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.     

Offspring desertion and parental care in the Whiskered Tern Chlidonias hybrida

In species with biparental care, a conflict of interest can arise if one mate tries to maximize its own reproductive success at the expense of the other's. One of the mates can desert the brood to accrue a number of benefits to enhance its own fitness, leaving parental care to the remaining parent. This study is the first to describe the desertion pattern in a tern species (Sternidae). We investigated offspring desertion in the Whiskered Tern Chlidonias hybrida, a species with semi‐precocial chicks. Offspring desertion was recorded in 52% of nests prior to fledging (n = 131 nests). Females also deserted during the post‐fledging period. Of the deserters, 97% were females. Desertions started when chicks were 5 days old and no longer required intense brooding. Desertions before fledging did not affect fledging success. Provisioning rates between pair members differed, and females supplied much less food than males. Female provisioning rate affected the chances of nest desertion significantly: daily desertion rates were lower when females supplied more food. After females had deserted, males increased their provisioning rates but compensated for the loss of female care only partly in two‐ and three‐chick broods. Only in small (one‐chick) broods was compensation full. We conclude that male and female Whiskered Terns adopt different reproductive strategies in the population studied here. Females invest much less in parental care than males, providing less food and deserting more frequently. Given the ready availability of food and low predation pressure, benefits appear to accrue to females that desert; selection forces may therefore not be acting against female desertion.     

Prolactin stress response does not predict brood desertion in a polyandrous shorebird

One of the fundamental principles of the life-history theory is that parents need to balance their resources between current and future offspring. Deserting the dependent young is a radical life-history decision that saves resources for future reproduction but that may cause the current brood to fail. Despite the importance of desertion for reproductive success, and thus fitness, the neuroendocrine mechanisms of brood desertion are largely unknown. We investigated two candidate hormones that may influence brood desertion in the Kentish plover Charadrius alexandrinus: prolactin ('parental hormone') and corticosterone ('stress hormone'). Kentish plovers exhibit an unusually diverse mating and parental care system: brood desertion occurs naturally since either parent (the male or the female) may desert the brood after the chicks hatch and mate with a new partner shortly after. We measured the hormone levels of parents at hatching using the standard capture and restraint protocol. We subsequently followed the broods to determine whether a parent deserted the chicks. We found no evidence that either baseline or stress-induced prolactin levels of male or female parents predicted brood desertion. Although stress-induced corticosterone levels were generally higher in females compared with males, individual corticosterone levels did not explain the probability of brood desertion. We suggest that, in this species, low prolactin levels do not trigger brood desertion. In general, we propose that the prolactin stress response does not reflect overall parental investment in a species where different parts of the breeding cycle are characterized by contrasting individual investment strategies.     

Cooperation, conflict, and crèching behavior in goldeneye ducks

Crèching behavior, or brood amalgamation, results in offspring being reared by adults other than their genetic parents. Although a variety of hypotheses have been proposed to explain this behavior, most assume either that brood amalgamation is accidental (i.e., nonselected) or that adoption of young is selected for because of social benefits to the young and/or adopting parents. We propose, instead, that brood amalgamation is a function of two separate processes: brood desertion and brood adoption. To examine brood desertion, we develop a graphic model to predict when parents should abandon their young and we test this model experimentally for the Barrow's goldeneye (Bucephala islandica). As predicted, females deserted their offspring when the size of the brood was experimentally reduced. Brood adoption occurred when deserted ducklings joined other broods. However, the success of ducklings in doing so was strongly dependent on the availability of potential host broods and on the age of the recipient broods. Foreign ducklings were readily accepted into young broods (<10 d old) but invariably were rejected from old broods. We could detect no benefits or costs of brood adoption to the host females, contrary to the expectations of a social benefit hypothesis. Our experiments indicate that Crèching behavior is driven by selection on adults to abandon their brood when the benefits of continued investment are outweighed by the reduction in future reproduction and selection on deserted ducklings to join other broods to obtain parental care. Rather than a form of cooperative brood care, Crèching in goldeneyes is perhaps best considered as a form of reproductive parasitism, entailing parent-offspring conflict over brood desertion and intergenerational conflict over adoption of abandoned young.     

