Fledgling sex ratios in relation to brood size in size-dimorphic altricial birds

In six species of dimorphic raptors (females larger than males)and one passerine (males larger than females), the sex ratioat fledging varied systematically with brood size at fledging.In all species the strongest bias toward the smaller sex wasestablished in the largest as well as the smallest broods; amore even distribution of males and females was observed inbroods of intermediate size. We explored a specific differentialmortality explanation for this sex ratio variation. Our hypothesispostulates that variation in mortality is caused by differencesin food demand between broods of the same size, due to theirsex composition. Data from the marsh harrier Circus aeruginosuson gender-related food demand and overall nestling mortalitywere used to predict the frequency of surviving males and femalesat fledging, assuming an even sex ratio at hatching and randommortality with respect to both sexes within broods. The modelquantitatively fits the marsh harrier data well, especiallyin broods originating from large dutches. Although we anticipatethat other mechanisms are also involved, the results supportthe hypothesis of sex-ratio-dependent mortality, differentialbetween broods, as the process generating the observed brood-sizedependence of fledgling sex ratios in sexually dimorphic birds.     

Post-fledging interactions between the Common Cuckoo Cuculus canorus and its cavity-nesting Common Redstart Phoenicurus phoenicurus host

Brood parasite–host interactions during the incubation and nestling stages have been well studied, but the post-fledging period remains virtually unknown. Using radiotracking, we provide the first detailed data on post-fledging interactions between the Common Cuckoo Cuculus canorus and its only regular cavity-nesting host, the Common Redstart Phoenicurus phoenicurus. Cuckoos raised alone (‘solitary’) fledged at higher mass, with higher wing and tarsus length and started to fly at a younger age than Cuckoos raised alongside young Redstarts (‘mixed’). However, a further 23 fledging and post-fledging parameters measured at five pre-determined times (fledging, first-flight, predation, starvation, independence) did not differ between solitary and mixed Cuckoos. In addition, none of the parameters measured during the post-fledging period (growth, dispersal distances, number of flights) differed between solitary and mixed Cuckoos. Redstart fledglings from non-parasitized broods (‘solitary’) showed generally similar fledging and post-fledging parameters to fledglings reared alongside a Cuckoo (‘mixed’). Surprisingly, there were no significant differences in post-fledging predation rate, starvation or overall survival rates between mixed and solitary Cuckoos or mixed and solitary Redstarts. Thus, during the post-fledging period, mixed Cuckoo fledglings successfully compensated for the poorer performance experienced during the nestling stage whereas mixed and solitary Redstarts did not differ in any measured parameters. This suggests that the regular occurrence of mixed broods in this host–parasite system – which is unique among the many Cuckoo hosts – is evolutionarily stable for both hosts and parasites.     

Unusual pattern of sex‐specific mortality in relation to initial brood sex composition in the black‐billed magpie Pica pica

In sexually size‐dimorphic species, brood sex composition may exert differential effects on sex‐specific mortality. We investigated the sex‐specific mortality and body condition in relation to brood sex composition in nestlings of the black‐billed magpie Pica pica. Neither significantly sex‐biased production at hatching nor overall sex‐biased mortality during the nestling period was found. Sex‐specific mortality as a function of brood sex composition, however, differed between female and male nestlings. We found higher mortality for females in male‐biased broods and higher mortality for males in female‐biased broods, a phenomenon that we call ‘rarer‐sex disadvantage’. As a result, fledging sex ratios became more biased in the direction of bias at hatching, a phenomenon that cannot be readily explained by previous hypotheses for sex‐specific mortality. Two temporal variables, fledging date and laying date, were also correlated with sex‐specific mortality: female nestlings in earlier broods experienced higher mortality than male nestlings whereas male nestlings in later broods experienced higher mortality. We suggest that this unusual pattern of mortality may be explained by adaptive adjustments of brood sex composition by parents, either through the effects of a slight sex difference in offspring dispersal patterns on parental fitness, or owing to sex differences as regards the benefits of early fledging.     

