Mate desertion by male Great Reed Warblers Acrocephalus arundinaceus at the end of the breeding season

Male Great Reed Warblers usually take part in the care of offspring as nest defenders and by feeding young, but at the end of the breeding season they desert their mates with eggs or nestlings. Deserted females continue offspring care. Desertion does not depend on the male's mated status (polygynous or monogamous) nor on his past breeding success. Deserted females compensate for the loss of their partners by increasing the frequency of food-bringing, resulting in a reduction in the amount of time the nestlings are brooded. Although desertion may lead to increased rates of offspring mortality through predation, breeding success of deserted females was high, especially if the male assisted during the early stages. Deserters pay costs by giving up the chance of additional matings and by lowering the reproductive success of existing mates. Male warblers reduce the former cost by choosing the season of desertion and the latter is lowered by the female's high parental ability. A deserter was found to start moulting while his mate was still feeding his nestlings, and an earlier start to the moult may be the primary benefit that he gains.     

Females compensate for moult‐associated male nest desertion in Hooded Warblers

Uniparental offspring desertion occurs in a wide variety of avian taxa and usually reflects sexual conflict over parental care. In many species, desertion yields immediate reproductive benefits for deserters if they can re‐mate and breed again during the same nesting season; in such cases desertion may be selectively advantageous even if it significantly reduces the fitness of the current brood. However, in many other species, parents desert late‐season offspring when opportunities to re‐nest are absent. In these cases, any reproductive benefits of desertion are delayed, and desertion is unlikely to be advantageous unless the deserted parent can compensate for the loss of its partner and minimize costs to the current brood. We tested this parental compensation hypothesis in Hooded Warblers Setophaga citrina, a species in which males regularly desert late‐season nestlings and fledglings during moult. Females from deserted nests effectively doubled their provisioning efforts, and nestlings from deserted nests received just as much food, gained mass at the same rate, and were no more likely to die from either complete nest predation or brood reduction as young from biparental nests. The female provisioning response, however, was significantly related to nestling age; females undercompensated for male desertion when the nestlings were young, but overcompensated as nestlings approached fledging age, probably because of time constraints that brooding imposed on females with young nestlings. Overall, our results indicate that female Hooded Warblers completely compensate for male moult‐associated nest desertion, and that deserting males pay no reproductive cost for desertion, at least up to the point of fledging. Along with other studies, our findings support the general conclusion that late‐season offspring desertion is likely to evolve only when parental compensation by the deserted partner can minimize costs to the current brood.     

Sexual differences in moult–breeding overlap and female reproductive costs in pied flycatchers, Ficedula hypoleuca

1. In this study I show that a sexual difference in timing of the post-nuptial moult frequently occurs in a sub-arctic population of the pied flycatcher.
2. Most pairs started to moult after fledging of their young, but an overlap between moult and nestling feeding was more common among males than females. This sexual difference in moult–breeding overlap increased as the season progressed.
3. Females with moult–breeding overlap laid smaller clutches than non-moulting females. In addition to many other factors explaining the seasonal decline in clutch size that has been found for many bird species, it is possible that females adjust their clutch size according to their own risk of having to start moulting while still feeding the nestlings.
4. Nearly 24% of the females were deserted by their mate before the young fledged. Desertion imposed no fitness costs to males in terms of fledgling number or quality, suggesting that their females managed to adjust their care for the loss of male care.
5. Deserted females started moulting later than aided females, which may be a result of their increased reproductive investment.
6. Deserted females and females aided by moulting males had lower survival rate than females aided by non-moulting males.
7. These findings suggests that delayed moult may be one mechanism causing inter-annual reproductive costs in birds, and the relationship between a sexual difference in timing of moult and its fitness consequences may be widespread among passerine birds.     

