Brood desertion in Kentish plover: sex differences in remating opportunities

To understand the evolution of parental care, one needs to estimatethe payoffs from providing care for the offspring and the payoffsfrom terminating care and deserting them. These payoffs arerarely known. In this study we experimentally estimated therewards from brood desertion in a species that has a variablepattern of parental care. In particular, either the female or themale parent may desert the brood in Kentish plover Charadrius alexandrinus,so some broods are attended by one parent of either sex, whereasin other broods both parents stay with the brood until the chicks fledge.We created single males and single females by experimentallyremoving the other parent and the clutch. The expected rematingtime of males was significantly higher (median: 25.4 days) thanthat of the females (5.3 days, p <.0001). The expected rematingtime tended to increase over the breeding season in both sexes,although the increase was significant only in females. The newnest of remated males was closer to their previous territory (mean± SE, 46 ± 8 m) than that of the remated females(289 ± 57 m, p <.001). Hatching success of new nestswas not different between remated males and females. Our resultsdemonstrate that the remating opportunities are different formale and female Kentish plovers and these opportunities varyover the season. We propose that the remating opportunitieswere influenced by the male-biased adult sex ratio and the seasonaldecrease in the number of breeders. However, we stress thatmeasuring remating times is a more direct measure of matingopportunities than calculating the operational sex ratio.     

Offspring sex ratio in the sequentially polygamous Penduline Tit Remiz pendulinus

Despite the growing literature on facultative sex-ratio adjustment in chromosomal sex-determining vertebrate taxa (birds, mammals), the consistency of results is often low between studies and species. Here, we investigate the primary and secondary offspring sex ratio of a small passerine bird, the Eurasian Penduline Tit (Remiz pendulinus) in three consecutive years. This species has a uniquely diverse breeding system, in which the male (and/or the female) abandons the nest during egg-laying, and starts a new breeding attempt. This allowed us to test (1) whether patterns of parental care, i.e., male-only care, female-only care or biparental desertion, influence offspring sex ratio, and (2) whether the offspring sex ratio is repeatable between successive clutches of males and females. Using molecular markers to sex 497 offspring in 176 broods, we show that (1) offspring sex ratio does not depend on which parent provides care, and (2) the offspring sex ratio is not repeatable between clutches of a given individual. The overall primary and secondary offspring sex ratio at a population level is not different from parity (54 ± 6% males, and 50 ± 3% (mean ± SE), respectively). We suggest that ecological and phenotypic factors, rather than individual traits of parents, may influence offspring’s sex, and conclude that there is currently no evidence for a facultative adjustment of offspring sex ratio in the Penduline Tit.     

Ecological constraints on breeding system evolution: the influence of habitat on brood desertion in Kentish plover

1. One of the fundamental insights of behavioural ecology is that resources influence breeding systems. For instance, when food resources are plenty, one parent is able to care for the young on its own, so that the other parent can desert and became polygamous. We investigated this hypothesis in the context of classical polyandry when females may have several mates within a single breeding season, and parental duties are carried out largely by the male. 2. We studied a precocial wader, the Kentish plover Charadrius alexandrinus, that exhibits variable brood care such that the chicks may be raised by both parents, only by the female or, more often, only by the male. The timing of female desertion varies: some females desert their brood at hatching of the eggs and lay a clutch for a new mate, whereas other females stay with their brood until the chicks fledge. Kentish plovers are excellent organisms with which to study breeding system evolution, as some of their close relatives exhibit classical polyandry (Eurasian dotterel Eudromias morinellus, mountain plover Charadrius montanus), whereas others are polygynous (northern lapwing Vanellus vanellus). 3. Kentish plovers raised their broods in two habitats in our study site in southern Turkey: saltmarsh and lakeshore. Food intake was higher on the lakeshore than in the saltmarsh as judged from feeding behaviour of chicks and adults. As the season proceeded and the saltmarsh dried out, the broods moved toward the lakeshore. 4. As the density of plovers increased on lakeshore, the parents spent more time defending their young, and female parents stayed with their brood longer on the lakeshore. 5. We conclude that the influence of food abundance on breeding systems is more complex than currently anticipated. Abundant food resources appear to have profound implications on spatial distribution of broods, and the social interactions between broods constrain female desertion and polyandry.     