The adaptive significance of brood size and time of breeding in the dipper Cinclus cinclus (Aves: Passeriformes) as seen from post-fledging survival

We assessed the post-fledging survival of dippers Cinclus cinclus from 743 broods in relation to brood size, time of hatching and territory quality. We paid particular attention to assessing whether contrasting breeding performance along unproductive (i.e. acidic) and productive (i.e. circumneutral) rivers represented strategies which optimized the number of surviving young.
For all brood sizes, post-fledging survival varied significantly through the breeding season, with most survivors coming from attempts in the peak period of hatching. After correcting for these seasonal effects, the most common brood size overall, of four, was also the most productive as seen from post-fledging survival; differences in the frequency of occurrence and survival between broods of four and five were marginal. Moreover, a change in the modal brood size from five to four occurred as the season progressed. consistent with a shift in brood productivity.
Broods at acidic sites were significantly smaller than at circumneutral sites; while brood size four was the most productive at both types of site, brood size three was the second most productive at acidic sites, while brood size five was the second most productive at circumneutral sites. Dippers at acidic sites bred significantly later than at circumneutral sites, but post-fledging survival declined most rapidly through the season at the former.
These survival data provide evidence from both seasonal and spatial patterns that brood sizes in the dipper may be optimized in ways consistent with the enhancement of productivity. By contrast, delayed breeding at acidic sites contrasted with the patterns expected from optimization, instead reflecting resource scarcity.     

Post-fledging recovery and dispersal of ringed Eurasian Sparrowhawks Accipiter nisus

Ring recoveries were used to explore the effect of early experience on the subsequent survival and dispersal of young Sparrowhawks in two areas in south Scotland. Young raised on high grade territories were recovered in greater proportion than young on low grade territories, implying that they had survived better after leaving the nest. Recovery rate tended to decline with increasing elevation of the nesting territory, implying that young from the high-ground territories survived less well than those from low-ground territories. Recovery rate showed no relationship with brood-size or sex composition, or with maternal age (yearling or older).
After the post-fledging period, dispersal distances were greater in females than in males and showed significant increases with increasing elevation of nesting territory (one area only) and with lateness of laying (or fledging) date. Regardless of laying date, young males from yearling mothers also dispersed further than young from older mothers, and young males from high elevation territories further than those from lower territories. No significant relationships emerged between dispersal distances and grade of natal territory, brood size or sex composition.     

Survival of common goldeneye Bucephala clangula ducklings in relation to weather, timing of breeding, brood size, and female condition

The survival of common goldeneye Bucephala clangula ducklings during their first week of life was studied in relation to hatching date, brood size, female condition, and weather (temperature and precipitation) during the first week post-hatch by using data from radio-marked females and their broods. Also, age-specific variation in the survival of the young was determined until fledging (over 50 d of age). Survival was lowest in the first week after hatching. Hatching date, brood size, and first week temperature and precipitation were poor predictors of duckling survival during the first week after nest exodus. Instead, the ducklings of females in a better body condition survived better in their first week of life. The results suggest that weather does not have a direct effect on downy ducklings' survival, but the condition of the female seems to be an important determinant of the survival of common goldeneye ducklings.     

Nesting and post-fledging predation risk influence diel patterns of songbird fledging