Re-use of courtship nests for quick remating in the polygynous Fan-tailed Warbler Cisticola juncidis

In a population of Fan-tailed Warblers Cisticola juncidis near Osaka, Japan, the males were polygynous, building a succession of elaborate courtship nests within their territories, and advertising them to females. Nest desertion by females frequently occurred early in the breeding cycle. Deserted nests were generally left untouched, but 17 out of 88 territorial males cleaned and re-used 20 of 79 nests deserted by females for their next courtship. The frequency of nest re-use increased with season. Of the re-used nests six (300%) were re-occupied by secondary females. Newly built nests were preferred to the 'second-hand' nests, probably because new nests were likely to last longer. Breeding success did not differ between the two categories of nests. In males, nest re-use seems to save time and energy expenditure for additional mating.     

A mathematical model on the optimal timing of offspring desertion

We consider the offspring desertion as the optimal strategy for the deserter parent, analyzing a mathematical model for its expected reproductive success. It is shown that the optimality of the offspring desertion significantly depends on the offsprings' birth timing in the mating season, and on the other ecological parameters characterizing the innate nature of considered animals. Especially, the desertion is less likely to occur for the offsprings born in the later period of mating season. It is also implied that the offspring desertion after a partially biparental care would be observable only with a specific condition.     

Morphological and genetic predictors of parental care in the North American barn swallow Hirundo rustica erythrogaster

Sexually dimorphic traits often signal the fitness benefits an individual can provide to potential mates. In species with altricial young, these signals may also predict the level of parental care an individual is expected to provide to shared offspring. In this study, we tested three hypotheses that traditionally relate sexually dimorphic traits to parental care in two populations of North American barn swallows Hirundo rustica erythrogaster. The good parent hypothesis predicts a positive relationship between an individual's ornamentation and his or her care whereas the differential allocation (more care given by individuals when paired to high quality mates) and reproductive compensation (more care given by individuals when paired to low quality mates) hypotheses predict that an individual's level of parental investment is relative to the quality of their mate. Male and female North American barn swallows have colorful ventral feathers and elongated tail streamers, but there is evidence that ventral color, not tail streamer length, predicts measures of seasonal reproductive success. Accounting for the positive correlation between within‐pair feeding rates and other potentially confounding variables in all of our models, we found no support for the good parent hypothesis because in both males and females, traits shown to be under sexual selection did not predict feeding rates in either sex. However, our data reveal that male coloration, and not streamer length, predicted a female's provisioning rate to shared offspring (females fed more when paired with darker individuals) in two separate populations, supporting the differential allocation, but not the reproductive compensation hypothesis. Because genetic traits have also been shown to affect parental investment, we evaluated this variable as well and found that a male's paternity did not have significant effects on either male or female feeding rates. Overall, our results suggest that females do not pair with darker males in order to gain direct benefits in terms of his expected levels of parental care to shared offspring, but do themselves invest greater levels of care when paired to darker males. Further, our results are consistent with previous studies which suggest that ventral feather color, not streamer length, is a target of sexual selection in North American populations of barn swallow because females invested more in their offspring when paired to darker mates.     

Female differential allocation in response to extrapair offspring and social mate attractiveness

Renewed debate over what benefits females might gain from producing extra‐pair offspring emphasizes the possibility that apparent differences in quality between within‐pair and extra‐pair offspring are confounded by greater maternal investment in extra‐pair offspring. Moreover, the attractiveness of a female''s social mate can also influence contributions of both partners to a reproductive attempt. Here, we explore the complexities involved in parental investment decisions in response to extra‐pair offspring and mate attractiveness with a focus on the female point of view. Adult zebra finches paired and reproduced in a colony setting. A male''s early‐life diet quality and his extra‐pair reproductive success were used as metrics of his mating attractiveness. Females paired with males that achieved extra‐pair success laid heavier eggs than other females and spent less time attending their nests than their mates or other females. Extra‐pair nestlings were fed more protein‐rich hen''s egg than within‐pair nestlings. Females producing extra‐pair offspring had more surviving sons than females producing only within‐pair offspring. Collectively, results show that females differentially allocate resources in response to offspring extra‐pair status and their social mate''s attractiveness. Females may also obtain fitness benefits through the production of extra‐pair offspring.     

