Trading up: the fitness consequences of divorce in monogamous birds

Social and genetic mating systems play an important role in natural and sexual selection, as well as in the dynamics of populations. In socially monogamous species different genetic mating patterns appear when individuals mate outside the breeding pair within a breeding season (extra‐pair mating) or when they change partners between two breeding seasons (widowing or divorce). Divorce can be defined as having occurred when two previously paired individuals are alive during the next breeding season and at least one of them has re‐mated with a new partner. In socially monogamous birds divorce is widespread, but it is not clear whether it is a behavioural adaptation to improve the quality of a mating decision or whether, alternatively, it results as a non‐selected consequence of other processes: existing studies suggest a heterogeneous set of results with respect to this central question. This heterogeneity could result from a number of factors, ranging from the methodological approaches used, to population‐ or species‐specific characters. In this review we use phylogenetic meta‐analyses to assess the evidence that divorce is adaptive (in terms of breeding success) across 64 species of socially monogamous birds. Second, we explore biological and methodological reasons for the heterogeneity in the results of previous studies. Results of our analyses supported the hypothesis that divorce is, in general, an adaptive behavioural strategy as: (1) divorce is triggered by relatively low breeding success; (2) there is a positive change in breeding success as a result of divorce. More specifically, while controlling for methodological moderators, we show that: (i) earlier stages of breeding are better predictors of divorce than later stages (r = 0.231; 95% CI: 0.061–0.391 for clutch size; similar for laying date); (ii) females benefited from divorce more than males in terms of increasing breeding success between successive breeding attempts, with different stages of the breeding cycle improving at different rates (e.g. r = 0.637; 95% CI: 0.328–0.817 for brood‐level measures). We show that the effect size was dependent on the methodological approach used across studies and argue that research on the adaptive nature of divorce should be cautious when designing the study and interpreting the results. Altogether, by providing strong evidence that divorce is an adaptive strategy across monogamous birds, the results of our analysis provide a firm ground for further exploration of external covariates of divorce (e.g. demographic factors) and the mechanisms underlying the differences in the effect sizes of the proximal fitness causes and consequences of divorce.     

Mate fidelity and breeding site tenacity in a monogamous sandpiper, the black turnstone

We examined the relationship between mate fidelity and breeding site tenacity during a 5-year study of the black turnstone, Arenaria melanocephala, a socially monogamous sandpiper breeding in subArctic Alaska. We tested the predictions of several hypotheses regarding the incidence of divorce and the benefits of fidelity to mate and breeding site. Interannual return rates to the breeding grounds (88% for males, 79% for females) were among the highest yet recorded for any scolopacid sandpiper, and 88% of returning birds nested on their previous year's territory. The annual divorce rate was only 11%, and mate fidelity was significantly linked to fidelity to territory but independent of sex and year. Males arrived in spring significantly earlier than their mates and interannual fidelity was influenced by the relative timing of arrival of pair members. Reunited pairs had significantly higher fledging success than new pairs formed after death or divorce. The incidence of divorce was unrelated to reproductive success the previous year, although birds nested significantly further away after failure than after a successful nesting attempt. Sightings of marked individuals suggested that members of pairs do not winter together, and breeding site tenacity provides a mechanism through which pair members can reunite. We reject the 'incompatibility' hypothesis for divorce in turnstones, and our data contradict predictions of the 'better option' hypothesis. Alternatively, we propose the 'bet-hedging' hypothesis to explain the occurrence of divorce, which transpires when an individual pairs with a new mate to avoid the cost of waiting for a previous mate to return. Such costs can include remaining unmated, if the former mate has died, or experiencing lower reproductive success because of delayed breeding. Copyright 2000 The Association for the Study of Animal Behaviour.     

Divorce and asynchronous arrival in common terns, Sterna hirundo

We investigated which of three hypotheses (better option, incompatibility or asynchronous arrival) best explains divorce in the common tern. One partner did not return the next year in 18.5% of 150 pairs. Among the 106 pairs in which both mates returned, the divorce rate was 18.9%. We found no significant differences in: breeding performance or condition in relation to the probability of divorce; quality of previous mates and new mates, mean age in relation to pair bond status; breeding success before and after divorce nor did this differ from breeding success of reunited pairs. Hence the better option and incompatibility hypotheses were not supported. However, divorce was more likely in pairs in which mates arrived asynchronously on the breeding grounds, supporting the asynchronous arrival hypothesis. Median arrival asynchrony for divorced pairs was 7.5 days and for reunited pairs 2 days; mates arriving more than 16 days apart always split up. About 20% of divorced birds lost breeding status in the year of divorce, probably as a consequence of their late arrival. Our results suggest that terns search for a new mate as soon as they arrive on the breeding grounds and that mates remain faithful to each other to avoid the costs of searching for a new partner. Thus, synchrony in arrival facilitates pair bond maintenance rather than asynchrony promoting divorce, since divorce appears to be a side-effect of asynchrony and not an active decision. Copyright 1999 The Association for the Study of Animal Behaviour.     

