Sex-biased hatching sequences in the cooperatively breeding Noisy Miner

The Noisy Miner Manorina melanocephala (Meliphagidae) is a cooperatively breeding bird species in which sons often remain on their natal home ranges and help one or both of their parents. In a population of Noisy Miners in SE Queensland, Australia, a molecular technique was used to explore adult and offspring sex ratios, and also hatching sequences. Among the adult population, there were 2.31 males for every female, and roughly 99% of helping was performed by males. At hatching and fledging, the population sex ratio was even, with exactly 57 males and 57 females. However, in 17 out of 18 broods the first egg to hatch was male. First-hatched males were significantly larger and heavier than their sisters just prior to fledging. Through their helping behaviour, large healthy sons could clearly enhance the future reproductive success of parents, and benefit the entire group. Sex-biased hatching sequences could potentially provide cooperatively breeding birds with a subtle and precise way of varying investment in the helping sex.     

Within territory abundance of red wood ants Formica rufa is associated with the body condition of nestlings in the Eurasian treecreeper Certhia familiaris

Studies on individual reproductive success in relation to interspecific competition between distantly related taxa are scarce. We studied whether the abundance of red wood ants Formica rufa -group is related to the breeding habitat selection, fecundity and offspring quality in the Eurasian treecreeper Certhia familiaris, an old-growth forest passerine. The nest-box occupancy data were gathered over a five-year study period, whereas the breeding performance analyses were based on a two-year data set. The abundance of wood ants, measured within 50  m around the nest-boxes, was not related to nest-box occupancy rate, fecundity or the physiological stress of nestlings. In contrast, the abundance of wood ants was negatively related to the offspring quality and life-history traits, such as lowered body mass, subcutaneous fat reserves, and tarsus length at fledging. Our results suggest that exploitative competition between distantly related taxa may have considerable and adverse influences on nestling quality as measured by body mass, subcutaneous, fat and tarsus length. Red wood ants may decrease the fitness of treecreepers as the lower body condition of nestlings has the potential to impair recruitment into the breeding population and, additionally, impair the future reproductive effort.     

Population sex ratio shift from fledging to recruitment: consequences for demography in a philopatric seabird

In many dimorphic bird species, offspring sex ratio is skewed towards the production of the smaller sex. Offspring sex ratio can be biased in monomorphic birds however, and the demographic consequences of such bias are unknown. Sex-specific mortality and dispersal are fundamental mechanisms of sex ratio adjustment at the population level, but evidence for adjustments is weak and feedback into population dynamics poorly understood. Here, we link sex ratio at fledging with sex-specific subadult return and recruitment at the Banter See common tern Sterna hirundo colony. Using molecular sexing methods and a remote detection system, we permanently tracked individuals from four complete cohorts (n=1171 fledglings) across these life-history stages at their natal colony site, which permitted a structured analysis of sex ratio across multiple seasons. Sex ratio shifted significantly from significant daughter dominance at fledging to higher proportions of natal males among recruits; return and recruitment rates of sons were significantly higher than daughters (p≤0.002). No significant between-year differences were detected. 47.4% of natal male recruits were paired with a non-natal female, but only 37.0% of natal female recruits had a non-natal partner. Elasticity analysis suggested that natal males have a greater influence on natal population growth rate than natal females. Sex biased dispersal is the most probable reason for these results indicating higher emigration to and immigration from other colonies in females, the less territorial and less philopatric sex. This pattern may be related to different gender roles in parental duties and with respect to competition for local resources.     

