Breeding success and chronology of Wood Storks Mycteria americana in northern and central Florida, U.S.A.

The breeding success and chronology of Wood Storks Mycteria americana were studied at eight colonies in northern and central Florida during 1981–1985. Mean ± s.d. clutch size for all colony-years was 3.07 ± 0.56 (n = 2694 nests), with three-egg clutches (72%) most frequent. Mean clutch size among all colonies and years ranged from 2.73 ± 0.55 to 3.41 ± 0.61. Many colonies exhibited significant negative trends in clutch size with, hatching date because of a proportional decrease in four-egg clutches later in the season. Mean colony clutch size was not correlated with nest numbers, nesting density or mean hatching date within most years. Mean ± s.d. number of fledglings for all colonies and years was 1.29 ± 1.16 fledglings per nest (n = 2812 nests). Mean annual fledging rates in colonies ranged from 0 (colony failed) to 2.66 fledglings per nest. Most breeding failure occurred prior to egg hatching, and the second highest mortality occurred between hatching and 2 weeks of age. Four-egg clutches fledged more storks than three-egg clutches, which in turn were more successful than two-egg clutches. However, all clutch sizes showed similar fledgling per egg rates. The seasonal decline in productivity was associated proportionally with smaller clutch sizes later in the breeding season. An increase in mean hatching date was correlated with an increase in latitude. There was greater within-year breeding synchrony among colonies than interyear breeding synchrony within each colony. Breeding synchrony was not correlated with mean hatching date, latitude, longitude, nest numbers or nesting density.     

Direct and indirect effects of an insect outbreak increase the reproductive output for an avian insectivore and nest‐cavity excavator,the red‐breasted nuthatch Sitta canadensis

Community‐wide food pulses may ameliorate food constraints but may also result in increased competition for other resources and predation rates. In cavity‐nesting vertebrate communities, where the availability of tree cavities can limit reproduction and the reuse of cavities can increase nest predation by squirrels, excavators may maximize their fecundity by creating new cavities in competitor‐ and predator‐rich habitats that undergo food pulses. The reproductive cost associated with excavation (i.e. increased energy allocation early in the breeding season that often delays laying and thereby reduces clutch size), may be reduced if food pulses allow for a longer breeding season and larger clutches. A large‐scale mountain pine beetle Dendroctonus ponderosae outbreak that occurred during our long‐term study (1995–2009) provided a natural food supplementation experiment across 27 sites in British Columbia, Canada. We examined the effects of a reduction in food constraints accompanied with increases in excavation rates, conspecific density and nest predation risk on the fecundity of a facultative excavator, the red‐breasted nuthatch Sitta canadensis. We found a total of 420 nests in tree cavities. Nuthatch clutch sizes ranged from two to nine eggs, and broods from one to nine fledglings per nest. Later clutches were larger at sites and in years with high beetle abundance (mean clutch size of six eggs did not decline later in the season), second broods were produced in outbreak years (usually only one nesting attempt/normal year), and the number of fledglings per successful nest increased with increasing beetle abundance and nuthatch densities, but declined with increased squirrel densities. Since fecundity did not differ between new and reused cavities, the costs and benefits of excavation versus cavity reuse may be neutralized for nuthatches during strong resource pulses. Overall, the beetle outbreak reduced food constraints for nuthatches and provided alternate food for nest predators, allowing increased annual fecundity.     

