Social carry‐over effects underpin trans‐seasonally linked structure in a wild bird population

Spatial structure underpins numerous population processes by determining the environment individuals' experience and which other individuals they encounter. Yet, how the social landscape influences individuals' spatial decisions remains largely unexplored. Wild great tits (Parus major) form freely moving winter flocks, but choose a single location to establish a breeding territory over the spring. We demonstrate that individuals' winter social associations carry‐over into their subsequent spatial decisions, as individuals breed nearer to those they were most associated with during winter. Further, they also form territory boundaries with their closest winter associates, irrespective of breeding distance. These findings were consistent across years, and among all demographic classes, suggesting that such social carry‐over effects may be general. Thus, prior social structure can shape the spatial proximity, and fine‐scale arrangement, of breeding individuals. In this way, social networks can influence a wide range of processes linked to individuals' breeding locations, including other social interactions themselves.     

Form,function and consequences of density dependence in a long‐distance migratory bird

The density dependence of demographic parameters and its implications for population regulation have long been recognized. Recent work has revealed potential effects of density on mating systems and sexual selection, but few studies concurrently assess the consequences of density on both demography and sexual selection. Such an approach is important because population processes and individual behaviors can interact to influence population growth and evolutionary trajectories. In this study, we tested the density dependence of breeding success, extra‐pair paternity, and the opportunity for sexual selection in a population of American redstarts Setophaga ruticilla using two different measures of density. To evaluate temporal patterns, we analyzed annual territory density, based on the total number of territories at our study site each year. To evaluate spatial patterns, we analyzed local territory density within years, based on the number of territories surrounding a focal territory. Greater annual density was associated with fewer offspring fledged per female, a reduced mean population rate of fledging success, and a lower relative contribution of extra‐pair paternity to male fitness. Greater local density was associated with fewer offspring fledged, reduced fledgling success, higher rates of nest loss, and higher rates of paternity loss on focal territories. Interestingly, greater local density was also associated with greater nestling mass on focal territories, which could imply that more densely‐packed territories contain superior resources. Overall, our results suggest that the effects of crowding via greater territory density reduce fecundity through increased nest predation, rather than reduced food availability, and increase rates of extra‐pair paternity. Thus, the selective pressures faced by individuals and their reproductive behaviors are likely to differ based on the annual and local density they experience, which may then feed back into population demography.     

Seasonal mortality and sequential density dependence in a migratory bird

Migratory bird populations may be limited during one or more seasons, and thus at one or more places, but there is a dearth of empirical examples of this possibility. We analyse seasonal survival in a migratory shellfish‐eating shorebird (red knot Calidris canutus islandica) during a series of years of intense food limitation on the nonbreeding grounds (due to overfishing of shellfish stocks), followed by a relaxation period when destructive harvesting had stopped and food stocks for red knots recovered. For the estimation of seasonal survival from the 15 yr‐long near‐continuous capture–resight dataset, we introduce a ‘rolling window’ approach for data exploration, followed by selection of the best season definition. The average annual apparent survival over all the years was 0.81 yr^?1. During the limitation period, survival probability of adult red knots was low in winter (0.78 yr^?1), but this was compensated by high survival in summer (0.91 yr^?1). During the relaxation period survival rate levelled out with a winter value of 0.81 yr^?1 and a summer survival of 0.82 yr^?1. The fact that during the cockle‐dredging period the dip in survival in winter was completely compensated by higher survival later in the annual cycle suggests sequential density dependence. We conclude that seasonal compensation in local survival (in concert with movements to areas apparently below carrying capacity) allowed the islandica population as a whole to cope, in 1998–2003, with the loss of half of the suitable feeding habitat in part of the nonbreeding range, the western Dutch Wadden Sea. As a more general point, we see no reason why inter‐seasonal density dependence should not be ubiquitous in wildlife populations, though its limits and magnitude will depend on the specific ecological contexts. We elaborate the possibility that with time, and in stable environments, seasonal mortality evolves so that differences in mortality rates between seasons would become erased.     