Sex roles, parental effort and offspring desertion in the monogamous Eurasian Curlew Numenius arquata

The reasons for female desertion of offspring and the evolution of predominantly male care among monogamous bird species are not clearly understood. We studied parental effort during the incubation and chick rearing periods in the Eurasian Curlew Numenius arquata in western Finland, and compared timing of brood desertion with other populations in Europe. Males and females contributed equally to incubation and showed no differences in the intensity of mobbing behaviour towards a potential nest predator (stuffed crow) shortly after hatching. However, females deserted their offspring approximately halfway through the brooding period ( c. 16 d after hatching), while males remained with chicks until independence ( c. 35 d). Females with late-laid clutches deserted their offspring sooner after hatching than those with clutches produced earlier in the season. Curlew females deserted younger chicks in northeast Europe, where laying dates were later, breeding seasons shorter and migration distances were longer, than in western and central Europe. We suggest that the most likely reasons for offspring desertion by females may be associated with increased female survivorship and maintenance of pairbond between years.     

Nest desertion: a trade-off between current and future reproduction

Life history theory predicts that parents should desert a reproductive attempt if the costs of rearing this brood exceed the expected benefits. Thus, if the value of the current breeding attempt is reduced, for example through an unexpected reduction in size, parents are expected to reconsider whether it is worth continuing investing in their brood. With regard to nest desertion behaviour two predictions can be made: individuals are (1) more likely to desert if the reduction in clutch size is large and (2) less likely to desert if the reduction is at a late stage of breeding. We investigated the threshold at which nest desertion takes place by experimentally reducing great tit, Parus major, clutches to different sizes and at different stages of the incubation period. The results were in accordance with the predictions: clutch desertion rates were negatively related to the number of eggs that remained in the nest, and nest desertion was less likely nearer the end of the incubation period. In addition, we estimated the fitness consequences of nest desertion behaviour. For this purpose we made one group of birds desert in favour of a replacement clutch and another group rear a reduced brood. The latter were more likely to produce a second clutch after the first-brood fledglings had left the nest. As a consequence, the number of fledglings produced over the entire breeding season did not differ between the two experimental groups. We also counted the number of recruits and breeding adults in the following breeding season and found that the experimental groups did not differ in local recruitment and adult survival. Therefore, the results did not indicate that parents improved their fitness by deserting a reduced clutch.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .     

Major benefits of guarding behavior in subsocial bees: implications for social evolution

Parental care is a behavior that increases the growth and survival of offspring, often at a cost to the parents' own survival and/or future reproduction. In this study, we focused on nest guarding, which is one of the most important types of extended parental care; we studied this behavior in two solitary bee species of the genus Ceratina with social ancestors. We performed the experiment of removing the laying female, who usually guards the nest after completing its provisioning, to test the effects of nest guarding on the offspring survival and nest fate. By dissecting natural nests, we found that Ceratina cucurbitina females always guarded their offspring until the offspring reached adulthood. In addition, the females of this species were able to crawl across the nest partitions and inspect the offspring in the brood cells. In contrast, several Ceratina chalybea females guarded their nests until the offspring reached adulthood, but others closed the nest entrance with a plug and deserted the nest. Nests with a low number of provisioned cells were more likely to be plugged and abandoned than nests with a higher number of cells. The female removal experiment had a significantly negative effect on offspring survival in both species. These nests frequently failed due to the attacks of natural enemies (e.g., ants, chalcidoid wasps, and other competing Ceratina bees). Increased offspring survival is the most important benefit of the guarding strategy. The abandonment of a potentially unsuccessful brood might constitute a benefit of the nest plugging behavior. The facultative nest desertion strategy is a derived behavior in the studied bees and constitutes an example of an evolutionary reduction in the extent of parental care.     

Nest desertion cannot be considered an egg‐rejection mechanism in a medium‐sized host: an experimental study with the common blackbird Turdus merula

Two main mechanisms of egg rejection, the main defence of hosts against brood parasites, have been described: ejection and desertion. Desertion of the parasitized nest is much more costly and is usually exhibited by small‐sized host species unable to remove the parasitic egg. However, nest desertion is frequently assumed to be an anti‐parasite strategy even in medium or large‐sized host species. This assumption should be considered with caution because: 1) large‐sized hosts able to eject the parasitic egg should eject it rather than desert the nest, and 2) breeding birds may desert their nests in response to different disturbances other than brood parasitism. This problem is especially important in the common blackbird Turdus merula because this is a medium‐sized species, potential host of the common cuckoo Cuculus canorus, in which desertion has been frequently reported as a response to cuckoo egg models. Here, we seek to determine whether nest desertion can be considered a response unequivocally directed to the parasitic egg in medium‐sized hosts using the blackbird as the study species. In an experimental study in which we have manipulated levels of mimicry and size of experimental eggs, we have found that both colour (mimetic and non‐mimetic; at least for human vision) and size (small, medium, and large) significantly affected ejection rates but not nest desertion rates. In fact, although large eggs disproportionally provoked nest desertion more frequently than did small or medium‐sized eggs, cuckoo‐sized parasitic eggs were not deserted allowing us to conclude that desertion is unlikely to be an adaptive response to brood parasitism at least for this species.     