Age and clutch size variation in Dusky Flycatcher nest survival

Examination of spatial and temporal factors that influence nest survival can provide insight into habitat selection, reproductive decisions (e.g., clutch size), population dynamics, and conservation requirements for species. We used nest survival data for the Dusky Flycatcher Empidonax oberholseri to examine several factors that may influence nesting success. Our prediction was that the number of nest initiations would be positively associated with period nest survival. We used a model selection framework and found that nesting success was a function of clutch size and a cubic effect of age. Clutches with one, two, three, and four eggs had period survival rates of 0, 0.05, 0.33, and 0.49, respectively. Daily survival rates decreased from the onset of egg-laying and increased during the later stages of incubation before remaining relatively constant through the later portions of the nestling stage. Model-selection criterion provided support for a date effect on daily survival (i.e., daily nest survival declined across the nesting season) although the 95% confidence interval for the estimate included zero. We found that the majority of nest initiations occurred early in the nest season and declined across the season as period nest survival declined. Our prediction concerning nest survival was partially supported. In addition, we found substantial positive associations between clutch size and nest survival. While low daily survival rates for clutches with one or two eggs suggested that individuals may have reduced reproductive effort in response to nest predation risk, we did not find strong evidence that individuals reduced their clutch sizes in subsequent nest attempts. Alternative predictions, including the preferential settlement of higher quality individuals (e.g., those with the ability to lay full clutches to replace depredated nests) into high-quality habitat and differences in behavior patterns (e.g., number of visits to provision nestlings), may provide more consistent explanations for these patterns.     

Geographical variation in reproductive ageing patterns and life‐history strategy of a short‐lived passerine bird

We investigated differences in ageing patterns in three measures of breeding performance in populations of barn swallows Hirundo rustica L. from Spain and Denmark differing in breeding latitude and hence migration distance and duration of the breeding season. We found differences in ageing patterns between populations. Generally, young (i.e. yearling) and old females (i.e. ≥ 5 years of age) laid their first eggs later and produced smaller clutches than middle‐aged females (i.e. 2–4 years of age) in both populations. The southernmost population (i.e. Spanish) showing the shorter migratory distance experienced a greater within‐individual increase in timing of breeding and clutch size in early life and a greater within‐individual decrease in laying date but not in clutch size during senescence compared with the northernmost population (i.e. Danish). We also found that the number of fledglings produced annually was related to the age of the two members of the breeding pairs with pairs composed of young and old females performing less well than breeding pairs composed of middle‐aged females. We did not find reproductive senescence for the age of the male while controlling for the age of the female on the number of fledglings produced annually by the breeding pair. Differential survival between individuals did not explain age effects on laying date or annual clutch size in neither population. However, the increase in the number of fledglings produced annually with age was partly explained by the disappearance of poor‐quality members of the pairs, mainly poor‐quality males. Age‐related breeding success (i.e. number of fledglings) was similar for barn swallows from Spain and Denmark. Therefore, the study of ageing patterns and life‐history strategies in free‐ranging animals from more than a single population can throw new light on life‐history theory, population dynamics and evolutionary studies of senescence.     

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

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

A comparative study of insect abundance and reproductive success of barn swallows Hirundo rustica in two urban habitats

Few studies have quantified the relative reproductive success of passerines in urban habitats. I studied food availability and reproductive success of barn swallows Hirundo rustica in two urban habitats during 2012–2015. Barn swallows breeding in the town center experienced lower insect densities than those in the town periphery. Lower food availability resulted in reduced feeding rates per capita, lower nestling body mass, longer nestling periods, longer inter‐clutch intervals, fewer first and second brood fledglings and a lower total number of fledglings produced during the breeding season in comparison to barn swallows breeding in the town periphery. I hypothesize that the lower intra‐specific competition for nest sites and fitness advantages linked to the solitary breeding in urban habitats balanced the apparent costs of reproduction in more urbanized habitats.     