Paternal influence on growth and survival of dark-eyed junco young: do parental males benefit?

Recent studies of typically monogamous passerine birds have suggested that the fitness benefits males derive by caring for their young may not be as great as was previously thought. This study was conducted to determine whether parental care by male dark-eyed juncos, Junco hyemalis, serves to increase either the quantity or quality of young that they produce. Over a 4-year period, males were caught at the time their eggs hatched, and the subsequent growth and survival of the young of unaided females and control pairs were compared. Broods raised by unaided females gained body mass more slowly and fledged at slightly lower mass than those raised by two parents. However, fledging mass was not correlated with survival to independence. There were no differences in tarsus growth between the two treatment groups. Entire brood loss to predators occurred as often among females without male help as it did among those with male help. However, partial brood loss was more common among female-only broods than among controls; this difference was largely attributable to higher rates of starvation and exposure in female-only broods. There appeared to be an interaction between growth and predation. Female-only broods that were below the median mass of combined treatment groups at 5 days of age were more likely than all other broods to experience partial or complete predation. Male impact on offspring survival varied with age of the offspring. When years were combined, males tended to increase survival during the first half of the nestling period, but their impact at the time of nest-leaving was minimal. In all years, from nest-leaving to independence (ca. 2 weeks), broods without male help survived only about half as well as did those with male help. Independent young raised by one parent were as likely to return the following spring as were young raised by two parents. Thus, paternal care benefits males by improving the survivorship of their fledglings, and may also act as a buffer against poor female parental quality and inclement weather. However, the magnitude of these benefits is such that bigamous males might achieve greater reproductive success than monogamous males. Various possible male strategies are discussed.     

The function of habitat change during brood-rearing in the precocial Kentish plover <Emphasis Type="Italic">Charadrius alexandrinus</Emphasis>

Food availability is often variable during the breeding season. Parents with nonmobile, altricial young have no choice but to accept changes in local food availability, whereas in precocial animals, the parents may lead their young away from poor sites to areas that have rich resources and/or are safe from predators. We investigated the latter hypotheses in the Kentish plover Charadrius alexandrinus, a precocial shorebird that raises its young in two habitats: on lakeshore and in saltmarsh. Parents move with their broods from saltmarsh to lakeshore, especially late in the breeding season, and we hypothesized that lakeshores provide more food than the saltmarsh. Consistent with our hypotheses, plover chicks grew faster on the shore, and the difference in growth rates between the two habitats was amplified later in the breeding season. In addition, brood survival was higher on lakeshore than in saltmarsh and decreased with hatching date. Taken together, our results suggest that Kentish plover parents increase their reproductive success by switching brood-rearing habitats strategically.     

No effect of parental quality or extrapair paternity on brood sex ratio in the blue tit (Parus caeruleus)

Sex allocation theory predicts that parents should manipulatebrood sex ratio in order to maximise the combined reproductivevalue of their progeny. Females mating with high quality malesshould, therefore, be expected to produce brood sex ratiosbiased towards sons, as male offspring would receive a relativelygreater advantage from inheritance of their father's characteristicsthan would their female siblings. Furthermore, it has been suggested that sex allocation in chicks fathered through extrapair fertilizations should also be biased towards sons. Contraryto these predictions, we found no evidence that the distributionof sex ratios in a sample of 1483 chicks from 154 broods ofblue tits (Parus caeruleus) deviated significantly from thatof a binomial distribution around an even sex ratio. In addition,we found no significant effect on brood sex ratio of the individualquality of either parent as indicated by their biometrics, feather mite loads, time of breeding, or parental survival. This suggeststhat females in our population were either unable to manipulateoffspring sex allocation or did not do so because selectionpressures were not strong enough to produce a significant shiftaway from random sex allocation. The paternity of 986 chicks from 103 broods was determined using DNA microsatellite typing.Extrapair males sired 115 chicks (11.7%) from 41 broods (39.8%).There was no significant effect of paternity (within-pair versusextrapair) on the sex of individual offspring. We suggest that,in addition to the weakness of selection pressures, the possiblemechanisms responsible for the allocation of sex may not besufficiently accurate to control offspring sex at the levelof the individual egg.     

