower at nests with helpers because helpers contribute about one-third of the feeding trips and increase overall feeding rates and fledging success (Dickinson et al., 1996
). I examined the breeder male's share of provisioning at 16 nests with helpers for which I had both feeding watches and paternity exclusion data. The trend, although not statistically significant, was opposite the direction predicted if breeder males reduce their share of provisioning in response to extrapair fertilization (nests with vs. without extrapair chicks: 
for an expected reduction in share of provisioning from 0.33 to 0.19). This result did not change when I included the breeder male's age as a covariate (ANOVA: extrapair offspring: 
for an expected reduction from 0.33 to 0.19). |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In this study, neither apparent nor actual loss of paternity caused a detectable reduction in the male's share of feeding. The males' proportional contribution to feeding chicks was not sensitive to capture and release, 1-h detention on the territory, or 24 h removal. In more fine-tuned analyses, I asked whether males reduced their feeding contribution when they experienced EP male intrusion or witnessed female acceptance of EPCs. I found no evidence that males responded to these behavioral cues of potential paternity loss by reducing their share of parental feeding.
An important caveat regarding these results is that this study may have insufficient power to detect small to medium effects on the male's share of provisioning. As the analyses became increasingly specific, the sample sizes (and thus the statistical power of the tests) declined (Table 2). Power ranged from 0.16 to 0.99 and was above 0.5 only when the expected effect was quite large (Figure 1, Tables 2 and 3). Hence, power was not always high enough to be confident that the conclusions would hold with larger sample sizes.
A second important caveat is that share of feeding trips is not a complete measure of parental effort. Males may not alter their provisioning rates, but may instead alter the types of prey they are feeding to young, opting to feed prey that are easier to catch but that are smaller and less nutritious (Wright and Cuthill, 1989). I examined only one measure of provisioning risk (tendency to enter the nest-box first when feeding in tandem with the female); this measure would not detect a tendency in which males that have lost paternity take fewer risks in foraging (Power, 1980). It is also possible that males reduce their propensity to engage in nest defense. Failure to include these measures of parental effort is a potential weakness of this study.
The results on behavioral cues are corroborated by observational data in which I found no detectable relationship between share of paternity and the male's share of provisioning trips. However, older males tended to feed proportionally more often than did younger males. In the sample of manipulated (detention) nests used in this study, receptive females had younger mates than unreceptive females (Dickinson, 2001), suggesting that there is a potential confound between EPP and age that may hamper interpretation of the relationship between paternal care and paternity in western bluebirds as well as in other taxa. If feeding becomes less costly with age and older males are less likely than younger males to suffer loss of paternity, this could lead to a spurious correlation between EPP and male investment in parental care. This potential confound should be addressed in future studies of the relationship between paternity and provisioning.
Even with a helper present, breeder males with extrapair chicks in their nests did not exhibit a significant reduction in their share of provisioning. Failure of the breeder male to reduce his feeding effort in response to EP fertilizations at nests with helpers is consistent with two possibilities: either the breeder male's investment in the current offspring still has fitness payoffs, or provisioning is functioning as mating effort, favored due to its positive impact on the male's future mating opportunities.
Detention of males during the female's fertile period has two effects: it prevents males from guarding, increasing the likelihood that extrapair males will come onto the territory and mate with the female, and it prohibits the detained male from copulating with his mate (Dickinson, 1997). Both these changes can influence a male's likelihood of paternity. Western bluebird pairs copulate at a high rate throughout laying and even copulate on the day the last egg is laid (Dickinson and Leonard, 1996). Copulations sometimes occur in bouts of up to four successful copulations, one right after another, a behavior that has been hypothesized to increase success in sperm competition (Møller and Birkhead, 1992). Even if multiple copulation is highly effective at overcoming paternity loss, 24-h removal is likely to circumvent paternity assurance for at least one egg. In spite of this, males denied access for 24 h on days 2–3 of laying did not reduce their care. The power was relatively low (0.43) for this analysis; however, males in all of the treatments and controls showed provisioning levels at or near 50%, suggesting that if there is an effect, it is extremely small.
Although paternity exclusion data are not available for the male detention experiments presented here, there is evidence from eastern bluebirds that temporarily detaining males for 48 h during laying reduces the male's paternity (MacDougall-Shackleton et al., 1996). Because eastern bluebird females were not observed systematically during male detention, it is not clear whether the reduction in paternity was due to denying the resident male opportunities to copulate with his mate or to EPCs occurring while the resident male was removed from his territory. The behavioral evidence I present here suggests that EPCs occur frequently enough during detention to influence paternity and that male detention in western bluebirds will produce similar effects on the resident male's paternity to its effects in eastern bluebirds.