Delayed egg‐laying and shortened incubation duration of Arctic‐breeding shorebirds coincide with climate cooling

Biological impacts of climate change are exemplified by shifts in phenology. As the timing of breeding advances, the within‐season relationships between timing of breeding and reproductive traits may change and cause long‐term changes in the population mean value of reproductive traits. We investigated long‐term changes in the timing of breeding and within‐season patterns of clutch size, egg volume, incubation duration, and daily nest survival of three shorebird species between two decades. Based on previously known within‐season patterns and assuming a warming trend, we hypothesized that the timing of clutch initiation would advance between decades and would be coupled with increases in mean clutch size, egg volume, and daily nest survival rate. We monitored 1,378 nests of western sandpipers, semipalmated sandpipers, and red‐necked phalaropes at a subarctic site during 1993–1996 and 2010–2014. Sandpipers have biparental incubation, whereas phalaropes have uniparental incubation. We found an unexpected long‐term cooling trend during the early part of the breeding season. Three species delayed clutch initiation by 5 days in the 2010s relative to the 1990s. Clutch size and daily nest survival showed strong within‐season declines in sandpipers, but not in phalaropes. Egg volume showed strong within‐season declines in one species of sandpiper, but increased in phalaropes. Despite the within‐season patterns in traits and shifts in phenology, clutch size, egg volume, and daily nest survival were similar between decades. In contrast, incubation duration did not show within‐season variation, but decreased by 2 days in sandpipers and increased by 2 days in phalaropes. Shorebirds demonstrated variable breeding phenology and incubation duration in relation to climate cooling, but little change in nonphenological components of traits. Our results indicate that the breeding phenology of shorebirds is closely associated with the temperature conditions on breeding ground, the effects of which can vary among reproductive traits and among sympatric species.     

Increased survival and breeding performance of double breeders in little penguins Eudyptula minor,New Zealand: evidence for individual bird quality?

The little penguin Eudyptula minor is unique among penguin species in being able to fledge chicks from two clutches in one breeding season. Pairs laying two clutches in a given season make a higher reproductive investment, and may be rewarded by a higher reproductive success as they may raise twice as many chicks as pairs laying one clutch. The higher effort made by pairs laying two clutches could correlate negatively with survival, future reproductive performance or offspring survival, indicating a cost of reproduction. Conversely, a positive relationship between the number of clutches produced in a given breeding season and survival, future reproductive performance or offspring survival would indicate that birds laying two clutches belonged to a category of birds with higher fitness, compared to birds laying only one clutch in the season. In this study we used a long‐term data set taken from an increasing population of little penguins in Otago, SE New Zealand. We modelled the relationship between the number of clutches laid in a breeding season and survival probability, reproductive performance in the next breeding season and first year survival of offspring using capture‐recapture modelling.
Birds laying two clutches produced 1.7 times more fledglings during a breeding season than pairs laying one clutch. We found that birds laying two clutches had a higher probability of breeding in the following breeding season, a higher probability of laying two clutches in the following breeding season and a higher survival probability. There was no overall difference in post‐fledging survival between the young of birds producing one clutch and the young of birds producing two clutches. However, the survival of young of single clutch breeders declined with laying date, whereas the young of double clutch breeders had the same survival rate irrespective of laying date. For a subset of data with birds of known age, we found evidence that the probability of laying two clutches increased with age. However, there were also indications for differences among birds in the tendency to lay two clutches that could not be attributed to age. We tentatively interpret our results as evidence of quality difference among little penguin breeders.     

Sprague's pipit breeding biology and reproductive success in planted and native grasslands

Much of the native grasslands in agricultural regions have been converted to cropland or tilled and seeded with non‐native grasses for livestock production. Several grassland songbird species occupy planted grasslands, but occupancy or density may not be a reliable indicator of habitat quality. I studied the breeding biology of Sprague's pipit Anthus spragueii from 2004 to 2008 in Saskatchewan, Canada. My objective was to determine the extent to which the breeding biology, density and reproductive success of pipits varied in planted and native grasslands. Peak clutch initiation occurred in mid‐ to late‐May in planted and native grassland. Peak pipit density also occurred in May, but density drastically declined over the breeding season in planted grassland. Clutch size varied among years and declined over the breeding season, but was similar in planted (4.7  0.1 SE) and native grasslands (4.5  0.1 SE). Daily nest survival rates varied with age of the nest and date, but the relationships differed in the two habitats and was likely a result of lower nestling survival in planted grassland compared to native grassland. The number of young fledged per nest increased as the season progressed and tended to be greater in native (1.2  0.1 SE) than planted (0.9  0.2 SE) grasslands. Seasonal productivity was much greater in native grassland. Only three nests were initiated after May in planted grassland and all were unsuccessful, whereas pipit young fledged at higher rates from nests initiated in native grassland in June and July than planted grassland nests initiated in May. The number of fledged young from successful nests did not vary strongly with habitat, date or year. This research indicates that planted grasslands attract pipits at the beginning of the breeding season, but habitat suitability and reproductive success substantially declines as the breeding season progresses compared to that found in native grassland.     