The benefits of uniparental versus biparental mouth brooding in Galilee St. Peter's fish

In Galilee St. Peter's fish Sarotherodon galilaeus the care system is naturally labile; biparental, male-only and female-only care all exist in one population. This unusual flexibility facilitates comparisons between the forms of care. The costs of parental care were considered in a previous study. Here, the benefits of parental care were quantified by observing wild fish, both held in pond enclosures and free-swimming in Lake Kinneret, Israel. Parental care was shown to be essential for offspring survival in St. Peter's fish. The reproductive success of parents who shared incubation duties was nearly twice as high as that of parents caring alone. However, per brood (or mouth cavity) reproductive success was 20% higher for uniparental parents. Both sexes were equally capable and efficient in care; when both sexes cared, they each incubated a similar number of eggs and released a similar number of fry. The results are discussed in terms of the relationship between caring Strategies and clutch size.     

The effect of parental age,experience and historical reproductive success on wandering albatross (Diomedea exulans) chick growth and survival

Growth and survival of altricial young are influenced by their parents’ abilities to invest in a breeding attempt. As a result, chick growth and survival in one breeding season may be indicative of their parents’ long-term reproductive potential. To determine whether variation in long-term reproductive success is driven by differential breeding investment, parental care and chick growth in wandering albatrosses (Diomedea exulans) were correlated with parental historical reproductive success. Effects of age and breeding experience (determined from past breeding attempts) and pre-laying body condition (mass–size indices) on chick growth and survival also were tested. Longer brooding of chicks increased their survival, but length of chick brooding did not differ between historically unproductive and successful breeders. Past reproductive success also was not correlated with chick growth rates or fledging mass or size. Chick brooding period, chick growth rates, final mass and size were independent of parental body condition. Older and more experienced parents brooded chicks for longer and their chicks grew faster, supporting previous findings that breeding competence is a learnt skill. Chick care and growth characteristics differed more between than within pairs, suggesting that differences in these characteristics are driven by variation among pairs.     

Brood Reduction in White Storks Mediated through Asymmetries in Plasma Testosterone Concentrations in Chicks

We hypothesized that increasing chick plasma testosterone concentrations, transmitted from the mothers via their eggs, enhances survival of their offspring and that the fitness of the young, depending on the maternal hormones, is influenced by parental quality. To test our hypotheses we distinguished the broods of white storks Ciconia ciconia L. where chicks died and those where all chicks survived. We analysed the plasma testosterone concentrations in the chicks, the ability of the chicks to be first to receive food and the mass of chicks before fledging in relation to their hatching order and recorded the body mass of parents and food mass delivered by them.
Female storks used the asymmetries in testosterone concentrations within a brood to control brood size and adjusted the number of young hatched to match the parental ability to rear offspring. Females of poor condition altered the testosterone concentrations to produce large differences between the chicks: The first-hatched chicks, which had high plasma testosterone levels, responded faster to the feeding parent and received more food than did their younger siblings. One or two later-hatched chicks, which had lower testosterone levels, died in these broods. Females in good condition produced small differences in testosterone concentrations between the chicks and all chicks survived in their brood. Chicks that were raised by the females of poor condition in reduced broods were heavier than chicks that were raised by females of good condition in broods where all chicks survived.
We suggest that the control of brood size by testosterone concentration, transmitted by the mother to the chicks, is a hormonal means of condition-dependent reproductive strategy in the white stork.     