Brood reduction in the Red-necked Grebe Podiceps grisegena

Brood reduction in Red-necked Grebes Podiceps grisegena breeding on fish ponds in south-eastern Poland occurred either through the desertion of the last-laid eggs after partial hatching of the clutch and/or the selective starvation of the smallest chicks. Abandonment of unhatched eggs was not influenced by the number of young already hatched or by the breeding date, but it was more likely in larger clutches and in families suffering chick starvation. Chicks from the largest broods had a higher probability of survival until fledging than those from single-chick broods. Larger chicks obtained food more successfully through better positioning during food delivery. In families that did not suffer brood reduction, chicks were better provisioned with food than in reduced broods. Although allocation of food among chicks in reduced broods was more skewed to the disadvantage of the younger siblings, dominant chicks obtained less food prior to brood reduction than dominant siblings in unreduced broods. Sibling aggression did not differ between unreduced and reduced broods before death of the weakest chicks. Post-laying adjustment of the number of offspring to prevailing feeding conditions occurred at two stages: by parental manipulation of the number of hatched eggs at the time when parents and chicks leave the nest and by competition between chicks. It is suggested that late egg desertion may be an adaptive mechanism of brood-size adjustment, when elimination of the weakest chicks through sibling competition is not very efficient.     

Telomere attrition and growth: a life‐history framework and case study in common terns

The relationship between growth and age‐specific telomere length, as a proxy of somatic state, is increasingly investigated, but observed patterns vary and a predictive framework is lacking. We outline expectations based on the assumption that telomere maintenance is costly and argue that individual heterogeneity in resource acquisition is predicted to lead to positive covariance between growth and telomere length. However, canalization of resource allocation to the trait with a larger effect on fitness, rendering that trait relatively invariant, can cause the absence of covariance. In a case study of common tern (Sterna hirundo) chicks, in which hatching order is the main determinant of variation in resource acquisition within broods, we find that body mass, but not telomere length or attrition, varies with hatching order. Moreover, body mass and growth positively predict survival to fledging, whereas telomere length and attrition do not. Using a novel statistical method to quantify standardized variance in plasticity, we estimate between‐individual variation in telomere attrition to be only 12% of that of growth. Consistent with the relative invariance of telomere attrition, we find no correlation between age‐specific body mass or growth and telomere attrition. We suggest that common tern chicks prioritize investment in long‐term somatic state (as indicated by canalization of telomere maintenance) over immediate survival benefits of growth as part of an efficient brood reduction strategy that benefits the parents. As such, interspecific variation in the growth–telomere length relationship may be explained by the extent to which parents benefit from rapid mortality of excess offspring.     

Great Spotted Cuckoo Nestlings but not Magpie Nestlings Starve in Experimental Age‐Matched Broods

Nestlings of non‐evicting avian brood‐parasites have to compete for food with foster parents' own nestlings. The outcome of these competitive contests is determined mainly by body size differences between parasitic and host nestlings. As part of the coevolutionary arms race between brood parasites and their hosts at the nestling stage, it has been reported that some host foster parents discriminate against parasitic chicks and are reluctant to feed them. Here, by experimentally creating size‐matched broods of different composition (only magpie Pica pica chicks, only great spotted cuckoo Clamator glandarius chicks or mixed broods), we show that great spotted cuckoo chicks starved in 20.2 per cent (17 of 84) of the parasitized magpie nests even in absence of size asymmetries, while in none (0 of 72) of the nests a magpie chick starved. As far as we know, this is the first record of non‐evictor brood parasitic nestlings starving without being smaller than their host nestmates in a frequently used host species. Nest composition had no effect on chick starvation. The cuckoo nestling starved even in two of the nests occupied by only one cuckoo chick. Our results could be explained by (1) magpies being reluctant to feed cuckoo chicks; (2) parasitic chicks receiving lower‐quality food items or cuckoo nestlings being sensitive to some particular component of the diet (e.g. cereal grains); and (3) the existence of cuckoo chick discrimination ability by magpie foster parents.     