Male feeding contribution facilitates multiple brooding in a biparental songbird

Despite the cost of lost mating opportunities, biparental care is routinely observed across different groups of animals. Theory predicts that paternal care will increase reproductive success, yet many empirical studies have failed to find a direct link. Most studies have focused on the effect of paternal care on current breeding attempts, but male contributions may benefit future reproduction. Species producing clutches or litters more than once per season (i.e. multiple breeding attempts per season) are suitable for investigating whether paternal care contributes to current or future reproductive success, or both. We investigated the importance of the male's feeding contribution to multiple brooding in Japanese Tits Parus minor in northern Japan, where approximately 50% of pairs were known to reproduce multiple times within a breeding season. Male feeding contribution (proportion of provisioning delivered by the male) was positively correlated with the probability of the female multiple brooding and with nestling body mass. However, it was not correlated with fledging success in the current breeding attempt. Our results demonstrate the importance of male parental care for annual reproductive success in this multi‐brooded species and provide insights into the evolution of biparental care.     

Multi-male mating and female choice increase offspring growth in the spiderNeriene litigiosa(Linyphiidae)

The purpose of this controlled-breeding study was to investigate the viability consequences of female choice and sequential polyandry for offspring in a way that would separate the influences of these two aspects of female sexual behaviour. Female sierra dome spiders,Neriene litigiosa(=Linyphia litigiosa) typically mate two to three times before production of their first batch of eggs, although some females (ca 16%) mate only once. Strong fighters are preferred as first mates and principal sires. Large males that give a vigorous performance during copulatory courtship are preferred as sires among a female's secondary mates. In this study, the number of matings by free-living females was experimentally controlled and the size and copulatory vigour of all the females’ mates were recorded. At the end of the breeding season, the females were collected and their broods were obtained in captivity. Randomly chosen subsets of 28 female's offspring were reared under standard conditions during the following spring. The spiderlings were reared for 27 days in the company of siblings under conditions that would encourage the expression of genetic variation in viability, and growth and survivorship were monitored. After controlling for maternal effects (i.e. female size and oviposition date) and variance in feeding opportunities among rearing groups, both mate number and mate size were positively and independently related to offspring growth rates and the size of offspring attained after emergence from the natal cocoon. The results support the hypotheses that the preference for large males yields viability benefits to offspring and that polyandry can augment the benefits of selective mating.     

Change in offspring sex ratio over a very short season in Lincoln's Sparrows: the potential role of bill development

ABSTRACT The sex ratios of offspring are targets of natural selection that can affect parental energy expenditure and fitness, adult sex ratios, and population dynamics. Parents may manipulate offspring sex ratios based on sex differences in their offsprings' potential for reproductive success. In Lincoln's Sparrows (Melospiza lincolnii), male bill shape is associated with the quality of songs, and song quality predicts female preferences in a reproductive context. Males and females that hatch later relative to brood mates or later in the breeding season tend to develop bill shapes that are, for males, associated with low‐quality song. Because females do not sing and do not experience this selection pressure, we predicted that the sex of offspring produced late relative to their brood mates or relative to the season should be biased toward females. Using a molecular technique to sex nestlings, we found no effects of hatching order or any interaction between date of clutch initiation (season) and hatching order on offspring sex. However, we found a seasonal decline in the proportion of male offspring, from approximately 0.8 at the beginning to 0.4 at the end of a clutch initiation season only 19 d in duration. To our knowledge, this is the shortest period over which the offspring sex ratio has been shown to change in a bird population. Moreover, these findings are consistent with the hypothesis that sex differences in the potential attractiveness of offspring ultimately influence offspring sex ratios.     