Within-season divorce in Blue Tits (Cyanistes caeruleus)

Divorce between breeding seasons, i.e. mate change while the old breeding partner is still alive, has been well-studied in several bird species. It can be viewed either as the result of reproductive decisions of individuals to maximize their own fitness or as the outcome of intra-sexual competition for mating opportunities. In contrast, divorce within a breeding season—also referred to as rapid mate switching—has received much less attention. Within-season divorce may occur after sudden changes in environmental conditions, such as those causing the disappearance or devaluation of nesting sites or territories, or when better breeding partners become available. Within-season divorce can be initiated by a member of the pair, or it can be the result of a take-over by an unpaired individual that competes for access to the resource. During a field experiment investigating the effects of limiting nesting sites on reproductive behaviour in Blue Tits, we recorded several cases of within-season divorce. The rate of divorce was not related to the experimental nest-site limitation, and pairs that changed their partner suffered reduced reproductive success compared to faithful pairs. Although there were no differences in the timing of breeding, clutch size or hatching success, pairs with a new partner also suffered a reduced fledging success, which was partly explained by complete brood failures. This study highlights that the pair bond prior to egg laying can be unstable when conditions force individuals to compete for a new partner or nest site and indicates the importance of the correct timing of divorce within the breeding cycle.     

Pair bond and breeding success in Blue Tits Parus caeruleus and Great Tits Parus major

Data from 939 nests of the Blue Tit Parus caeruleus and 1008 nests of the Great Tit P. major from nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.     

Fitness consequences of divorce in the azure-winged magpie depends on the breeding experience of a new mate

Sexual conflict in producing and raising offspring is a critical issue in evolutionary ecology research.Individual experience affects their breeding performance,as measured by such traits of provisioning of offspring and engagement in extra-pair copulations,and may cause an imbalance in sexual conflict.Thus,divorce is hypothesized to occur within aged social pairs,irrespective of current reproductive success.This concept was explored in the azure-winged magpie Cyanopica cyanus by investigating the divorce of a social pair and its relationship to their changes in breeding performance with prior experience.Females engaging in extra-pair copulation may intensify sexual conflicts and may be the main reason for divorce.Once divorced,females repairing with an inexperienced male realized higher reproductive success than that repairing with an experienced male;males repairing with an experienced female realized higher reproductive success than that repairing with an inexperienced female.This finding indicates that the fitness consequence of divorce depends on the breeding experience of new mates.Divorced females can obtain more extra-pair copulations,whereas divorced males cannot,when they repair with inexperienced breeders.Divorced females provisioned a brood at lower rates than inexperienced females whereas divorced males had no such difference.It appears that divorced females can obtain an advantage in sexual conflicts with inexperienced mates in future reproduction.Consequently,females are probably more active than males in divorcing their aged mates so as to select an inexperienced male as a new mate.Azure-winged magpies thus provide novel insights into the implicaticns of sexual conflict in birds.     

Is Divorce in Young Common Terns,Sterna hirundo,after Recruitment just a Question of Timing?

The probability of divorce in birds has been linked with age, breeding experience, reproductive output and synchrony in return. Here, we investigate the consequences of first breeding attempts in common terns for mating in the subsequent season. Nearly 20% of all first‐time breeders disappeared or skipped at least one season after recruitment. In 84 pairs, which consisted of at least one recruit and of which both partners returned to the colony, the divorce rate was 45%. We compared reproductive success, arrival dates, and asynchrony in arrival dates of pairs of the first breeding season against the second season, for both reunited and divorced pairs and males and females separately. First, in pairs of which both members came back to the colony, we found an increase of reproductive success most pronounced in males. In the second season reproductive success of divorced compared with reunited pairs was higher, as only divorced pairs significantly improved the number of fledglings, and again this relation was stronger in males. Secondly, females of reunited pairs arrived significantly earlier from the first to the second season and by far more days than their males. However, in divorced pairs former mates did not differ in the number of days they advance their arrivals. Finally, divorced males arrived on average 4 d earlier than their former mates, whereas divorced females arrived 5 d later compared with their former mates of the recruitment season. Contradictory to nearly all other divorce studies in birds so far, we found a clear fitness gain in divorced males. We suggest that the improvement in reproductive success of young males stems from a side‐effect of the birds’ quality and ability to reach the breeding site in appropriate time and earlier as potential competitors. In long‐lived bird species the heterogeneity among young individuals in the timing of arrival at the colony seems to explain why former recruit‐pairs reunite or split. For young males we suggest as best explanation of divorce that they profit from ‘pushing for an empty chair’, while females seem to profit from their choosiness and may actively decide between former and other mates.     