The provisioning results reported here are consistent with findings for tree swallows, Tachycineta bicolor, and eastern bluebirds, S. sialis. In tree swallows, experimental manipulation of the male's ability to witness EPCs did not influence parental provisioning (Whittingham et al., 1993). In eastern bluebirds, removal for 48 h during the laying period did not influence male feeding, even though it reduced the male's share of paternity (MacDougall-Shackleton and Robertson, 1998). In both species, removal or sequestration of the female from early morning until 1 h after laying did not cause males to reduce their care (Kempenaers et al., 1998). These studies provide compelling evidence against adjustment of paternal care with paternity loss in eastern bluebirds and tree swallows, and even account for the possibility that EPCs occur while the female is off territory.
The results of the current study are also consistent with past conclusions based on the behavior of replacement males in this population. By not adjusting their care in accordance with share of paternity, social fathers are essentially feeding normally just as replacement males do when they have at least some copulatory access (Dickinson and Weathers, 1999). Not all avian species use the same chick-feeding rules. Male dunnocks and accentors (Prunella spp.) use share of copulatory access (Davies et al., 1992; Hartley et al., 1995), while some birds that breed cooperatively with relatives appear to use complex rules that depend on social status (Whittingham and Dunn, 1998). In superb fairy-wrens, a species with kin-directed helping and extragroup paternity, dominant males reduce their care with loss of paternity when they have helpers, but not when they breed as pairs. This suggests that male care is costly and that the impact of male care on the fitness of current young is a major selective force governing whether and how much to feed (Dunn and Cockburn, 1996). I failed to find such a pattern in western bluebirds. Both males and females reduced their provisioning rates to a small degree when helpers were present, but the male's reduction was not greater at nests with EPP than nests without (Dickinson et al., 1996).
Missing from all of the above-mentioned studies is information on the survival and reproductive costs and benefits of partial withdrawal of male care for male parents and their offspring. Although female eastern and western bluebirds are unable to fully compensate for a complete lack of male care (Dickinson and Weathers, 1999; Meek and Robertson, 1994), it is possible that they could compensate for a moderate reduction in the male's contribution. However, a moderate reduction may not have much benefit for the male. It is currently not possible to assess the relative importance of current and future costs of small increments of provisioning for social fathers or even to tell whether available cues are reliable predictors of EPP (Mauck et al., 1999).
In western bluebirds, breeding opportunities are limited due to an apparent shortage of females (Dickinson and Weathers, 1999). In spite of this, studies based on replacement males suggest that provisioning is predominantly parental effort rather than mating effort. Nonfeeding replacement males do not have reduced return rates and appear as likely to breed with the female in a subsequent attempt as are feeding replacement males (Dickinson and Weathers, 1999). Furthermore, survival of unassisted females was not reduced, suggesting that withdrawing feeding effort will not influence the male's chances of mating in the future. In contrast, because males cannot recognize their own chicks (Leonard et al., 1995), partial withdrawal of care may reduce fledgling survival and reduce the success of sons in competing locally for mates the following spring. Breeder males usually have at least some offspring in the current nest and only have a 50% chance of surviving to breed the next year. As female survival is similar, males have at most a 25% chance of breeding with the female during a subsequent attempt. These odds suggest that investment in current offspring is an evolutionarily sound priority for western bluebird males.
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ACKNOWLEDGEMENTS |
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This work was performed at the University of California Natural Reserve System's Hastings Reservation and the adjacent Oak Ridge Ranch. I thank two anonymous reviewers and the editor of Behavioral Ecology, Ron Ydenberg, for helpful comments on earlier drafts of this paper. I am indebted to a large number of field interns whose involvement in banding, nest monitoring, and behavioral observations made this study possible. I thank T. Abe, J. Akre, D. Barber, A. Berg, R. Bower, J. Brewer, D. Christian, K. Dean, V. Demas, M. Dietz, J. DePiero, M. Eichholz, K. Ellison, R. Etemad, J. Goldstein, K. Gordon, S. Harding, K. Hondrick, B. Hahn, N. Hazle, M. Koopman, K. Kraaijeveld, J. Kranz, L. Kummer, M. Lewis, L. McGraw, D. Monk, R. Morlen, G. Morse, J. Nesbitt, K. Petersen, J. Rombouts, J. Rosenthal, D. Ruthrauff, L. Scinto, P. Shepherd, F. Smit-Kraaijeveld, M. Soenksen, M. Stapleton, and K. Truman for their dedicated efforts in the field. The Kirk family graciously allowed access to Oak Ridge Ranch. Present and past directors of the MVZ (Craig Moritz, Jim Patton and David Wake) and the administrative staff of the MVZ and Hastings Reservation provided logistical support throughout the years covered by this project. This study was supported by National Science Foundation (NSF) and National Institutes of Health (NRSA) postdoctoral fellowships and grants from the North American Bluebird Society, Ellis Bird Farm, the University of California's Genetic Resources Conservation Program, NSF grant IBN-9507365, and NSF grant IBN-0097027.
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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