The effects of rainfall on different components of seasonal fecundity in a tropical forest passerine

Seasonal fecundity is a composite metric that is determined by component parameters such as clutch size, nest survival and re‐nesting probability. Many of these component parameters are known to vary with environmental conditions, in particular rainfall prior to or during the breeding season. In some species, seasonal fecundity is positively related to rainfall, but little is known about which component parameters of seasonal fecundity respond most strongly to rainfall. We used intensive nest monitoring of a multi‐brooded tropical forest passerine, the Montserrat Oriole Icterus oberi, to examine the effects of rainfall during the pre‐breeding season on component parameters of annual fecundity. We monitored all nests of a total of 42 pairs over 5 years in which rainfall varied substantially. We then related clutch size, nest survival, onset and length of the breeding season, re‐nesting probability and re‐nesting interval to pre‐breeding season rainfall using generalized linear mixed models that accounted for random variation across sites and individual pairs, and incorporated other variables known to affect the response. Higher pre‐breeding season rainfall led to an increase in clutch size and a decrease in re‐nesting interval, but nest survival, re‐nesting probability and length of the breeding season were not affected by variation in rainfall. The onset of the breeding season was delayed in very dry years. We conclude that higher rainfall is likely to increase food availability and thus body condition of female Montserrat Orioles, leading to an increase in fecundity due to larger clutch sizes.     

Elevational gradient in clutch size of Red‐faced Warblers

Our understanding of life history evolution has benefited from debates regarding the underlying causes, and geographic ubiquity, of spatial patterns in avian clutch sizes. Past studies have revealed that birds lay smaller clutch sizes at higher elevation. However, in most previous studies, investigators have failed to adequately control for elevational differences in breeding phenology. To better understand the elevational gradient in avian clutch size, we need to know how clutch size changes across the entire elevational breeding range of a species (i.e., the shape of the relationship between elevation and clutch size), and whether the elevational gradient in clutch size is merely an artifact of elevational gradients in breeding phenology or breeding season length. We examined the relationship between breeding elevation and clutch size of Red‐faced Warblers (Cardellina rubrifrons) along a 1000‐m elevational gradient in Arizona. Our objectives were to determine how clutch size changed with elevation, and if the relationship between clutch size and elevation merely reflected elevational changes in breeding season length or phenology. The proportion of 5‐egg clutches decreased and the proportion of 3‐ and 4‐egg clutches increased non‐linearly with increasing elevation, even after controlling for the elevational gradient in nest initiation date. Thus, average clutch size declined across the elevational breeding range of Red‐faced Warblers, but this decline was not due to elevational variation in breeding phenology. Timing of breeding changed, but the duration of the breeding season did not change appreciably across the elevational gradient. Hence, elevational differences in breeding season length or breeding phenology cannot explain why Red‐faced Warblers (and perhaps other birds) breeding at higher elevations have smaller clutches.     