No evidence of immediate fitness benefits of within-season divorce in monogamous birds

Individuals of socially monogamous species can correct for suboptimal partnerships via two secondary mating strategies: divorce and extra-pair mating, with the former potentially providing both genetic and social benefits. Divorcing between breeding seasons has been shown to be generally adaptive behaviour across monogamous birds. Interestingly, some pairs also divorce during the breeding season, when constraints on finding a new partner are stronger. Despite being important for a comprehensive understanding of the evolution of social monogamy, whether within-season divorce is adaptive and how it relates to extra-pair mating remains unknown. Here, we meta-analysed 90 effect sizes on within-season divorce and breeding success, extracted from 31 studies on 24 species. We found no evidence that within-season divorce is adaptive for breeding success. However, the large heterogeneity of effect sizes and strong phylogenetic signal suggest social and environmental factors—which have rarely been considered in empirical studies—may play an important role in explaining variation among populations and species. Furthermore, we found no evidence that within-season divorce and extra-pair mating are complementary strategies. We discuss our findings within the current evidence of the adaptiveness of secondary mating strategies and their interplay that ultimately shapes the evolution of social monogamy.     

Influence of reproductive output on divorce rates in polar seabirds

The high occurrence of social monogamy in birds has led to questions about partner fidelity, or the perennial nature of monogamy from one breeding season to another. Despite the evolutionary advantages of partner fidelity, divorce occurs among 95% of bird species. We aimed to describe patterns of divorce and partner fidelity in five seabird species breeding in Arctic and Antarctic regions and investigated the influence of breeding status on pair bond maintenance. For four out of the five species considered, we observed low divorce rates (respectively 1.9%, 3.3%, 2.5%, and 0.0% for Brünnich''s guillemot, glaucous gull, Antarctic petrel, and south polar skua), while the divorce rate was much higher (19.1%) for the black‐legged kittiwake. For kittiwakes, the divorce rate was lower for pairs that managed to raise their chick to 15 days of age, while the effect of breeding success on divorce in the four other species could not be tested due to the rareness of divorce events. Our results emphasize the potentially large temporal (interannual) variations that should be taken into account in understanding divorce and partner fidelity in seabirds.     

Breeding biology during establishment of a reintroduced Griffon Vulture Gyps fulvus population

We studied the breeding parameters of a reintroduced population of individually marked Griffon Vultures Gyps fulvus in the Grand Causses region of southern France from the time of first reintroduction in 1982 to 1992. Among nesters, 65% of the birds released as immatures or born in the wild recruited into the breeding population when 4 years old, i.e. 1 year earlier than previously described. The proportion of birds nesting each year was relatively high and increased with time, suggesting that conspecific attraction favoured recruitment. We detected a permanent adverse effect of long-term captivity on the nesting success of birds released when more than 2 years old: compared to the natural population, birds which had been kept in captivity showed a reduced breeding success during the whole study period. The breeding success of released immatures and wild-born birds was similar to the highest values observed in a natural population in the Spanish and French Pyrenees. Breeding failures did not usually result in mate change but affected nest-site fidelity. Birds not born in the wild were more likely to recruit to the largest subcolonies available, which highlights the role of social attraction. The observed philopatry of wild-born birds probably resulted from such a social attraction since most of them were born in the largest subcolony. Conspecific attraction maintained the spatial aggregation of nests, whereas nest changes after a breeding failure favoured the spatial expansion of the colony.     