Multiple Paternity in Polyandrous Barn Owls (Tyto alba)

In polyandrous species females produce successive clutches with several males. Female barn owls (Tyto alba) often desert their offspring and mate to produce a 2^nd annual brood with a second male. We tested whether copulating during chick rearing at the 1^st annual brood increases the male''s likelihood to obtain paternity at the 2^nd annual breeding attempt of his female mate in case she deserts their brood to produce a second brood with a different male. Using molecular paternity analyses we found that 2 out of 26 (8%) second annual broods of deserting females contained in total 6 extra-pair young out of 15 nestlings. These young were all sired by the male with whom the female had produced the 1^st annual brood. In contrast, none of the 49 1^st annual breeding attempts (219 offspring) and of the 20 2^nd annual breeding attempts (93 offspring) of non-deserting females contained extra-pair young. We suggest that female desertion can select male counter-strategies to increase paternity and hence individual fitness. Alternatively, females may copulate with the 1^st male to derive genetic benefits, since he is usually of higher quality than the 2^nd male which is commonly a yearling individual.     

Parental Effort in Relation to Structural Plumage Coloration in the Mountain Bluebird (Sialia currucoides)

Indicator models of sexual selection suggest that costly ornaments signal reliable information regarding an individual's quality to potential mates. In species that produce altricial offspring, the amount of parental care provided by both males and females can impact reproductive success. The Good Parent Hypothesis proposes that ornamentation in biparental species can act as an honest signal of parental ability to potential mates. We tested this hypothesis using the mountain bluebird (Sialia currucoides), a sexually dichromatic, socially monogamous species in which both sexes have structurally based ornamental plumage coloration. A male's plumage color predicted neither the rate at which it provisioned nestlings nor brood growth rate. The same was true for females. We also found no indication of assortative mating by color or body condition. Feeding rates within pairs were positively correlated, which we suggest may be due to pairs responding similarly to the perceived needs of nestlings or to local area prey availability. In sum, our results do not support the Good Parent Hypothesis as an explanation for the evolution of ornamental plumage color in mountain bluebirds. We suggest alternative hypotheses for the evolution of ornamental plumage color in this species.     

Extra-pair young in house wren broods are more likely to be male than female

Sex-allocation theory predicts that females should preferentially produce offspring of the sex with greater fitness potential. In socially monogamous animal species, extra-pair mating often increases the variance in fitness of sons relative to daughters. Thus, in situations where offspring sired by a female''s extra-pair mate(s) will typically have greater fitness potential than offspring sired by the within-pair mate, sex-allocation theory predicts that females will bias the sex of offspring sired by extra-pair mates towards male. We examined the relationship between offspring sex and paternity over six breeding seasons in an Illinois population of the house wren (Troglodytes aedon), a cavity-nesting songbird. Out of the 2345 nestlings that had both sex and paternity assigned, 350 (15%) were sired by extra-pair males. The sex ratio of extra-pair offspring, 0.534, was significantly greater than the sex ratio of within-pair offspring, 0.492, representing an increase of 8.5 per cent in the proportion of sons produced. To our knowledge, this is the first confirmed report of female birds increasing their production of sons in association with extra-pair fertilization. Our results are consistent with the oft-mentioned hypothesis that females engage in extra-pair mating to increase offspring quality.     

Breeding ecology of the Aquatic Warbler Acrocephalus paludicola on the Biebrza marshes, northeast Poland

Aquatic Warblers Acrocephalus paludicola were studied in a natural fen mire in the Biebrza River valley, the main breeding ground of the species in its entire distribution range. The number of males present and singing at the sample plot changed considerably during the breeding season. Individual identified colour-ringed males sang with unchanged intensity throughout the breeding season. The period of daily singing activity differed from other species of Acrocephalus ; males sang at dusk rather than at dawn. Males took no part in rearing nestlings but remained on territory and showed vigilance behaviour. The density of males in the breeding season ranged from 1.0 to 10.9 per 10 ha. The density of nesting females ranged from 1.3 to 15.7 per 10 ha. In the most suitable habitat females were more numerous than males. The distribution of females (nests) was clumped where potential food resources were higher. Nests were well hidden in places with deeper water between sedge tufts and an abundance of old dry sedge. Females feeding nestlings collected most food within a 5–60-m radius (mean 31.7 m). The return rate of males was higher than that of females. The results suggest a mating system that is intermediate between facultative polygyny and promiscuity.     