Habitat Selection and Reproductive Success of Lewis's Woodpecker (Melanerpes lewis) at Its Northern Limit

Lewis''s Woodpecker (Melanerpes lewis) has experienced population declines in both Canada and the United States and in 2010 was assigned a national listing of threatened in Canada. We conducted a two-year study (2004–2005) of this species at its northern range limit, the South Okanagan Valley in British Columbia, Canada. Our main objective was to determine whether the habitat features that influenced nest-site selection also predicted nest success, or whether other factors (e.g. cavity dimensions, clutch initiation date or time of season) were more important. Nest tree decay class, density of suitable cavities and total basal area of large trees were the best predictors of nest-site selection, but these factors were unrelated to nesting success. Estimates of demographic parameters (mean ± SE) included daily nest survival rate (0.988±0.003, years combined), nest success (0.52±0.08), clutch size (5.00±0.14 eggs), female fledglings per successful nest (1.31±0.11), and annual productivity (0.68±0.12 female fledglings per nest per year). Although higher nest survival was associated with both early and late initiated clutches, early-initiated clutches allowed birds to gain the highest annual productivity as early clutches were larger. Nests in deep cavities with small entrances experienced lower predation risk especially during the peak period of nest predation. We concluded that nest-site selection can be predicted by a number of easily measured habitat variables, whereas nest success depended on complicated ecological interactions among nest predators, breeding behaviors, and cavity features. Thus, habitat-based conservation strategies should also consider ecological factors that may not be well predicted by habitat.     

A more informative method for analyzing reproductive success

ABSTRACT.   Few investigators have examined how a female's prior history (i.e., number of clutches laid previously and whether those nesting attempts were successful or not) might influence the success of a particular clutch. Our objective was to determine the seasonal change in the mean number of fledglings successfully leaving nests in a population of Prairie Warblers ( Dendroica discolor ). To do so, we calculated the success of each clutch in the laying sequence (i.e., first, second, third, …, n ^th) and sorted them further by whether they were laid before (first-brood clutches) or after (second-brood clutches) the first brood fledged. In our sample population of 70 females we knew the fate of all clutches laid during a breeding season. Although the number of fledglings per successful clutch varied slightly, the probability that a clutch would be successful (i.e., produce at least one fledgling) and the number of fledglings per nest attempt increased during the breeding season. Thus, later clutches were more productive than earlier clutches. The low probability of producing a successful first clutch early in the breeding season would seem to favor selection for females that wait until later in the season to begin breeding. However, the seasonal reproductive success of females may be more dependent on breeding early (increasing the number of clutches laid in a season) than on the higher probability of success with later clutches. Our results indicate that further study of seasonal changes in reproductive success and their causes is needed.     

Breeding season weather determines long‐tailed tit reproductive success through impacts on recruitment

Productivity is a key demographic trait that can be influenced by climate change, but there are substantial gaps in our understanding of the impact of weather on productivity and recruitment in birds. Weather is known to influence reproductive success in numerous species, although such effects have not been reported in all studies, perhaps because they are masked by high nest predation rates or buffered by density dependence. Here, we use a 19‐yr study of a population of individually marked long‐tailed tits Aegithalos caudatus to quantify the impacts of weather on productivity in the nest (from eggs to fledging) and subsequent recruitment, while taking nest predation rates and density dependence into account. We find that weather has negligible effects on clutch size, hatching success, brood size, probability of fledging and number of fledglings. Annual variation in nest predation rates is a strong predictor of productivity, but we find no evidence that the magnitude of nest predation is determined by weather. Recruitment was strongly associated with breeding season weather, even when taking density dependence effects into account. This contrasts with the conventional view that first year survival of temperate passerines is primarily determined by winter weather. Recruitment was reduced when March temperatures were high, perhaps caused by earlier peaks in caterpillar abundance and thus reduced food availability at the time of fledging. Recruitment increased following high May temperatures, perhaps due to an improved thermo‐regulatory environment for young fledglings. These opposing effects of warm March and May temperatures highlight the importance of considering asymmetrical rates of warming in different months when predicting climate change impacts.     