High divorce rates in Corsican blue tits: how to choose a better option in a harsh environment

We investigate which hypothesis, the “better mate hypothesis” or the “better territory hypothesis” best explains the unusually high divorce rate (59%) in a population of blue tits (Parus caeruleus) living in a sclerophyllous habitat characterised by severe environmental constraints (trophic, parasitic, climatic) on the island of Corsica, France. Using data from the breeding seasons 1985–1998 and from a brood size experiment (1990–1993) we examined the causes of divorce and their consequences on breeding performance, mate assortment and territory choice. Breeding performance had no significant effect on whether birds re‐united or divorced in the next breeding season. Re‐uniting pairs did better than divorced females and the latter improved their breeding performance compared to prior to divorce, but this was mainly due to age and territory effects. There were no differences in male performance depending on whether they re‐united or divorced. The age combination of pairs did not differ between re‐uniting and divorcing pairs, but mate assortment changed after divorce with males re‐mating more often with older partners than females. Manipulation of brood size showed a trend for birds with enlarged broods to divorce more. Pairs responded significantly to territory quality by divorcing more often in poor than in good breeding sites. Both faithful pairs and male divorcees had shorter breeding dispersal distances than female divorcees. Divorce rates were determined by the large differences in quality among breeding sites. Males, whatever their status, usually retained their previous territory whereas divorced females moved significantly longer distances and improved their breeding site. Moving to a better territory after divorce benefits only females which appear to be the choosing sex in the decision to divorce. This study strongly supports the “habitat mediated hypothesis” and we suggest that the large observed intraspecific variation in the magnitude of divorce rates in many species of birds is mostly determined by habitat characteristics.     

Divorce in the barn owl: securing a compatible or better mate entails the cost of re‐pairing with a less ornamented female mate

Two nonmutually exclusive hypotheses can explain why divorce is an adaptive strategy to improve reproductive success. Under the ‘better option hypothesis’, only one of the two partners initiates divorce to secure a higher‐quality partner and increases reproductive success after divorce. Under the ‘incompatibility hypothesis’, partners are incompatible and hence they may both increase reproductive success after divorce. In a long‐term study of the barn owl (Tyto alba), we address the question of whether one or the two partners derive fitness benefits by divorcing. Our results support the hypothesis that divorce is adaptive: after a poor reproductive season, at least one of the two divorcees increase breeding success up to the level of faithful pairs. By breeding more often together, faithful pairs improve coordination and thereby gain in their efficiency to produce successful fledglings. Males would divorce to obtain a compatible mate rather than a mate of higher quality: a heritable melanin‐based signal of female quality did not predict divorce (indicating that female absolute quality may not be the cause of divorce), but the new mate of divorced males was less melanic than their previous mate. This suggests that, at least for males, a cost of divorce may be to secure a lower‐quality but compatible mate. The better option hypothesis could not be formally rejected, as only one of the two divorcing partners commonly succeeded in obtaining a higher reproductive success after divorce. In conclusion, incompatible partners divorce to restore reproductive success, and by breeding more often together, faithful partners improve coordination.     

Mate fidelity in a polygamous shorebird,the snowy plover (Charadrius nivosus)

Social monogamy has evolved multiple times and is particularly common in birds. However, it is not well understood why some species live in long‐lasting monogamous partnerships while others change mates between breeding attempts. Here, we investigate mate fidelity in a sequential polygamous shorebird, the snowy plover (Charadrius nivosus), a species in which both males and females may have several breeding attempts within a breeding season with the same or different mates. Using 6 years of data from a well‐monitored population in Bahía de Ceuta, Mexico, we investigated predictors and fitness implications of mate fidelity both within and between years. We show that in order to maximize reproductive success within a season, individuals divorce after successful nesting and re‐mate with the same partner after nest failure. Therefore, divorced plovers, counterintuitively, achieve higher reproductive success than individuals that retain their mate. We also show that different mating decisions between sexes predict different breeding dispersal patterns. Taken together, our findings imply that divorce is an adaptive strategy to improve reproductive success in a stochastic environment. Understanding mate fidelity is important for the evolution of monogamy and polygamy, and these mating behaviors have implications for reproductive success and population productivity.     