Evidence of negative seasonal carry‐over effects of breeding ground mercury exposure on survival of migratory songbirds

Mercury (Hg) is a well‐known global contaminant that persists in the environment. The organic form, methylmercury (MeHg) has been shown to adversely affect bird immune function, foraging behavior, navigation, and flight ability, which individually or together could reduce migration performance, and ultimately survival. Nestlings grow feathers at their natal site, and in North America many adult passerines undergo a complete feather molt prior to autumn migration at or near their breeding location. Body Hg is redistributed into growing feathers, and remains stable following feather growth. As flight feathers are retained in most species over the non‐breeding season until molt in the following summer, tail feathers can be used at other times and places as indicators of Hg body burden on the breeding grounds. In five migratory passerine species, we compared Hg concentrations in tail feathers that were grown prior to autumn migration and retained until the following spring. We predicted that we would observe a shift in the distribution of species‐specific feather Hg values towards lower means in the spring if Hg reduced survival over the migration and winter periods. We found reductions in mean feather Hg between autumn and spring in two long‐distance migratory insectivores (blackpoll warbler Setophaga striata; American redstart Setophaga ruticilla). Most significantly, spring‐returning blackpoll warblers, a species that undertakes long non‐stop flights to South America during autumn migration, had nearly 50 percent lower Hg concentrations than those that departed in the autumn. Our finding suggests that Hg exposure on the breeding areas could have a carry‐over effect to influence migration success and survival of insectivorous songbirds that undergo extensive and demanding migratory journeys. More investigation is needed to fully understand the relationships among Hg exposure, migration performance, and survival of songbirds.     

Comparing the seasonal survival of resident and migratory oystercatchers: carry‐over effects of habitat quality and weather conditions

Events happening in one season can affect life‐history traits at (the) subsequent season(s) by carry‐over effects. Wintering conditions are known to affect breeding success, but few studies have investigated carry‐over effects on survival. The Eurasian oystercatcher Haematopus ostralegus is a coastal wader with sedentary populations at temperate sites and migratory populations in northern breeding grounds of Europe. We pooled continental European ringing‐recovery datasets from 1975 to 2000 to estimate winter and summer survival rates of migrant and resident populations and to investigate long‐term effects of winter habitat changes. During mild climatic periods, adults of both migratory and resident populations exhibited survival rates 2% lower in summer than in winter. Severe winters reduced survival rates (down to 25% reduction) and were often followed by a decline in survival during the following summer, via short‐term carry‐over effects. Habitat changes in the Dutch wintering grounds caused a reduction in food stocks, leading to reduced survival rates, particularly in young birds. Therefore, wintering habitat changes resulted in long‐term (>10 years) 8.7 and 9.4% decrease in adult annual survival of migrant and resident populations respectively. Studying the impact of carry‐over effects is crucial for understanding the life history of migratory birds and the development of conservation measures.     

Non‐trophic plant–animal interactions mediate positive density dependence among conspecific saplings

Trophic plant–animal interactions (e.g. browsing by ungulates, insect attack) are an important and well‐studied source of mortality in many tree populations. Non‐trophic tree–animal interactions (e.g. deer antler rubbing) also frequently lead to tree death, and thus have significant effects on forest ecosystem functioning, but they are much less well studied than trophic interactions are. As deer populations have increased in recent decades in the Northern Hemisphere, their impact on tree populations via browsing and antler rubbing will increase. The aim of the study was to illustrate the potential ability of non‐trophic plant–animal interactions to regulate the dynamics of a natural forest. Specifically, we wanted to determine whether and how density and distance‐dependent processes affect sapling mortality caused by an antler rubbing by red deer Cervus elaphus. We used a spatially explicit approach to examine density and distance‐dependent mortality effects in almost two thousand Picea abies saplings over 20 years, based on a fully mapped permanent 14.4 ha plot in a natural subalpine old‐growth spruce forest. Antler rubbing by deer was the main identified cause of sapling mortality, and it showed a strong spatial pattern: positive density dependence of survival among spruce saplings. Deer selectively killed spruce saplings that were isolated from conspecifics. In consequence, non‐trophic plant–deer interactions were a major driver of the spatial pattern of P. abies sapling survival. The other mortality causes (e.g. breaking, overturning) did not show density‐dependent patterns or their effects were much weaker. In the medium and long term, the density‐dependent pattern of sapling mortality due to antler rubbing can alter the tree stand structure. Our results highlight the ecological relevance of non‐trophic plant–animal interactions for forest ecosystem functioning.     