Developmental stress increases reproductive success in male zebra finches

There is increasing evidence that exposure to stress during development can have sustained effects on animal phenotype and performance across life-history stages. For example, developmental stress has been shown to decrease the quality of sexually selected traits (e.g. bird song), and therefore is thought to decrease reproductive success. However, animals exposed to developmental stress may compensate for poor quality sexually selected traits by pursuing alternative reproductive tactics. Here, we examine the effects of developmental stress on adult male reproductive investment and success in the zebra finch (Taeniopygia guttata). We tested the hypothesis that males exposed to developmental stress sire fewer offspring through extra-pair copulations (EPCs), but invest more in parental care. To test this hypothesis, we fed nestlings corticosterone (CORT; the dominant avian stress hormone) during the nestling period and measured their adult reproductive success using common garden breeding experiments. We found that nestlings reared by CORT-fed fathers received more parental care compared with nestlings reared by control fathers. Consequently, males fed CORT during development reared nestlings in better condition compared with control males. Contrary to the prediction that developmental stress decreases male reproductive success, we found that CORT-fed males also sired more offspring and were less likely to rear non-genetic offspring compared with control males, and thus had greater overall reproductive success. These data are the first to demonstrate that developmental stress can have a positive effect on fitness via changes in reproductive success and provide support for an adaptive role of developmental stress in shaping animal phenotype.     

Sex-bias and timing of natal dispersal in cooperatively breeding Puff-throated Bulbuls Alophoixus pallidus

While natal dispersal can have a significant impact on population dynamics, it is typically difficult to quantify. We investigated timing of natal dispersal of the cooperatively breeding Puff-throated Bulbul Alophoixus pallidus in a tropical evergreen forest by modelling the probability of staying in or dispersing from their natal territory whilst taking into account the effects of sex, group size, and the presence of helper(s). Birds did not disperse until the beginning of and during the breeding season following the hatching year. Dispersal was strongly female-biased both in frequency and distance: most females (95%) dispersed away from their natal territories, and of those relocated, traversed 2–7 territories. In contrast, 50% of males remained in the natal territory as helpers in their second year, while relocated dispersing males crossed 1–2 territories. Natal dispersal was not influenced by either group size or the presence of helpers. Males that fledged earlier in the breeding season exhibited higher rates of philopatry than the males that fledged later, but no correlation between fledging date and philopatry was observed in females. The probability of staying in the natal territory during the second year was 0.58 ± 0.14 SE and 0.05 ± 0.04 for males and females, respectively. These findings may add to our understanding of how natal dispersal can reflect social patterns and kin structure in cooperative breeding species from a little-studied tropical forest region.     

Variation in the magnitude of sexual size dimorphism in nestling Coal Tits (Periparus ater)

The magnitude of sexual size dimorphism can be affected by sex differences in environmental sensitivity early in ontogeny that result in differential growth rates of male and female nestlings. Here, the larger sex might either be more sensitive because of higher food demands or less sensitive due to greater competitive ability. When environmental conditions deteriorate during the breeding season, this “environmental stress” hypothesis predicts differential seasonal declines in the performance of male and female offspring. Based on a sample of molecularly sexed Coal Tit (Periparus ater) nestlings from 2 years, we investigated sexual size dimorphism in body mass, condition (i.e. size-corrected mass), tarsus and wing length and whether its magnitude changed from early to late broods. Male offspring were heavier, larger (in terms of tarsus and wing length) and had higher size-corrected mass than their female nest mates (the same was evident in adult breeders). In 2002 (the year with the longer effective breeding season), body mass and condition declined with progressing hatching date and this effect was significantly more pronounced in male than in female nestlings. There was also a seasonal decline in male wing length, while female wing length remained relatively constant, which resulted in males having shorter wings than females in late broods. Tarsus length was unaffected by time of breeding, except that the difference between males and females was relatively smaller in late (i.e. second) broods in 2002. While these results are in accordance with the idea of an increased environmental sensitivity of the larger males, confounding effects of sex-differential hatching order cannot be ruled out. Dedicated to Doris Winkel.     