Factors affecting divorce in the Great Tit Parus major

In a population of the Great Tit Parus major in Sagamihara, the divorce rate was 17.8% over 10 years from 1984/85 to 1993/94. Most of the divorces occurred in the year following pair formation. There were no differences in breeding success between faithful and divorced pairs. The birds were divided into three groups in relation to their home ranges: residents, emigrants and migrants. The residents established their home ranges within their flock ranges and remained there for life. Non-residents (of which two sorts were identified) left their flock ranges to breed, but returned to their original flock ranges after breeding. Therefore, their home ranges were separated into breeding and previous flock ranges. Most of the faithful pairs consisted of residents whose home ranges overlapped extensively. The divorce rate was higher in the pairs consisting of residents and non-residents and in those of non-residents. Since the non-breeding home ranges of emigrants and migrants did not overlap with their mates' non-breeding home ranges, an important factor affecting divorce seems to be the non-overlap of non-breeding home ranges between both mates of pairs after breeding.     

Effects of Aging and Mate Retention on Reproductive Success of Captive Female Peregrine Falcons

SYNOPSIS. Separating ecological (extrinsic) factors affectingreproduction from physiological and experiential (intrinsic)factors can be problematic in free-ranging animals. This studyexamined age-related changes in six measures of reproductivesuccess (clutch size, fertility, hatchability, brood size athatching, survivorship of nestlings, and brood size at fledging)for captive female peregrine falcons (Falco peregrinus) whereecological factors (i.e., access to mates, nesting sites, andfood) were not limiting and were similar across years and betweenindividuals. Mean nestling survivorship increased throughoutthe lifespan of the female while all other measures of reproductionpeaked at about seven years of age and decreased thereafter.Birds with prior breeding experience had higher productivitythan inexperienced birds of the same age. Productivity increasedwith increasing experience of the pair. Productivity droppedby an average of 53% when females experienced a change of mate,and then subsequently increased over a period of several years.Productivity was not affected when birds were moved to differentbreeding sites. There was no evidence that early improvementsin reproduction were related to chronological aging in the absenceof experiential differences. Birds that began breeding earlierproduced more fledglings during their lifetimes as a resultof higher annual productivity. Lifetime reproduction was notcorrelated with longevity because birds with higher maximalegg production had shorter breeding lifespans. Birds retainingmates produced more fledglings during their lifetime than birdsthat changed mates, but birds that changed mates more than oncedid not have lower lifetime reproduction than birds that hadonly one mate change. These results suggest that 1) age-relatedchanges in reproduction are not necessarily resource-mediated,2) in the absence of resource limitation, experience of thepair is a primary factor determining annual reproductive success,3) benefits of increasing experience may be offset by the onsetof senescence, 4) the cost of present egg production on futurereproductive potential supports a "pleiotropic" theory of aging,and 5) costs associated with mate changes may encourage selectionfor low "divorce" rates (i.e., lifetime monogamy) in this species.     

The Impact of Increased Food Availability on Reproduction in a Long-Distance Migratory Songbird: Implications for Environmental Change?

Many populations of migratory songbirds are declining or shifting in distribution. This is likely due to environmental changes that alter factors such as food availability that may have an impact on survival and/or breeding success. We tested the impact of experimentally supplemented food on the breeding success over three years of northern wheatears (Oenanthe oenanthe), a species in decline over much of Europe. The number of offspring fledged over the season was higher for food-supplemented birds than for control birds. The mechanisms for this effect were that food supplementation advanced breeding date, which, together with increased resources, allowed further breeding attempts. While food supplementation did not increase the clutch size, hatching success or number of chicks fledged per breeding attempt, it did increase chick size in one year of the study. The increased breeding success was greater for males than females; males could attempt to rear simultaneous broods with multiple females as well as attempting second broods, whereas females could only increase their breeding effort via second broods. Multiple brooding is rare in the study population, but this study demonstrates the potential for changes in food availability to affect wheatear breeding productivity, primarily via phenotypic flexibility in the number of breeding attempts. Our results have implications for our understanding of how wheatears may respond to natural changes in food availability due to climate changes or changes in habitat management.     

Divorce in cooperatively breeding long-tailed tits: a consequence of inbreeding avoidance?

The decision of whether to divorce a breeding partner between reproductive attempts can significantly affect individual fitness. In this paper, we report that 63% of surviving pairs of long-tailed tits Aegithalos caudatus divorced between years. We examine three likely explanations for the high divorce rate in this cooperative breeder. The 'better option' hypothesis predicts that divorce and re-pairing increases an individual's reproductive success. However, divorcees did not secure better partners or more helpers and there was no improvement in their reproductive success following divorce. The 'inbreeding avoidance' hypothesis predicts that females should disperse from their family group to avoid breeding with philopatric sons. The observed pattern of divorce was consistent with this hypothesis because, in contrast to the usual avian pattern, divorce was typical for successful pairs (81%) and less frequent in unsuccessful pairs (36-43%). The 'forced divorce' hypothesis predicts that divorce increases as the number of competitors increases. The pattern of divorce among failed breeders was consistent with this hypothesis, but it fails to explain the overall occurrence of divorce because divorcees rarely re-paired with their partners' closest competitors. We discuss long-tailed tits' unique association between divorce and reproductive success in the context of dispersal strategies for inbreeding avoidance.     

Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment

Reproductive success of the cooperative breeding Seychelleswarbler (Acrocephalus sechellensis) increases with age. Thisage effect is not due to differential survival or increasedreproductive effort, but to accumulated helping and breedingexperience. In their first year of breeding, reproductive performanceof inexperienced warblers with neither helping nor breedingexperience was significandy lower than that of warblers of thesame age with either previous helping or breeding experience.Reproductive performance was the same for primiparae with helpingexperience and for birds with breeding experience. Female primiparaewith helping experience or breeding experience built betternests and spent more time incubating than inexperienced females,which led to increased hatching success. Male primiparae withhelping experience or males with breeding experience guardedthe clutch better than inexperienced males, which led to reducedegg predation. Even-aged warblers with different previous experienceswere transferred to unoccupied islands, where birds startedbreeding immediately in high-quality territories. The experimentshowed that birds with helping experience produced their firstfledgling as fast as experienced breeders, and significandyfaster than inexperienced birds. Breeding performance did notimprove further with experience after the first successful breedingattempt. Only birds with previous breeding experience who pairedwith inexperienced birds, were likely to change mate. The otherpair combinations remained stable. Thus, primiparous birds withhelping experience have greater lifetime reproductive successthan inexperienced primiparae of the same age. This experimentshows that helping behavior has not only been selected for inthe context of promoting an individual's indirect fitness, butalso in the context of gaining helping experience which translatesinto improved reproductive success when a helper becomes a breeder.[Behav Ecol 7: 326-333 (1996)]     

Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus

We investigated the fitness consequences of divorce in oystercatchers. We made a distinction between two types of divorce: in desertions the disruption of the pair bond is initiated by one of the pair members, and in usurpations by a conspecific individual. Survival and reproduction prospects for oystercatchers are largely determined by their social status (nonbreeder or breeding bird at a site of a specific quality). Changes in social status in relation to divorce showed that birds taking the initiative to leave their mate increased in fitness, relative to birds that were forced to leave their partner. Status of individuals that remained in their territory after divorce was unaffected if their mate was expelled, but declined if their mate deserted. Survival after divorce was significantly lower for birds that were expelled than for those deserting. Divorce rate, and especially desertion rate, was higher among occupants of low- than high-quality territories. In general, divorce rate increased following elevated mortality. In high-quality territories usurpations increased with increasing breeder mortality, but at low-quality territories this relation was absent. Desertion rates were similarly related to mortality in both territory types. Divorce participants thus differed strongly in their fitness prospects, depending on the type of divorce, the role played in the divorce and the quality of the territory where divorce took place. Studies that do not observe the birds during divorce cannot determine the type of divorce and the role played by the individuals, and this may lead to misleading conclusions on the costs and benefits of divorce. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Reduced reproductive performance prior to death in the Black‐legged Kittiwake: senescence or terminal illness?

The breeding of Black-legged Kittiwakes Rissa tridactyla in their last breeding season has been compared with that of pairs which survived to the next breeding season. The time of laying was not affected but the clutch size, breeding success and productivity were significantly lower and a higher proportion of pairs totally failed to rear any young in pairs where one of the partners died before the next breeding season. When both partners died before the next breeding season, the breeding performance was reduced to a greater extent, suggesting an additive influence. The depressive effect on the last breeding attempt was confirmed by comparing the breeding success of individual females in their ultimate and penultimate breeding attempts. The effect was present in breeding birds of all ages, from first time breeders to the oldest individuals, and so could not be interpreted as an effect of senescence. Most of the adults which died apparently did so several months after breeding, during the winter half of the year when the kittiwake is oceanic. It is suggested that this depressive effect on breeding is the result of terminal illness which caused the birds to lose condition progressively over several weeks or months. It is possible that stress in the form of disease or parasite infections is primarily responsible for the death of many of the individuals. Attention is drawn to the possible confusion between the effect reported here and claims that senescence leads to lower reproductive output in old birds. It is suggested that more critical examinations are needed of claims that senescence directly affects the breeding success of birds.