Climate alters the movement ecology of a non‐migratory bird

Global climate change is causing increased climate extremes threatening biodiversity and altering ecosystems. Climate is comprised of many variables including air temperature, barometric pressure, solar radiation, wind, relative humidity, and precipitation that interact with each other. As movement connects various aspects of an animal''s life, understanding how climate influences movement at a fine‐temporal scale will be critical to the long‐term conservation of species impacted by climate change. The sedentary nature of non‐migratory species could increase some species risk of extirpation caused by climate change. We used Northern Bobwhite (Colinus virginianus; hereafter bobwhite) as a model to better understand the relationship between climate and the movement ecology of a non‐migratory species at a fine‐temporal scale. We collected movement data on bobwhite from across western Oklahoma during 2019–2020 and paired these data with meteorological data. We analyzed movement in three different ways (probability of movement, hourly distance moved, and sinuosity) using two calculated movement metrics: hourly movement (displacement between two consecutive fixes an hour apart) and sinuosity (a form of tortuosity that determines the amount of curvature of a random search path). We used generalized linear‐mixed models to analyze probability of movement and hourly distance moved, and used linear‐mixed models to analyze sinuosity. The interaction between air temperature and solar radiation affected probability of movement and hourly distance moved. Bobwhite movement increased as air temperature increased beyond 10°C during low solar radiation. During medium and high solar radiation, bobwhite moved farther as air temperature increased until 25–30°C when hourly distance moved plateaued. Bobwhite sinuosity increased as solar radiation increased. Our results show that specific climate variables alter the fine‐scale movement of a non‐migratory species. Understanding the link between climate and movement is important to determining how climate change may impact a species’ space use and fitness now and in the future.     

Linking warming effects on phenology,demography, and range expansion in a migratory bird population

Phenological changes in response to climate change have been recorded in many taxa, but the population‐level consequences of these changes are largely unknown. If phenological change influences demography, it may underpin the changes in range size and distribution that have been associated with climate change in many species. Over the last century, Icelandic black‐tailed godwits (Limosa limosa islandica) have increased 10‐fold in numbers, and their breeding range has expanded throughout lowland Iceland, but the environmental and demographic drivers of this expansion remain unknown. Here, we explore the potential for climate‐driven shifts in phenology to influence demography and range expansion. In warmer springs, Icelandic black‐tailed godwits lay their clutches earlier, resulting in advances in hatching dates in those years. Early hatching is beneficial as population‐wide tracking of marked individuals shows that chick recruitment to the adult population is greater for early hatched individuals. Throughout the last century, this population has expanded into progressively colder breeding areas in which hatch dates are later, but temperatures have increased throughout Iceland since the 1960s. Using these established relationships between temperature, hatching dates and recruitment, we show that these warming trends have the potential to have fueled substantial increases in recruitment throughout Iceland, and thus to have contributed to local population growth and expansion across the breeding range. The demographic consequences of temperature‐mediated phenological changes, such as the advances in lay dates and increased recruitment associated with early hatching reported here, may therefore be key processes in driving population size and range changes in response to climate change.     

Dispersal polymorphisms from natal phenotype–environment interactions have carry‐over effects on lifetime reproductive success of a tropical parrot

We examined how interactions between an individual's phenotype and its environment affect natal dispersal at multiple scales and the effects on lifetime reproductive success using a 22‐year study of green‐rumped parrotlets (Forpus passerinus). Dispersal increased or decreased lifetime reproductive success depending upon an individual's natal environment and phenotype. Many of the phenotypic traits and environmental conditions that influenced lifetime reproductive success also influenced dispersal, such as clutch size and competition, and this differed with scale. By examining phenotype–environment interactions, we observed both positive and negative effects of rainfall, habitat quality and competition on dispersal depending upon phenotype. The dispersal behaviours of juveniles typically resulted in higher lifetime reproductive success. Thus, individuals commonly exhibit ideal free behaviour and results provide support for the occurrence and maintenance of dispersal polymorphisms. This study highlights the long‐term, carry‐over effects of natal environment, natal phenotype and dispersal tactic on lifetime reproductive success.     