Among stages of avian ontogeny, the act of nest departure or fledging is an abrupt transition into a new environment and a major leap toward independence for offspring. In altricial birds, the timing (i.e. time of day) of fledging is notable in that many species tend to fledge early in the morning. Past studies have proposed nest predation as a key factor driving birds to fledge earlier in the morning (the ‘survival hypothesis’), whereby offspring avoid peak times of nest predation that occur later in the day. A natural extension of this hypothesis is the predation of offspring post-fledging, whereby offspring are also timing their fledging with future survival prospects outside of the nest. However, few studies have investigated fledging behaviour in the context of both nesting and post-fledging predation. To help fill this knowledge gap, we investigated factors driving the timing and duration of fledging across six songbird species in the context of offspring predation: daily nest mortality, post-fledging mortality and diel patterns of nest predation risk. We found that > 60% of songbirds fledged early in the morning, whereas the peaks in nest predation risk occurred several hours post-fledging. Furthermore, species under greater risk of nest predation fledged earlier in the day and in closer succession to their siblings. Parameters of post-fledging mortality were poor predictors of fledging timing, but individuals from broods of species under higher risk of post-fledging mortality fledged in closer succession to their siblings. These results provide evidence in support of the survival hypothesis, and suggest that songbirds fledge in the morning to avoid peak times of nest predation risk that occur later in the day (~ 8 h after civil dawn). Such results corroborate past research highlighting predation on dependent offspring as a key factor driving variation in life histories across animal taxa; however, estimates of post-fledging mortality suggest that nest predation alone does not fully explain variation in fledging behaviour among species. Future research is therefore needed to investigate the contribution of other factors, such as energetics, parent–offspring conflict and diel patterns of post-fledging survival, which may help to mediate diel patterns of fledging within and among songbird species.     

Post-fledging body mass increase in Common Terns Sterna hirundo: influence of age, sex and year

We studied the inter-year and inter-sex variation of the post-fledging body mass development of Common Terns Sterna hirundo in 2000 and 2001 at the Banter See colony, northern Germany. Here, post-fledglings can be identified and weighed remotely and automatically by a transponder system that makes use of automated balances installed at the colony. Individuals were sexed with PCR amplification methods. After fledging, young generally continued to increase their mass. However, in 2000, the young did not significantly increase their mass during the post-fledging period. In 2001, conditions were more favourable and body mass increased continuously. Further, in 2001, male post-fledglings were significantly heavier than female post-fledglings. Once having left the colony area (on average 18–23 days after fledging in 2000, and 14–16 days after fledging in 2001), post-fledgling body mass had still not reached adult body mass. The longer a juvenile stayed at the colony, the higher was its final body mass, which if acting as a threshold level may control departure time. Neither brood size nor hatching order affected post-fledging mass or period. In the unfavourable year 2000, when many individuals were found dead after fledging, fledging age but not fledging mass was found to be a predictor of post-fledging survival before departure: individuals fledging when older had a lower survival probability. Our results stress the importance of the post-fledging period for body mass increase and survival prior to departure. The variation in post-fledging mass growth between years and between the sexes is discussed with respect to parental effort and a possible selective provisioning of sons over daughters.     

An experimental approach to the brood reduction hypothesis in Magellanic penguins

In many bird species, eggs in a brood hatch within days of each other, leading to a size asymmetry detrimental to younger siblings. Hatching asynchrony is often thought of as an adaptive strategy, and the most widely studied hypothesis in relation to this is the ‘brood reduction hypothesis’. This hypothesis states that when food resources are unpredictable, hatching asynchrony will allow the adjustment of the brood size maximizing fledging success and benefitting parents. The Magellanic penguin Spheniscus magellanicus is an appropriate species to test this hypothesis because it has a 2‐egg clutch that hatches over a 2‐d interval with a broad range of variation (–1 to 4 d), it shows facultative brood reduction, and food abundance between breeding seasons is variable. We performed a manipulative study at Isla Quiroga, Argentina, during three breeding seasons (2010–2012) by forcing broods to hatch synchronously (0 d) or asynchronously (2 or 4 d). Years were categorized based on estimated food abundance. Our study provided mixed results because in the low estimated food abundance year asynchronous broods did not have higher nestling survival than synchronous broods, and the second‐hatchling in asynchronous broods did not die more often than those in synchronous broods. On the other hand, younger siblings of 4‐d asynchronous broods starved earlier than those of synchronous broods, and 2‐d asynchronous broods fledged heavier young than synchronous broods. Asynchronous hatching would seem to benefit reproduction in this species, not with respect to survival, but in terms of the advantages it can accord to nestlings and, in terms of lower costs, for parents raising nestlings.     