Female brood desertion increases with number of available mates in the Rock Sparrow

We studied the reproductive strategy of a Rock Sparrow Petronia petronia population, breeding in nest boxes in the Western Alps (Italy). Over seven years of study (1991–1997) 19% of the females laid second clutches after successfully fledging the first one. Among these, about 50% deserted the first nest when nestlings were 14.3 d old (range=8–19 d), 3.6 d before fledging (range=1–8 d). In all these cases the primary male mate took over all parental duties and successfully reared the young. Inter-clutch time of deserting females was 8.1 d shorter than that of non-deserting double-brooded females. The breeding success of deserting females was significantly greater than that of both single-brooded females and double-brooded females that did not desert their first brood. The fledging success of the second clutches depended on the status of the secondary male: females paired with previously unpaired males had a higher fledging success than those that paired with a polygynous male. The frequency of deserting females varied among years from 0 to 16%, and was significantly and positively correlated with the frequency of males available as mates at the time of desertion. In this study we showed that sequential polyandry with brood desertion is a regularly occurring strategy in the female Rock Sparrow.     

杂色山雀双亲差异性育雏策略

社会性单配制鸟类的配偶双方在抚育子代时常存在性别差异,不同鸟种的雌雄双亲往往采取不同的育雏策略。以杂色山雀(Sittiparus varius)为研究对象,2017年3—7月对繁殖巢箱进行录像监测,记录杂色山雀育雏期亲代投入情况。分析结果显示:1)双亲递食率在育雏前期(4—6日龄)无显著差异,而育雏后期(10—12日龄)雌性的递食率显著高于雄性。2)雌性亲鸟后期递食率较育雏前期显著增加;而雄性亲鸟育雏前期和后期递食率无显著差异。3)雌性递食率与自身喙宽呈极显著正相关,雄性递食率与双亲体征参数均无相关关系。总的来说,在育雏阶段,杂色山雀雌性亲鸟的递食率随着雏鸟的需求和自身身体质量发生调整,雌性在育雏后期递食率显著升高,而雄性亲鸟递食率无变化,这可能与育雏期双亲投入分工不同有关。     

Divorce and its consequences in the Common Blackbird Turdus merula

An urban population of Common Blackbirds Turdus merula was studied between 1995 and 1998 in the Botanic Garden in Bonn, Germany. The number of breeding pairs varied from 32 to 39 per breeding season. Mean divorce rate (pair separation with remating of at least one of the partners) between two breeding seasons was 19.3% and 5.1% within breeding seasons. The number of fledglings produced per breeding season, of pairs which subsequently divorced, was significantly lower than that of pairs that stayed together. Females, but not males, produced more fledglings in the season after divorce than with their former mates the year before, and as many as females of the same age in stable pair bonds. The increase in reproductive success was thus unlikely to be due to age effects. Nestling mass correlated positively with offspring survival to the following year. Females had heavier nestlings after divorce than in their former pair bond. There was no difference in nestling mass for males before and after divorce. The data were used to test the predictions of several hypotheses to explain divorce. Because divorce was more likely after low reproductive success in the previous year, and only females benefited, the findings best support the 'better option' hypothesis.     