Brood adoption and deceit among African Marsh Harriers Circus ranivorus

Complete adoption of unrelated broods by foster parents is surprisingly common among birds and the behaviour appears to be maladaptive in several species. In a small, marked, southern African population of African Marsh Harriers Circus ranivorus, studied over 4 years, a young male took over the complete parenting (provisioning and defence) of an unrelated brood, and several times fed the brood in preference to the soliciting maternal parent. This female then deserted and deceitfully courted (out of season) a third male, whose provisioned food she took to feed to her own young. Since the adopting male retained the territory and bred successfully for at least two further years, he suffered no expected loss in fitness and hence this case of fostering was neither maladaptive nor apparently altruistic.     

Clutch desertion and re-nesting in pied flycatchers: an experiment with progressive clutch removal

Clutch desertion and re-nesting are important components of fitness when predation is frequent. In nestbox populations however, nest predation and desertion are rare but can be studied by experimental manipulations. We experimentally reduced clutches of pied flycatcher,Ficedula hypoleucaby removing one egg per day until desertion occurred. The size of the clutch at desertion and whether females re-nested or not were used as measures of the female response. Of the deserting females, 74% re-nested in our study area. Re-nesting frequency was correlated with date but not with the size of the clutch laid. The majority of the non-re-nesting females deserted empty nests, while the majority of re-nesting females deserted one egg. Clutch size at desertion was not correlated with the size of the clutch laid nor with laying date; it was smaller than the size predicted by an optimality analysis of the value of both the current (deserted) and the replacement clutch. For the re-nesting females, there was a negative correlation between fledging rate of the replacement clutch and the size of the clutch at desertion. Our predictions, made under the hypothesis that desertion and re-nesting are adaptive behaviours, were partly supported by the data; we explain the discrepancy by the constraint of searching for a new nest site or mate for re-nesting.     

Do great tits (Parus major) starve to reproduce?

To test whether nest abandonment is associated with parental health state, reproductive parameters and parental condition indices were examined in relation to brood desertion in great tits. Before desertion, pairs that abandoned their broods in the second half of the nestling period had significantly higher nestling mortality as well as lower average weight of nestlings and entire broods. Independently of brood size, female great tits that deserted their broods on average weighed 1 g (>5%) more than non-deserters. Comparison of metabolic profiles revealed that deserting females were in better nutritional condition (inclined to fat deposition) than non-deserters, which showed symptoms of postresorptive catabolic state, as indicated by a lower level of plasma triglycerides, very low density lipoproteins, and a higher level of free fatty acids and β-hydroxy-butyrate. These results suggest that desertion can be regarded as a reproductive restraint and that non-deserting females invested at least some of their maintenance resources on brood rearing. We found no evidence that desertion or non-desertion was associated with age- or disease-related differences in residual reproductive values. Male condition was not related to brood abandonment, suggesting that desertions were primarily initiated by females. Received: 16 November 1998 / Accepted: 1 February 1999     

An experimental test on time constraint and sexual conflict over parental care

Because parental care is costly, a sexual conflict between parents over parental investment is expected to arise. Parental care behavior is an adaptive decision, involving trade‐offs between remating, and consequently desertion of the brood, and continuing parental effort. If the main advantage of desertion is remating, then this will be a time constraint, because the deserting individual will require a certain minimum period of time to breed again in the same breeding season. So, a short breeding season should force certain individuals to desert the first brood to have enough time to successfully complete their second breeding attempt. The rock sparrow, Petronia petronia, is an unusual species in which brood desertion can occur in both sexes and the breeding season is quite short so it is a good species to investigate the role of time constraint on brood desertion. For 3 years, I investigated the brood desertion modality of the rock sparrow. Then, for 2 years, I removed a group of experimental nest boxes during the autumn. Later, I re‐installed the experimental nest boxes after the start of the breeding season (2 weeks after the first egg was laid), mimicking a shortening of the breeding season for the (experimental) pairs that used experimental nest boxes. I found that in the experimental pairs, the percentage of deserting individuals was significantly higher than in the control groups, and the deserting individuals were older females. This experiment adds to our knowledge of timing of reproduction effects on individual decisions to desert by showing that a short and delayed breeding season may have different effects on males and females. To my knowledge, this is the first experimental study that demonstrates a direct link between time constraint and brood desertion.     

A Cue-Isolation Experiment to Determine if Caspian Tern Parents Learn Their Offspring's Down Color

A ground-nesting gull or tern (Laridae) parent will attack a foreign conspecific chick that is substituted for the parent's chick at about the time the parent's chick is mobile. Presumably, parents discriminate among chicks using some combination of vocal, morphologic, and behavior cues. It is not known which cues are used. Morphologic variation in chicks' down potentially provides discrimination cues in some species of Laridae, such as the Caspian tern (Sterna caspia). To test discrimination based on the down color of chicks, a parent's chick was replaced sequentially with foreign conspecific chicks that were similar and dissimilar to the down color of the parent's chick. This cue-isolation experiment indicated that parents rejected dissimilar chicks more frequently. Thus, parents learn the down color of their chicks and then use this information as a basis for aggressive rejection of foreign chicks. A recognition system based on morphology allows parents to detect and reject foreign chicks with a probability greater than chance.     