Seasonal patterns in reproductive success of temperate‐breeding birds: Experimental tests of the date and quality hypotheses

For organisms in seasonal environments, individuals that breed earlier in the season regularly attain higher fitness than their late‐breeding counterparts. Two primary hypotheses have been proposed to explain these patterns: The quality hypothesis contends that early breeders are of better phenotypic quality or breed on higher quality territories, whereas the date hypothesis predicts that seasonally declining reproductive success is a response to a seasonal deterioration in environmental quality. In birds, food availability is thought to drive deteriorating environmental conditions, but few experimental studies have demonstrated its importance while also controlling for parental quality. We tested predictions of the date hypothesis in tree swallows (Tachycineta bicolor) over two breeding seasons and in two locations within their breeding range in Canada. Nests were paired by clutch initiation date to control for parental quality, and we delayed the hatching date of one nest within each pair. Subsequently, brood sizes were manipulated to mimic changes in per capita food abundance, and we examined the effects of manipulations, as well as indices of environmental and parental quality, on nestling quality, fledging success, and return rates. Reduced reproductive success of late‐breeding individuals was causally related to a seasonal decline in environmental quality. Declining insect biomass and enlarged brood sizes resulted in nestlings that were lighter, in poorer body condition, structurally smaller, had shorter and slower growing flight feathers and were less likely to survive to fledge. Our results provide evidence for the importance of food resources in mediating seasonal declines in offspring quality and survival.     

Annual and seasonal reproductive trends in the Lesser Spotted Woodpecker Dendrocopos minor

Annual and seasonal variation in reproductive timing and performance were studied in a population of the Lesser Spotted Woodpecker Dendrocopos minor over 10 years in southern Sweden. The median laying date of the first egg varied by up to 17 days between years, being generally larger than the variation of laying dates within years. Neither clutch size, brood size in successful nests, fledging success in successful nests nor mean nestling weight differed significantly between years. There was no trend for mean clutch size to vary between early and late years. In spite of a more than threefold variation in population size, no reproductive variable demonstrated an apparent density-dependence. Within the season, clutch size declined steeply with increasing clutch initiation date, whereas fledging success and nesting success did not, leading to a trend in brood size almost identical to the trend in clutch size. The survival prospects of fledged young declined with increasing clutch initiation date, and it is argued that the clutch size laid is a strategic adjustment to laying date. Out of 124 breeding attempts, 34% did not produce fledged young. In 9% of the breeding attempts, pairs laid no eggs. At least 20% of the breeding attempts failed after egg-laying. The most common cause of breeding failure was loss of the breeding partner followed by nest abandonment (40% of the failures). Only 16–28% of the failures were due to predation on the nest. Most complete failures, and also partial losses from nests, occurred at the early breeding stages. It is argued that the early nestling phase may be a critical stage, which the woodpeckers adjust to coincide with the seasonal food peak, explaining the strikingly late breeding season compared with other non-migrant species.     

Delayed timing of breeding as a cost of reproduction

Timing of breeding is a trait with considerable individual variation, often closely linked to fitness because of seasonal declines in reproduction. The drivers of this variation have received much attention, but how reproductive costs may influence the timing of subsequent breeding has been largely unexplored. We examined a population of northern wheatears Oenanthe oenanthe to compare three groups of individuals that differed in their timing of breeding termination and reproductive effort to investigate how these factors may carry over to influence reproductive timing and reproductive output in the following season. Compared to females that bred successfully, females that put in less effort and terminated breeding early due to nest failure tended to arrive and breed earlier in year 2 (mean advancement = 2.2 and 3.3 d respectively). Females that spent potentially more effort and terminated breeding later due to production of a replacement clutch after nest failure, arrived later than other females in year 2. Reproductive output (number of fledglings) in year 2 differed between the three groups as a result of group‐level differences in the timing of breeding in combination with the general seasonal decline in reproductive output. Our study shows that the main cost of reproduction was apparent in the timing of arrival and breeding in this migratory species. Hence, reproductive costs can arise through altered timing of breeding since future reproductive success (including adult survival) is often dependent on the timing of breeding in seasonal systems.     