Adaptive sex allocation in relation to hatching synchrony and offspring quality in house wrens

Increased variance in the reproductive success of males relative to females favors mothers that optimally allocate sons and daughters to maximize their fitness return. In altricial songbirds, one influence on the fitness prospects of offspring arises through the order in which nestlings hatch from their eggs, which affects individual mass and size before nest leaving. In house wrens (Troglodytes aedon), the influence of hatching order depends on the degree of hatching synchrony, with greater variation in nestling mass and size within broods hatching asynchronously than in those hatching synchronously. Early-hatching nestlings in asynchronous broods were heavier and larger than their later-hatching siblings and nestlings in synchronous broods. The effect of hatching order was also sex specific, as the mass of males in asynchronous broods was more strongly influenced by hatching order than the mass of females, with increased variation in the mass of males relative to that of females. As predicted, mothers hatching their eggs asynchronously biased first-laid, first-hatching eggs toward sons and late-laid, late-hatching eggs toward daughters, whereas females hatching their eggs synchronously distributed the sexes randomly among the eggs of their clutch. We conclude that females allocate the sex of their offspring among the eggs of their clutch in a manner that maximizes their own fitness.     

Sex-related effects of maternal egg investment on offspring in relation to carotenoid availability in the great tit

1. Maternal carotenoids in the egg yolk have been hypothesized to promote maturation of the immune system and protect against free radical damages. Depending on availability, mothers may thus influence offspring quality by depositing variable amounts of carotenoids into the eggs. Sex allocation theory predicts that in good quality environments, females should invest into offspring of the sex that will provide larger fitness return, generally males. 2. In a field experiment we tested whether female great tits bias their investment towards males when carotenoid availability is increased, and whether male offspring of carotenoid-supplemented mothers show higher body condition. We partially cross-fostered hatchlings to disentangle maternal effects from post-hatching effects, and manipulated hen flea Ceratophyllus gallinae infestation to investigate the relationship between carotenoid availability and resistance to ectoparasites. 3. As predicted, we found that carotenoid-supplemented mothers produced males that were heavier than their sisters at hatching, while the reverse was true for control mothers. This suggests that carotenoid availability during egg production affects male and female hatchlings differentially, possibly via a differential allocation to male and female eggs. 4. A main effect of maternal supplementation became visible 14 days after hatching when nestlings hatched from eggs laid by carotenoid-supplemented mothers had gained significantly more mass than control nestlings. Independently of the carotenoid treatment, fleas impaired mass gain of nestlings during the first 9 days in large broods only and reduced tarsus length of male nestlings at an age of 14 days, suggesting a cost to mount a defence against parasites. 5. Overall, our results suggest that pre-laying availability of carotenoids affects nestling condition in a sex-specific way with potentially longer-lasting effects on offspring fitness.     

Egg size and offspring performance in the collared flycatcher (<Emphasis Type="Italic">Ficedula albicollis</Emphasis>): a within-clutch approach

Adaptive within-clutch allocation of resources by laying females is an important focus of evolutionary studies. However, the critical assumption of these studies, namely that within-clutch egg-size deviations affect offspring performance, has been properly tested only rarely. In this study, we investigated effects of within-clutch deviations in egg size on nestling survival, weight, fledgling condition, structural size and offspring recruitment to the breeding population in the collared flycatcher (Ficedula albicollis). Besides egg-size effects, we also followed effects of hatching asynchrony, laying sequence, offspring sex and paternity. There was no influence of egg size on nestling survival, tarsus length, condition or recruitment. Initially significant effect on nestling mass disappeared as nestlings approached fledging. Thus, there seems to be limited potential for a laying female to exploit within-clutch egg-size variation adaptively in the collared flycatcher, which agrees with the majority of earlier studies on other bird species. Instead, we suggest that within-clutch egg-size variation originates from the effects of proximate constraints on laying females. If true, adaptive explanations for within-clutch patterns in egg size should be invoked with caution.     

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.     