Parental behaviour in the Lapwing Vanellus vanellus

Parental behaviour of monogamous and polygynous Lapwings was studied during incubation and brood care. Both parents attended the nest in 86% of monogamous pairs ( n = 29 pairs). In 14% of pairs, only the male parent continued incubation until the eggs hatched, whereas the female deserted the clutch before or at the end of incubation. There was a clear division of parental roles during incubation. Females spent more time incubating (64% of time) than their mates (27%), whereas males spent more time defending the nest (3%) than females (>1%). Time spent incubating did not differ between monogamous and polygynous males. However, polygynous females spent more time incubating (primary females: 95%; secondary females: 97%) than monogamous females. Biparental care was the most common pattern of post-hatching care, although in some broods either the male or the female parent deserted before the chicks fledged. Division of sex roles was less pronounced in brood care than during incubation. Females spent more time brooding (21%) than males (3%), and females attended their chicks more closely than males. Nevertheless, males and females spent similar amounts of time defending the brood from predators and conspecifics. We suggest that the apparent division of parental roles may be explained by sexual selection, i.e. the remating opportunities for male Lapwings might be reduced if they increase their share in incubation. However, the different efficiency of care provision, for example in ability to defend offspring, may also influence the roles of the sexes in parental care.     

Trade-off between mating opportunities and parental care: brood desertion by female Kentish plovers.

Why do some parents care for their young whereas others divorce from their mate and abandon their offspring? This decision is governed by the trade-off between the value of the current breeding event and future breeding prospects. In the precocial Kentish plover Charadrius alexandrinus females frequently, but not always, abandon their broods to be cared for by their mate, and seek new breeding partners within the same season. We have shown previously that females'' remating opportunities decline with date in the season, so brood desertion should be particularly favourable for early breeding females. However, the benefits are tempered by the fact that single-parent families have lower survival expectancies than those where the female remains to help the male care for the young. We therefore tested the prediction that increasing the value of the current brood (by brood-size manipulation) should increase the duration of female care early in the season, but that in late breeders, with reduced remating opportunities, desertion and thus the duration of female care should be independent of current brood size. These predictions were fulfilled, indicating that seasonally modulated trade-offs between current brood value and remating opportunities can be important in the desertion decisions of species with flexible patterns of parental care.     

Chick begging as a signal: are nestlings honest?

Begging by dependent avian offspring is known to correlate withhunger level, and parents use this as a signal of brood demandto adjust their chick feeding behavior. While there is informationon how each chick adjusts its begging to its own condition,little is known of how chicks adjust to the state of their nestmates. In two experiments we manipulated the competitive environmentof individual European starling (Sturnus vulgaris) chicks byaltering the state of nest mates while holding the state oftarget chicks constant In the first experiment we placed thetarget chick's nest mates in neighboring nests with brood sizesof two, five, or eight chicks. Following the manipulation wereturned them to their own nests and recorded begging behavioron videotape. In the second experiment we separated a targetchick from its siblings and manipulated feeding level in thelaboratory. The siblings were fed at one of three levels; meanwhile,all the target chicks were fed at the intermediate level. Afterthe manipulation we placed the target chicks with their siblingsand recorded their begging in response to an artificial stimulus.In neither experiment was the begging effort of the unmanipulatedtarget chicks affected by the changes in begging behavior oftheir siblings. This result supports the view that begging isa reliable signal of individual chick state and does not involveresponses to the effort of nest mates.     

A recognition-free mechanism for reliable rejection of brood parasites

Hosts often discard eggs of avian brood parasites, whereas parasitic chicks are typically accepted. This can be explained theoretically by fitness losses associated with adults learning to recognize parasitic young and mistakenly rejecting their own young. A new experimental study confirms that rejection of parasitic chicks, without relying on memory to discriminate between foreign and own young, is a feasible and potentially cost-free mechanism used by reed warblers to reject common cuckoo chicks. By abandoning broods that are in the nest longer than is typical for their own young, parents can reliably reject parasite nestlings and reduce fitness losses owing to having to care for demanding parasitic young. Discrimination without recognition has important implications for the realized trajectories of host-parasite coevolutionary arms races.