High nest failure but better nestling quality for early breeders in an alpine population of Northern Wheatear (Oenanthe oenanthe)

Climate change is leading to the advancement of spring conditions, resulting in an earlier snowmelt and green-up, with highest rates of change in highly seasonal environments, including alpine habitats. Migratory birds breeding at high elevations need to time their arrival and lay dates accurately with this advancement, but also with the annually variable spring conditions at their breeding sites, to maximize nest survival probabilities and reproductive output. Nest survival probability and mean nestling mass were analysed in relation to lay date and habitat conditions in an alpine population of the migratory Northern Wheatear Oenanthe oenanthe collected over six consecutive breeding seasons in the Western Italian Alps. This open grassland species showed the lowest nest survival probability in years with an early onset of spring conditions. Within-season, nest survival was highest when breeding late, at lower elevations, and when grass cover and grass height were higher. Both across- and within-season, severe weather conditions may indirectly lead to higher early season nest failure rates by increasing predation risk. By contrast, mean nestling mass, and thus the quality of the fledglings, was lower when breeding late. This might be driven by a mismatch with the peak in food abundance. Breeding early is thus generally advantageous in terms of chick quality in our high-elevation population, but reproductive success is limited by the risk of nest failure that is higher in early springs and early in the season. This trade-off between breeding early and late may thus allow Northern Wheatears to maximize fitness under highly variable spring conditions. However, climate change may cause disruption to this trade-off, and shifts in phenology could become a threat for migratory alpine birds that might not be able to keep track of advancing spring conditions.     

Group composition and reproductive success of cooperatively breeding white‐winged choughs (Corcorax melanorhamphos) in urban and non‐urban habitat

Abstract: White‐winged choughs (Corcorax melanorhamphos, Corcoracidae) are a common, breeding resident in and around the city of Canberra in the Australian Capital Territory. We compared five measures of reproductive success between the urban and non‐urban populations of choughs to investigate the effect of urbanization on this cooperatively breeding species. Urban choughs initiated breeding earlier than their non‐urban counterparts and were more likely to suffer nest failures. However, there was no difference in the number of successful nests in a season or the number of fledglings produced per successful nesting attempt. A greater proportion of fledglings survived their first 12 months in the non‐urban habitat. We suggest that increased rates of nest predation and fledgling mortality in the urban environment may have a negative effect on reproductive success and remove any advantage that might be gained through a longer breeding season. Possible effects of urbanization on the social and genetic structure of white‐winged choughs are also discussed.     

Why do earlier‐arriving migratory birds have better breeding success?

In migratory birds, early arrival on breeding sites is typically associated with greater breeding success, but the mechanisms driving these benefits are rarely known. One mechanism through which greater breeding success among early arrivers can potentially be achieved is the increased time available for replacement clutches following nest loss. However, the contribution of replacement clutches to breeding success will depend on seasonal variation in nest survival rates, and the consequences for juvenile recruitment of hatching at different times in the season. In particular, lower recruitment rates of late‐hatched chicks could offset the benefits to early arrivers of being able to lay replacement clutches, which would reduce the likelihood of replacement clutch opportunities influencing selection on migratory timings. Using a simulation model of time‐constrained capacity for replacement clutches, paramaterized with empirically‐derived estimates from avian migratory systems, we show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers, even when later renesting attempts contribute fewer recruits to the population. However, these relationships vary depending on the seasonal pattern of nest survival. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important but overlooked source of selection on migratory timings. Empirical measures of seasonal variation in nest survival, renesting, and juvenile recruitment rates are therefore needed in order to identify the costs and benefits associated with individual migration phenology, the selection pressures influencing migratory timings, and the implications for ongoing shifts in migration and breeding phenology.