The adaptive significance of stealing in a marine bird and its relationship to parental quality

Kleptoparasitism (food theft) is a tactic used opportunisticallyby many foraging birds, but little is known about its fitnessbenefits. Here we show that habitual kleptoparasitism by individualparent roseate terns (Sterna dougallii) is associated with consistentlysuperior reproductive performance relative to nonkleptoparasitic("honest") parents, as measured by growth and survival to fledgingamong their offspring. In broods of two, both chicks of kleptoparasiticparents exhibited superior growth performance during the middleand later stages of the rearing period, relative to chicks ofhonest parents. This difference was especially pronounced insecond-hatched chicks, whose survival is highly variable amongyears and dependent on food availability. Over a 10-year period,average productivity (number of chicks fledged per pair) wassignificantly higher among kleptoparasites than among honestparents, with a larger relative difference during years of foodshortage. Our study indicates that kleptoparasitism in roseateterns is an important component of parental quality and providesthe first evidence that food stealing is associated with enhancedfitness in a facultatively kleptoparasitic seabird.     

Asynchronous hatching provides females with a means for increasing male care but incurs a cost by reducing offspring fitness

In species with biparental care, sexual conflict occurs because the benefit of care depends on the total amount of care provided by the two parents while the cost of care depends on each parent's own contribution. Asynchronous hatching may play a role in mediating the resolution of this conflict over parental care. The sexual conflict hypothesis for the evolution of asynchronous hatching suggests that females adjust hatching patterns in order to increase male parental effort relative to female effort. We tested this hypothesis in the burying beetle Nicrophorus vespilloides by setting up experimental broods with three different hatching patterns: synchronous, asynchronous and highly asynchronous broods. As predicted, we found that males provided care for longer in asynchronous broods whereas the opposite was true of females. However, we did not find any benefit to females of reducing their duration of care in terms of increased lifespan or reduced mass loss during breeding. We found substantial negative effects of hatching asynchrony on offspring fitness as larval mass was lower and fewer larvae survived to dispersal in highly asynchronous broods compared to synchronous or asynchronous broods. Our results suggest that, even though females can increase male parental effort by hatching their broods more asynchronously, females pay a substantial cost from doing so in terms of reducing offspring growth and survival. Thus, females should be under selection to produce a hatching pattern that provides the best possible trade‐off between the benefits of increased male parental effort and the costs due to reduced offspring fitness.     

Competition with a host nestling for parental provisioning imposes recoverable costs on parasitic cuckoo chick's growth

Chicks of the brood parasitic common cuckoo (Cuculus canorus) typically monopolize host parental care by evicting all eggs and nestmates from the nest. To assess the benefits of parasitic eviction behaviour throughout the full nestling period, we generated mixed broods of one cuckoo and one great reed warbler (Acrocephalus arundinaceus) to study how hosts divide care between own and parasitic young. We also recorded parental provisioning behaviour at nests of singleton host nestlings or singleton cuckoo chicks. Host parents fed the three types of broods with similar-sized food items. The mass of the cuckoo chicks was significantly reduced in mixed broods relative to singleton cuckoos. Yet, after the host chick fledged from mixed broods, at about 10-12 days, cuckoo chicks in mixed broods grew faster and appeared to have compensated for the growth costs of prior cohabitation by fledging at similar weights and ages compared to singleton cuckoo chicks. These results are contrary to suggestions that chick competition in mixed broods of cuckoos and hosts causes an irrecoverable cost for the developing brood parasite. Flexibility in cuckoos' growth dynamics may provide a general benefit to ecological uncertainty regarding the realized successes, failures, and costs of nestmate eviction strategies of brood parasites.     