Carry‐over effects provide linkages across the annual cycle of a Neotropical migratory bird,the Louisiana Waterthrush Parkesia motacilla

Population limitation models of migratory birds have sought to include impacts from events across the full annual cycle. Previous work has shown that events occurring in winter result in some individuals transitioning to the breeding grounds earlier or in better physical condition than others, thereby affecting reproductive success (carry‐over effects). However, evidence for carry‐over effects from breeding to wintering grounds has been shown less often. We used feather corticosterone (CORT_f) levels of the migratory Louisiana Waterthrush Parkesia motacilla as a measure of the physiological state of birds at the time of moult on the breeding territory to investigate whether carry‐over effects provide linkages across the annual cycle of this stream‐obligate bird. We show that birds arriving on wintering grounds with lower CORT_f scores, indicating reduced energetic challenges or stressors at the time of moult, occupied higher quality territories, and that these birds then achieved a better body condition during the overwinter period. Body condition, in turn, was important in determining whether adult birds returned the following winter, with birds in better condition returning at higher rates. Together these data suggest a carry‐over effect from the breeding grounds to the wintering grounds that is further extended with respect to annual return rates. Very few other studies have linked conditions during the previous breeding season with latent effects during the subsequent overwintering period or with annual survival. This study shows that the effects of variation in energetic challenges or stressors can potentially carry over from the natal stream and accumulate over more than one life‐history period before being manifested in reduced survival. This is of particular relevance to models of population limitation in migratory birds.     

Apparent survival of an Arctic‐breeding migratory bird over 44 years of fluctuating population size

Following increases in numbers during the second half of the 20th century, several Arctic‐breeding migrant bird species are now undergoing sustained population declines. These include the northwest European population of Bewick's Swan Cygnus columbianus bewickii, which declined from c. 29 000 birds on the wintering grounds in 1995 to 18 000 in 2010. It is unclear whether this decrease reflects reduced survival, emigration to a different area, or a combination of both. Furthermore, the environmental drivers of any demographic changes are also unknown. We therefore used an information‐theoretic approach in RMark to analyse a dataset of 3929 individually marked and resighted Bewick's Swans to assess temporal trends and drivers of survival between the winters of 1970/71 and 2014/2015, while accounting for effects of age, sex and different marker types. The temporal trend in apparent survival rates over our study period was best explained by different survival rates for each decade, with geometric mean survival rates highest in the 1980s (leg‐ring marked birds = 0.853, 95% confidence interval (CI) 0.830–0.873) and lowest in the 2010s (leg‐ring = 0.773, 95% CI 0.738–0.805; neck‐collar = 0.725, 95% CI 0.681–0.764). Mean (±95% CI) resighting probabilities over the study period were higher for birds marked with neck‐collars (0.91 ± 0.01) than for those marked with leg‐rings (0.70 ± 0.02). Weather conditions in different areas across the flyway, food resources on the winter grounds, density‐dependence and the growth of numbers at a relatively new wintering site (the Evros Delta in Greece) all performed poorly as explanatory variables of apparent survival. None of our 18 covariates accounted for more than 7.2% of the deviance associated with our survival models, with a mean of only 2.2% of deviance explained. Our results provide long‐term demographic information needed to help conservationists understand the population dynamics of Bewick's Swans in northwest Europe.     

Spatial and temporal variation in population trends in a long‐distance migratory bird

Aim Over the past three decades, evidence has been growing that many Afro‐Palaearctic migratory bird populations have suffered sustained and severe declines. As causes of these declines exist across both the breeding and non‐breeding season, identifying potential drivers of population change is complex. In order to explore the roles of changes in regional and local environmental conditions on population change, we examine spatial and temporal variation in population trajectories of one of Europe’s most abundant Afro‐Palaearctic summer migrants, the willow warbler, Phylloscopus trochilus. Location Britain and Ireland. Methods We use national survey data from Britain and Ireland (BBS: BTO/RSPB/JNCC Breeding Bird Survey and CBS: BWI/NPWS/Heritage Council Countryside Breeding Survey) from 1994 to 2006 to model the spatial and temporal variation in willow warbler population trends. Results Across Britain and Ireland, population trends follow a gradient from sharp declines in the south and east of England to shallow declines and/or slight increases in parts of north and west England, across Scotland and Ireland. Decreasing the spatial scale of analysis reveals variation in both the rate and spatial extent of population change within central England and the majority of Scotland. The rates of population change also vary temporally; declines in the south of England are shallower now than at the start of the time series, whereas populations further north in Britain have undergone periods of increase and decline. Main conclusion These patterns suggest that regional‐scale drivers, such as changing climatic conditions, and local‐scale processes, such as habitat change, are interacting to produce spatially variable population trends. We discuss the potential mechanisms underlying these interactions and the challenges in addressing such changes at scales relevant to migratory species.     