The significance of clutch overlap in Great Tits Parus major

We studied the effects of manipulation of the size of first broods in the Great Tit Parus major on the size and breeding success of second clutches and its relation to the degree of clutch overlap. The rearing of first brood fledglings always overlapped with the laying of the second clutch and in most cases also with the incubation period of the latter. The degree of clutch overlap depended on the size of the first brood, being less when the first brood was large. Clutch overlap also increased with season. Mechanisms affecting the timing of laying of second clutches are discussed. A large first brood imposed reproductive costs. It affected the size of the second clutch by causing it to be delayed; second clutch size decreases with season. It affected the post-fledging survival of second brood young as, in this population, this decreases with fledging date. The breeding success of second clutches was, however, not affected by the size of the first brood, but instead by the weight of the female when rearing the first brood.     

Sex-specific fledgling success and brood sex ratio in the Great Reed Warbler (Acrocephalus arundinaceus)

We examine sex ratio variation and sex-specific probability of successful fledgling in relation to hatching date across 376 broods of Great Reed Warblers (Acrocephalus arundinaceus). The sex ratio of complete broods as well as broods with partial mortality did not deviate significantly from parity (0.5 and 0.53, respectively). Variation in sex ratio between broods was not larger than expected from binomial distribution, thus females seem not to manipulate the sex ratio of their broods in the studied population. As a consequence, sex ratio did not vary in relation to hatching date, years and fishponds. Female offspring showed lower fledgling success than their brothers, but the relationship between probability of successful fledgling and hatching date differed between sexes. Fledgling success of female offspring declined with hatching date more strongly than the success of male offspring. Thus, our study shows that Great Reed Warblers do not adjust offspring sex to match observed seasonal sex-specific variation in survival.     

Hatching asynchrony,sibling hierarchies and brood reduction in the Chinstrap penguin Pygoscelis antarctica

We studied patterns of chick growth and mortality in relation to egg size and hatching asynchrony during two breeding seasons (1991 and 1992) in a colony of chinstrap penguins sited in the Vapour Col rookery, Deception Island, South Shetlands. Intraclutch variability in egg size was slight and not related to chick asymmetry at hatching. Hatching was asynchronous in 78% (1991) and 69% (1992) of the clutches, asynchrony ranging from 1 to 4 days (on average 0.9 in 1991 and 1.0 days in 1992). Chicks resulting from oneegg clutches grew better than chicks in families of two in 1991. In 1992, single chicks grew to the same size and mass at 46 days of age as chicks of broods of two, suggesting food limitation in 1991 but not in 1992. In 1991, asymmetry between siblings in mass and flipper length was significantly greater in asynchronous than in synchronous families during the initial guard stage, but these differences disappeared during the later créche phase. In 1992, asymmetry in body mass increased with hatching asynchrony and decreased with age. Only the effect of age was significant for flipper length and culmen. Asymmetries at 15 days were similar in both years, but significantly lower in 1992 than in 1991 at 46 days of age. There were relatively frequent reversals of size hierarchies during both phases of chick growth in the two years, reversals being more common in 1991 than in 1992 for small chicks. In 1991, survivors of brood reduction grew significantly worse than chicks in nonreduced broods. In both years, chicks of synchronous broods attained similarly large sizes before fledging as both A and B chicks of asynchronous broods. In 1991, chick mortality rate increased during the guard stage due to parental desertions, decreased during the transition to crèches (occurs at a mean age of 29 days) and returned to high constant levels during the crèche stage, when it is mostly due to starvation (in total 66% of hatched chicks survived to fledging). In contrast, in 1992, mortality was relatively high immediately after hatching and almost absent for chicks older than 3 weeks (87% of chicks survived to fledging). Mortality affected similarly one- and two-chick families. In 1991, asynchronous families suffered a significantly greater probability of brood reduction than synchronous families, but this probability was not significantly related to degree of asymmetry between siblings. No association between asynchrony and mortality was found in 1992. These results show that there is food limitation in this population during the crèche phase in some years, that asynchronous hatching does not facilitate early brood reduction and that it does not ensure stable size hierarchies between siblings. Brood reduction due to starvation is not associated to prior asymmetry and does not facilitate the survival or improve the growth of the surviving chick. Asynchronous hatching may be a consequence of thermal constraints on embryo development inducing incubation of eggs as soon as they are laid.     