No evidence for sibling or parent–offspring coadaptation in a wild population of blue tits,despite high power

Parent and offspring behaviors are expected to act as both the agents and targets of selection. This may generate parent–offspring coadaptation in which parent and offspring behaviors become genetically correlated in a way that increases inclusive fitness. Cross‐fostering has been used to study parent–offspring coadaptation, with the prediction that offspring raised by non‐relatives, or parents raising non‐relatives, should suffer fitness costs. Using long‐term data from more than 400 partially crossed broods of blue tits (Cyanistes caeruleus), we show that there is no difference in mass or survival between crossed and non‐crossed chicks. However, previous studies for which the evidence for parent–offspring coadaptation is strongest compare chicks from fully crossed broods with those from non‐crossed broods. When parent–offspring coadaptation acts at the level of the brood then partial cross‐fostering experiments are not expected to show evidence of coadaptation. To test this, we performed an additional experiment (163 broods) in which clutches were either fully crossed, non‐crossed, or partially crossed. In agreement with the long‐term data, there was no evidence for parent–offspring coadaptation on offspring fitness despite high power. In addition there was no evidence of effects on parental fitness, nor evidence of sibling coadaptation, although the power of these tests was more modest.     

Willow tit Parus montanus extrapair offspring are more heterozygous than their maternal half‐siblings

Through extrapair matings, males can sire additional offspring with low cost and females may look for direct benefits in form of food or additional paternal care or gain genetic benefits that increase offspring fitness. We studied the patterns of female mate choice and frequency of extrapair paternity in the socially monogamous willow tit Parus montanus using microsatellites. We also examined the effect of heterozygosity on the growth rate and survival of the chicks. We found 25 mixed‐paternity broods out of 117 broods of which both parents were sampled. Altogether, 6.7% of sampled chicks were classified as extrapair young. The pairwise relatedness of social pairs did not correlate with the percentage of extrapair young in the brood and there was no difference in heterozygosity between promiscuous and monogamous parents. However, the extrapair young were more heterozygous than the within‐pair young in the mixed‐paternity broods. The maternal half‐siblings in mixed paternity broods were similar in body size. Thus, there was no indication for different growth rate between the siblings, but there were indications that heterozygosity affects survival.     

Equivocal Responses of Feeding Parents to Experimental Brood Sizes in a Hawk–Cuckoo Host: Brood Size as a Reference for Parental Provisioning Decisions?

The number of offspring surviving until independence is the fundamental drive in the evolution of parental care. Because of the related costs, parental investment must be balanced with essential resources for parents themselves, among the resources available in the environment under the current parental condition. It is advantageous for parents to adjust their level of investment to the number of offspring; however, there is little evidence that parents employ numerical competence in adjusting their investment level. We investigated how parents respond to experimentally manipulated brood sizes in a passerine species, known as a host of a brood parasitic cuckoo whose chicks presumably deceive their hosts numerically. Parents reduced their provisioning to broods of reduced sizes, whereas parents did not increase provisioning to enlarged broods compared to that in the control condition. These parental responses can be attributed to the response of chicks to the experimental treatments compared to that in the control: chicks lowered begging intensity in the reduced condition, while they did not intensify being in the enlarged condition. Further analyses revealed that eagerness of parents to respond to chick begging intensity differed between the experimental treatments: strong parental response was detected toward begging chicks only in the reduced condition. We propose that the detected equivocality of parental responses might be related to the difference in the number of chicks between the unmanipulated and experimentally manipulated broods, the former reflecting the initial parental decision on the amount of resources to allocate to the brood.     