Watching sexy displays improves hatching success and offspring growth through maternal allocation

Male attractiveness can have tremendous effects on the fitness of his offspring via good genes, but also via enhanced maternal allocation of resources. Yet the proximate mechanisms influencing differential maternal allocation in relation to male sexiness are poorly known. Here, we studied the importance of visual stimulation for maternal allocation in the Houbara bustard, a vulnerable bird species bred in captivity to support wild populations. Artificial insemination allowed controlling for potential confounding factors, such as a male''s territory quality, social interactions or sperm quality/quantity, probably linked to mate attractiveness. We show that artificially inseminated females stimulated by highly displaying males increased their hatching success, owing to increased fertilization success. The females also increased the allocation of maternal androgens in their eggs, leading to an increase of circulating testosterone and growth rate in chicks. Hence, visual stimulation of the females can promote differential maternal allocation and favour offspring fitness. Our results further suggest that using artificial insemination for species conservation without appropriate stimulation of the breeding females probably has negative impacts on their breeding performance and therefore on population viability.     

Double brooding and offspring desertion in the barn owl Tyto alba

Many bird species produce two annual broods during a single breeding season. However, not all individuals reproduce twice in the same year suggesting that double brooding is condition‐dependent. In contrast to most raptors and owls, the barn owl Tyto alba produces two annual clutches in most worldwide distributed populations. Nevertheless, the determinants of double brooding are still poorly studied. We performed such a study in a Swiss barn owl population monitored between 1990 and 2014. The annual frequency of double brooding varied from 0 to 14% for males and 0 to 59% for females. The likelihood of double brooding was higher when individuals initiated their first clutch early rather than late in the season and when males had few rather than many offspring at the first nest. Despite the reproductive benefits of double brooding (single‐ and double‐brooded individuals produced 3.97 ± 0.11 and 7.07 ± 0.24 fledglings, respectively), double brooding appears to be traded off against offspring quality because at the first nest double‐brooded males produced poorer quality offspring than single‐brooded males. This might explain why females desert their first mate to produce a second brood with another male without jeopardizing reproductive success at the first nest. Furthermore, the reproductive cycle being very long in the barn owl (120 d from start of laying to offspring independence), selection may have favoured behaviours that accelerate the initiation of a second annual brood. Accordingly, half of the double‐brooded females abandoned their young offspring to look for a new partner in order to initiate the second breeding attempt, 9.48 d earlier than when producing the second brood with the same partner. We conclude that male and female barn owls adopt different reproductive strategies. Females have more opportunities to reproduce twice in a single season than males because mothers are not strictly required during the entire rearing period in contrast to fathers. A high proportion of male floaters may also encourage females to desert their first brood to re‐nest with a new male who is free of parental care duties.     

Sex-related parental care strategies in the lesser spotted woodpecker Picoides minor: of flexible mothers and dependable fathers

We investigated sex-specific parental care behaviour of lesser spotted woodpeckers Picoides minor in the low mountain range Taunus, Germany. Observed parental care included incubation, nest sanitation as well as brooding and feeding of nestlings. Contributions of the two sexes to parental care changed in progress of the breeding period. During incubation and the first half of the nestling period, parental care was divided equally between partners. However, in the late nestling stage, we found males to feed their nestlings irrespective of brood size while females considerably decreased feeding rate with the number of nestlings. This behaviour culminated in desertion of small broods by females shortly before fledging. The fact that even deserted nests were successful indicates that males were able to compensate for the females' absence. Interestingly, the mating of one female with two males with separate nests could be found in the population, which confirms earlier findings of polyandry in the lesser spotted woodpecker. We conclude that biparental care is not essential in the later stage and one partner can reduce effort and thus costs of parental care, at least in small broods where the mate is able to compensate for that behaviour. Reduced care and desertion appears only in females, which might be caused by a combination of two traits: First, females might suffer higher costs of investment in terms of mortality and secondly, male-biased sex ratio in the population generally leads to higher mating probabilities for females in the following breeding season. The occurrence of polyandry seems to be a result of these conditions.