Secondary sex ratio covaries with demographic trends and ecological conditions in the barn swallow

Parents are expected to invest more in the sex that benefits most from the local environment. When the quality of breeding sites varies spatially and natal dispersal of males and females differs, parents in high-quality habitats should skew their progeny sex ratio in favor of the less dispersing sex. We tested this prediction in the barn swallow (Hirundo rustica L.), by relating the proportion of male offspring around fledging (secondary sex ratio) of first and second broods to the ecological quality (presence of livestock farming and relative surface of hayfields in the foraging range) and local demographic trends of the farms where the colonies were located. Consistent with our predictions, the proportion of male offspring, which are more philopatric than females, increased with the extent of hayfields, which are high quality, preferred foraging habitats. Moreover, the proportion of male offspring in second broods was smaller in colonies with positive demographic trends, possibly indicating density-dependent effects on sex ratio. Independent of the mechanism generating uneven sex ratio (zygote sex ratio adjustment or sex-related pre-fledging mortality), barn swallows breeding under favorable conditions overproduced the sex that is more likely to benefit from such conditions.     

Seasonal variation in sex ratio and sexual egg dimorphism favouring daughters in first clutches of the spotless starling

We investigated possible pre‐hatching mechanisms of sex‐differential investment by females that may contribute to offspring sex‐ratio adjustment enhancing the fitness return from reproductive effort in the spotless starling (Sturnus unicolor). We found a seasonal shift in sex ratio from daughters to sons as the season advances. Furthermore, the probability of breeding at 1‐year old and recruitment into the breeding population in daughters is associated with laying date but not with mass at fledging. The reverse is true for males which rarely bred at 1‐year old. We also found that eggs containing female embryos are significantly heavier than those containing males in spite of the slight sexual dimorphism in favour of males. This suggests maternal control of provisioning, favouring daughters that may balance sibling mortality and competition with their brothers. Our results on seasonal variation in sex ratio and differential egg provisioning are consistent with an adaptive tactic in which mothers increase their reproductive return by enhancing the probability that daughters survive and breed in their first year of life.     

Discrepancy between factors affecting nestling growth and survival and maternal success in Common Grackles

ABSTRACT Multiple factors potentially affect nestling survival and maternal reproductive success. However, little is known about the relative importance of different factors when operating simultaneously or whether the same factors are important for nestlings and their mothers. We determined the effect of hatching asynchrony, individual egg size, mean egg size, nestling sex, and clutch initiation date on the survival of individual nestlings and on maternal reproductive success in Common Grackles (Quiscalus quiscula) from 2004 to 2006 in central Illinois. Factors most important to maternal success differed from those important for individual nestling growth and survival. Hatching asynchrony had the greatest within‐nest influence on the fate of nestlings; the earlier a nestling hatched relative to siblings, the greater its mass and likelihood of fledging. Clutch size had the greatest influence on maternal reproductive success, with females with larger clutches fledging more young. Thus, both nestling survival and maternal success were largely determined by a single, albeit different, factor. A possible explanation for the apparent unimportance of most factors we measured in determining maternal success is that we did not consider variation among females. Individual variation in maternal attributes such as condition, size, age, experience, or mate quality may result in females tailoring clutch attributes (i.e., egg size, sex, and degree of hatching asynchrony) in ways that allow them to maximize their reproductive success. The discordance between factors that benefited mothers versus their offspring illustrates the importance of considering the maternal consequences of any factor that appears to affect offspring survival. Factors that increase the mass and survival of some offspring may not result in increased maternal reproductive success.     

Nestling erythrocyte resistance to oxidative stress predicts fledging success but not local recruitment in a wild bird

Stressful conditions experienced by individuals during their early development have long-term consequences on various life-history traits such as survival until first reproduction. Oxidative stress has been shown to affect various fitness-related traits and to influence key evolutionary trade-offs but whether an individual''s ability to resist oxidative stress in early life affects its survival has rarely been tested. In the present study, we used four years of data obtained from a free-living great tit population (Parus major; n = 1658 offspring) to test whether pre-fledging resistance to oxidative stress, measured as erythrocyte resistance to oxidative stress and oxidative damage to lipids, predicted fledging success and local recruitment. Fledging success and local recruitment, both major correlates of survival, were primarily influenced by offspring body mass prior to fledging. We found that pre-fledging erythrocyte resistance to oxidative stress predicted fledging success, suggesting that individual resistance to oxidative stress is related to short-term survival. However, local recruitment was not influenced by pre-fledging erythrocyte resistance to oxidative stress or oxidative damage. Our results suggest that an individual ability to resist oxidative stress at the offspring stage predicts short-term survival but does not influence survival later in life.     