Interaction between parental care and sibling competition: parents enhance offspring growth and exacerbate sibling competition

Species with elaborate parental care often also show intense sibling competition over resources provided by parents, suggesting joint evolution of these two traits. Despite this, the evolution of elaborate parental care and the evolution of intense sibling competition are often studied separately. Here, we examine the interaction between parental food provisioning and sibling competition for resources through the joint manipulation of the presence or absence of parents and brood size in a species with facultative parental care: the burying beetle Nicrophorus vespilloides. The effect of the interaction between the presence or absence of parents and brood size was strong; brood size had a strong effect on growth when parents provided care, but no effect when parents were absent. As in previous studies, offspring grew faster when parents were present than when parents were absent, and offspring grew faster in smaller broods than in larger broods. Our behavioral observations showed that brood size had a negative effect on both the amount of time parents spent providing resources to individual offspring and the offspring's effectiveness of begging, confirming that the level of sibling competition increased with brood size. Furthermore, offspring in larger broods shifted more from begging toward self-feeding as they grew older compared to offspring in small broods. Our study provides novel insights into the joint evolution of parental care and sibling competition, and the evolution of offspring begging signals. We discuss the implications of our results in light of recent theoretical work on the evolution of parental care, sibling competition, and offspring begging signals.     

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.     

Strong evidence for selection for larger brood size in a great tit population

We measured the selection pressure on brood size in a recentlyestablished population of great tits (Parus major L.) in thenorthern Netherlands by manipulating brood size in three years(1995: n = 51, 1997: n = 66, 1998: n = 51), and we estimatedfitness consequences in terms of local survival of both offspringand parents. Enlarged broods had highest fitness; the offspringfitness component was positively affected by manipulation andthe parental fitness component was unaffected. Parental survivaland the probability that parents produced a second clutch werenot affected by the treatment. However, parents that had raisedenlarged broods produced their second clutch later in the season.Clutch size, brood size, and laying date of birds recapturedin the next breeding season were largely independent of thetreatment. We conclude that there is strong evidence for selectionfor larger brood size and reject the individual optimizationhypothesis for this population because the number of young inthe nest predicts fitness independently of the manipulationhistory.     

Bi-parental care in willow ptarmigan: a luxury?

Willow ptarmigan, Lagopus lagopus, usually pair monogamously and both parents invest considerable time and energy in the care of offspring. However, the species is occasionally polygynous (5–9%), and in these cases only the females care for offspring. To examine the consequences of uni-parental care for the production of offspring and the body condition of mates, either the male or the female parent was removed at the onset of incubation and at hatch. Natural removals were also observed. Lone hen willow ptarmigan (widows) defended eggs and incubated clutches successfully, whereas single parent males (widowers) did not. After hatch, widows (mates removed either at the onset of incubation or at hatch) and pairs raised equivalent numbers of chicks to fleging, but widowers (mate removed at hatch) were less successful. There were no differences between parental types in numbers of offspring returning in a subsequent season or in the mating status of offspring in the first year of return. Widows and paired hens had similar rates of survival during the breeding season, and, in subsequent years, similar return rates and mate fidelity. Willow ptarmigan, after the onset of incubation, provided more parental care than is required to produce offspring and maintain survival and body condition of their mates. Thus, these hypotheses can be eliminated as probable factors maintaining monogamy for willow ptarmigan at this site during 1981–1984. Bi-parental care in willow ptarmigan may have developed as a salvage strategy for males as a consequence of their remaining throughout incubation to guard their mates and capitalize on the potential to re-nest.     

Evolutionary transitions in parental care in cichlid fish

Cichlid fishes (Cichlidae) are well suited for testing theories of the evolution of vertebrate parental care. These freshwater teleost fish provide parental care for their offspring, display many different forms of care and have interspecific variation in which sex stays with the young. Here, we assemble the first family-wide composite phylogeny based on morphological and molecular studies, and trace two sets of character evolution: form of care (substrate guarding and mouthbrooding), and sex of care-giver (biparental, female-only, and male-only). Mouthbrooding has evolved from ancestral substrate guarding with 10 to 14 transitions and 0 to 3 reversals. The data support hypothesized transitions in the sex of care-giver, with uniparental female care having arisen from biparental care 21 to 30 times with 0 to 10 reversals. There is also evidence that male-only care evolved once from biparental care. These transitions in parental care characters are the most numerous reported for any family of vertebrates and, to our knowledge, provide the first quantitative support for models of parental care evolution in fish.