An experimental manipulation of clutch and brood size of White-rumped Swiftlets Aerodramus spodiopygius of Fiji

By monitoring hatching success, chick growth and fledging success in normal sized and experimentally enlarged clutches and broods of the White rumped Swiftlet Aerodramus spodiophygius in Fiji this paper demonstrates the inability of this species to raise more young than is normal.     

Long-term brood division and exclusive parental care in a cooperatively breeding passerine

In some species of birds, individual adults feed different subsets of the brood after the young have left the nest. However, few studies have shown that such ‘brood division’ represents long-term separation into subfamilies, rather than short-term biases in care, and the function of brood division is unknown. Most functional hypotheses assume that brood division is an adult adaptation to increase reproductive success; others suggest that fledglings are responsible for brood division, or that brood division is the outcome of conflicts of interest within families. We tested for long-term brood division in the white-browed scrubwren, Sericornis frontalis, a cooperatively breeding passerine, and assessed functional hypotheses, using behavioural, morphological and molecular evidence. We found that 71% of scrubwren broods became divided in the second week after fledging, and remained divided for the following 4-5 weeks of care. Furthermore, most individual young were fed almost exclusively by a single adult, even in undivided broods and broods of one. Brood division did not arise through adults caring for young of a specific sex, nor did genetic paternity explain division. Tests of other hypotheses were indirect, relying for example on testing mechanisms rather than predictions. We rejected the hypotheses that brood division was primarily an adaptation to reduce the mean or variance in predation, or that it was likely to increase the rate at which young were provisioned. Fledglings probably had an active role in maintaining brood division, with fledglings that had been biggest in the nest tending to be fed by adults that had fed nestlings at the greatest rate. This suggests that the most dominant fledgling may sequester the care of the best feeder. We also found evidence that brood division might be an adult adaptation to reduce the effects of sibling competition, or that it might arise through conflicts of interest among adults. We further suggest that social specialization, resulting from learning about a particular individual, might bring additional advantages to long-term brood division. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Sex ratio and fledging success of supplementary-fed Tengmalm's owl broods

A nest box population of Tengmalm's owls (Aegolius funereus) in northern Sweden was studied to investigate the effects of extra food on the sex ratio between hatching and fledging in this sexually size-dimorphic species. The brood size and brood sex ratio of supplementary-fed and control broods were compared. Newly hatched nestlings were blood sampled and sexed by polymerase chain reaction (PCR) amplification of the sex-linked CHD1Z and CHD1W genes. The brood sex ratio at hatching was strongly male biased (65%); this was also the case in broods where all eggs hatched (72%). There was no relationship between hatch order and sex ratio, and hatching sex ratio did not vary significantly with laying date. Brood size decreased between hatching and fledging, but did not differ between fed and control broods at either stage. Brood sex ratio did not differ between hatching and fledging, and fledging sex ratio did not differ between fed and control broods. It was concluded that, at least during the year in which the study was carried out, feeding had no effect on brood reduction, and that male and female nestlings did not show any differential mortality. The mechanisms behind the male-biased sex ratio at hatching, and any possible adaptive reasons for it, are not known.     

Are there trade-offs between pre- and post-fledging survival in black brent geese?