Predator-Specific Effects on Incubation Behaviour and Offspring Growth in Great Tits

In birds, different types of predators may target adults or offspring differentially and at different times of the reproductive cycle. Hence they may also differentially influence incubation behaviour and thus embryonic development and offspring phenotype. This is poorly understood, and we therefore performed a study to assess the effects of the presence of either a nest predator or a predator targeting adults and offspring after fledging on female incubation behaviour in great tits (Parus major), and the subsequent effects on offspring morphological traits. We manipulated perceived predation risk during incubation using taxidermic models of two predators: the short-tailed weasel posing a risk to incubating females and nestlings, and the sparrowhawk posing a risk to adults and offspring after fledging. To disentangle treatment effects induced during incubation from potential carry-over effects of parental behaviour after hatching, we cross-fostered whole broods from manipulated nests with broods from unmanipulated nests. Both predator treatments lead to a reduced on- and off-bout frequency, to a slower decline in on-bout temperature as incubation advanced and showed a negative effect on nestling body mass gain. At the current state of knowledge on predator-induced variation in incubation patterns alternative hypotheses are feasible, and the findings of this study will be useful for guiding future research.     

Dietary calcium limits size and growth of nestling tree swallows Tachycineta bicolor in a non-acidified landscape

Much previous research has focussed on the role of food supply in determining the growth and the survival of avian offspring. More recently, acid deposition in some ecosystems has demonstrated that in addition to energy, birds also need to acquire sufficient nutrients such as calcium to be successful. Whether procurement of adequate levels of calcium can limit reproductive success in areas that have not been impacted by acid rain remains equivocal. We tested whether calcium affected reproductive success of tree swallows Tachycineta bicolor by feeding extra calcium to nestlings during the brood-rearing period. Our manipulation did not enhance the survival of offspring, however, provisioning of extra calcium resulted in nestlings showing enhanced rates of growth of mass (all nests) and of ninth primary flight feathers (nests with after-second year female parents), compared to control nestlings. Calcium supplementation also resulted in nestlings having longer feathers and tarsi at age 16 days, and there was evidence that some nestlings receiving extra calcium were heavier at 16 days old. As offspring that have faster growth, or that are in good condition at fledging, often survive better after leaving the nest, these results suggest that calcium availability can limit fitness. Our results are noteworthy because our experiment was conducted in an area with abundant soil calcium where acid deposition has not occurred. The role of calcium in limiting the reproductive performance of avian species may therefore be more pervasive than previously thought.     

Philopatry and natal dispersal in a sedentary population of western marsh harrier

Dispersal is a key life-history trait because it influences population dynamics and population genetic structure. From a behavioural perspective, the study of natal dispersal requires some understanding of the mechanisms that affect individual movements, because movements of an animal form a path that is continuous throughout its life. Our aim was to investigate juvenile dispersal strategies in the western marsh harrier Circus aeruginosus , between fledging and first breeding attempt (from 1 to 4 years later, depending on the sex and individual). Using radio tracking, we monitored dispersing juvenile harriers and their home-range size variations within a sedentary population in central western France from 2001 to 2007. Juvenile dispersal strategy was mainly characterized by a very high natal philopatry (i.e. birds that remained within the study area) and short-distance dispersal. All but one bird (out of 39) remained within the study area between their first winter and their first spring, and 96.0% during their first spring. The distance moved at 2 years of age was significantly larger for males than for females (3800± sd 3593 m and 935± sd 481 m for seven males and six females, respectively), in contrast to most bird species studied so far. Home-range size was not sex biased and significantly decreased with age. In addition, non-breeding birds had larger home ranges (1603± sd 2128 ha) than breeders (349± sd 185 ha). Using data obtained from other populations, juvenile marsh harrier dispersal strategies appeared to be determined by migratory status (migratory birds dispersing farther) and demographic parameters (juvenile survival or fecundity).     