1.?The growth period is an important determinant of fitness later in life through its effects on first-year survival and future reproduction. Choices by adult females about where to rear their offspring strongly affect growth rates and offspring fitness in geese. 2.?Individual female black brent (Branta bernicla nigricans) tend to raise their broods in the same areas each year, and these areas are consistently ranked with respect to growth rates of goslings. Therefore, some females consistently rear their broods on areas resulting in lower post-fledging fitness. 3.?We explore the potential that growth rates of offspring (and associated fitness consequences) are traded off against other vital rates influencing fitness of either adult females or goslings. Growth of goslings primarily influences fitness after fledging, so one hypothesis is that survival before fledging, which is influenced by predation, is traded off against growth rates and post-fledging survival. 4.?We estimated pre-fledging and post-fledging survival for goslings reared on areas used by broods from the Tutakoke River black brent colony. We examined recaptures, recoveries by hunters and resightings of brent marked as goslings with webtags and standard leg rings. These data were analyzed using capture-mark-recapture models in program mark to derive separate estimates of pre- and post-fledging survival for 18 cohorts (1987-2004) of black brent goslings across seven brood rearing areas (BRAs). 5.?Estimates of pre-fledging survival probability varied from 0·00?±?0·00 (mean?±?95% confidence interval) to 0·92?±?0·1; and estimates of post-fledging survival probability varied from 0·00?±?0·00 to 1·00?±?0·08. Substantial variation existed both among BRAs and years but post-fledging survival declined substantially during the study. 6.?Pre- and post-fledging survival were positively correlated, exhibiting a quadratic relationship (?(post-fledging survival) =?1·00 (±0·47)x-0·83 (±0·480)x(2) , where x?=?pre-fledging survival). Therefore, we did not find a trade-off between pre- and post-fledging survival in black brent goslings across BRAs, suggesting that factors other than foraging conditions and predation on goslings must influence selection of BRAs.     

Optimal brood size and its limiting factors in the Rufous Turtle DoveStreptopelia orientalis

Optimal brood size and its limiting factors of the Rufous Turtle Dove,Streptopelia orientalis, were studied at the campus of the University of Tsukuba, Japan, during the breeding season in 1990–92. The dove usually lays two eggs in a nest. I made nests of a brood size of one and three by transferring a hatchling from one nest to the other, and compared their fledging success, factors of breeding failure, weight and tarsus length at fledging, growth rate and nestling period with those of a brood of two. The index of fitness (fledgling weight multiplied by average number of fledglings per nest) was almost the same in broods of two and three. However, the highest variation in fledging weight within the brood and the extension of nestling period were observed in broods of three, which caused the extension of inter-brood interval and consequently the smaller number of broods in the total breeding season. Therefore, broods of three would not have an advantage in producing more offspring than broods of two. Crop milk production had an effect on the growth of nestlings in the early phase of the nestling period, but the rapid growth in the granivorous phase compensated for the growth delay of the smallest nestling in broods of three. Small brood size and a large number of broods in a season would also be more effective under high predation pressure.     

Post-fledging brood and care division in the roseate tern (Sterna dougallii)

Extended post-fledging parental care is an important aspect of parental care in birds, although little studied due to logistic difficulties. Commonly, the brood is split physically (brood division) and/or preferential care is given to a subset of the brood by one parent or the other (care division). Among gulls and tern (Laridae), males and females generally share parental activities during the pre-fledging period, but the allocation of parental care after fledging is little documented. This study examined the behaviour of male and female roseate terns (Sterna dougallii) during the late chick-rearing and early post-fledging periods, and in particular the amount of feeds and the time spent in attendance given to individual chicks/fledglings. Pre-fledging parental care was biparental in all cases. Post-fledging parental care was dependent on the number of fledglings in the brood. Males and females continued biparental care in clutches with one surviving fledgling, while in two-fledgling clutches, males fed the A-fledgling while females fed the B-fledgling. Overall, there was no difference in attendance, only in feeds. This division of care may be influenced by the male only being certain of the paternity of the A-chick but not by chick sex.     

Effects of loss of male parent on brood survival in a biparental cichlid fish

Synopsis When brood-guarding males of the biparental convict cichlid, Cichlasoma nigrofasciatum, held in experimental ponds, deserted their mate and brood, or were removed from the pond, survival of the young depended on their age when the father left. If they were embryos or free embryos, few broods survived, but if they were up to seven days into the free-swimming juvenile period, most broods survived. The cause of brood mortality is not certain, but predation by conspecifics is most likely (no other fish species were in the ponds). The willingness of a deserted, brood-guarding female to continue defending her young probably depends, in part, on their age. If they are very young, the benefit/cost ratio of guarding them to independence may be so low that she should give up that breeding attempt and begin another. If they are older, the benefit/cost ratio is higher and she should continue to guard them alone.     

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.