Laying date,body mass and tick infestation of nestling tropical Roseate Terns Sterna dougallii predict fledging success,first‐year survival and age at first return to the natal colony

Both intrinsic and extrinsic factors recorded at individual nests can predict offspring fitness and survival but few studies have examined these effects in the tropics. We recorded nestling survival, post‐fledging survival and age at first return of Roseate Terns breeding at Aride Island, Seychelles, over a 12‐year period (1998–2009). Nest data recorded at the egg, nestling and fledging stages were collected during six breeding seasons (1998, 2001–2005) and a capture‐mark‐recapture dataset of six cohorts of fledglings was obtained from 2001–2009. Logistic regression models were used to assess the predictive effect of reproductive variables on fledging success, while multistate capture‐mark‐recapture models were used to estimate post‐fledging survival and return–recruitment probabilities to the natal site. Nestling survival probability increased with earliness of laying and was negatively affected by tick infestation during the growth period (0–23 days). Fledging probability was also positively related to chick body condition, whereas other pre‐fledging reproductive parameters such as clutch size and egg size were not influential. A multistate modelling of age‐specific survival and return–recruitment (transition) rates found that first‐year survival differed between cohorts and was also negatively affected by tick infestation. Annual survival stabilized from age 2 onwards at 0.83 ± 0.02. Transition rates were positively related to body condition at fledging, with heavier individuals returning for the first time to the natal colony at a younger age compared with lighter individuals. These results highlight the importance of local conditions encountered by tropical seabirds during the breeding season in shaping demographic parameters.     

Breeding success in Blue Tits: good territories or good parents?

Territorial quality and parental quality are usually assumed to be the main sources of variation in the reproductive success of passerine birds. To evaluate their relative importance for variation in breeding time (itself an important factor for breeding success), clutch size and offspring condition at fledging, we analysed six years of data from a Blue Tit Parus caeruleus population breeding on the island of Gotland, Sweden. Hatching dates and the condition of offspring were consistent for territories between years and accounted for 30% and 33% of the variation in these variables, respectively. After removing the effect of territory quality, none of the breeding parameters were significantly repeatable for individual females, but offspring condition was repeatable for males, accounting for 28% of variation. For females breeding on the same territory in subsequent seasons (combined effect of individual and territory quality) only hatching date was repeatable (45% of variation accounted for). In males, the combined effect of individual and territory quality was repeatable for offspring condition and accounted for 33% of variation, but this result was only marginally significant. Consistency of the peak frass-fall date for individual frass collectors over the study period suggests that repeatable hatching dates on territories may be related to the relationship between timing of breeding and timing of peak food availability on territories. Our results suggest that territory quality is more important than parental quality for breeding success in the Blue Tit, and that male (but not female) quality makes a considerable contribution to reproductive success.     

Long‐distance dispersal in migratory pied flycatchers Ficedula hypoleuca is relatively common between the UK and the Netherlands

Long‐distance dispersal can potentially have important consequences for evolutionary change, but is difficult to quantify. We present quantitative estimates of natal dispersal between the UK and the Netherlands in a long‐distance migratory bird, the pied flycatcher. Due to over 90 000 individual ringed adults caught and 730 000 ringed nestlings we are able to estimate that dispersal of young born in the UK to breeding in the Netherlands occurs on average 43 times yr^?1, and probably even more recently. We estimated that between 2000 and 2009 about 0.70% of the Dutch adult population could be immigrants from the UK. No cases of dispersal from the Netherlands to the UK were observed. Dispersing individuals bring new genes to the Netherlands, as males in the UK have a darker plumage than Dutch males. We found a good qualitative match between the proportion of dark males breeding in two Dutch populations and the estimated immigration rate from the UK. This and potentially other genetic differences may allow for new evolutionary directions if these genes are beneficial in the new population. The causes of dispersal are unknown, but it is striking that especially birds from the northern range limit dispersed.