Stable‐hydrogen isotope turnover in red blood cells of two migratory thrushes: application to studies of connectivity and carry‐over effects

ABSTRACT Understanding turnover rates of stable isotopes in metabolically active tissues is critical for making spatial connections for migratory birds because samples provide information about pre‐migratory location only until the tissue turns over to reflect local values. We calculated stable‐hydrogen isotope (δ²H) turnover rate in the red blood cells of two long‐distance migratory songbirds, Bicknell's Thrushes (Catharus bicknelli) and Swainson's Thrushes (Catharus ustulatus), using samples collected at a breeding site in New Brunswick, Canada. Blood from both species captured early in the breeding site was more positive in δ²H than blood sampled later in the summer, but did not match blood values for wintering Bicknell's Thrushes. An asymptotic exponential model was used to estimate turnover of red blood cell δ²H and yielded a half‐life estimate of 21 days and 14 days for Bicknell's and Swainson's thrushes, respectively. Red blood cells of both species approached the local breeding site value one month after the first individuals were detected at the site. For Bicknell's Thrushes, estimated δ²H in blood at arrival (?72‰) was closer to blood collected at wintering sites (mean ?61‰) than to expected breeding site δ²H (?120‰). Discrimination values calculated for red blood cells collected at the breeding site for both species were greater than expected based on studies using keratin. Turnover during migration currently limits the use of blood sampled early in the breeding season for connectivity/carry‐over effect studies. However, direct tracking technology such as geolocators can provide information about migration duration, timing, and stopovers that can be used to improve isotopic turnover equations for metabolically active tissues.     

Environmental and plant genetic effects on tri‐trophic interactions

The effects of plant genotype and environmental factors on tri‐trophic interactions have usually been investigated separately, limiting our ability to compare the relative strength of these effects as well as their potential to interactively shape arthropod communities. We studied the interactions among the herb Ruellia nudiflora, a seed predator, and its parasitoids using 14 maternal plant families grown in a common garden. By fertilizing half of the plants of each family and subsequently recording fruit number, seed predator number, and parasitoid number per plant, we sought to compare the strength of plant genetic effects with those of soil fertility, and determine if these factors interactively shape tri‐trophic interactions. Furthermore, we evaluated if these bottom–up factors influenced higher trophic levels through changes in abundance across trophic levels (density‐mediated) or changes in the function of species interactions (trait‐mediated). Plant genetic effects on seed predators and parasitoids were stronger than fertilization effects. Moreover, we did not find plant genetic variation for fertilization effects on fruit, seed predator, or parasitoid abundance, showing that each factor acted independently on plant resources and higher trophic levels. Both bottom–up forces were transmitted via density‐mediated effects where increased fruit number from fertilization and plant genetic effects increased seed predator and parasitoid abundance; however, seed predator attack was density‐dependent, while parasitoid attack was density‐independent. Importantly, there was evidence (marginally significant in one case) that fertilization modified the function of plant‐seed predator and seed predator–parasitoid interactions by increasing the number of seed predators per fruit and decreasing the number of parasitoids per seed predator, respectively. These findings show that plant genetic and soil fertility effects cascaded up this simple food chain, that plant genetic effects were stronger across all trophic levels, and that these effects were transmitted independently and through contrasting mechanisms.     

Trans‐generational effects on ageing in a wild bird population

Ageing, long thought to be too infrequent to study effectively in natural populations, has recently been shown to be ubiquitous, even in the wild. A major challenge now is to explain variation in the rates of ageing within populations. Here, using 49 years of data from a population of great tits (Parus major), we show that offspring life‐history trajectories vary with maternal age. Offspring hatched from older mothers perform better early in life, but suffer from an earlier onset, and stronger rate, of reproductive senescence later in life. Offspring reproductive lifespan is, however, unaffected by maternal age, and the different life‐history trajectories result in a similar fitness payoff, measured as lifetime reproductive success. This study therefore identifies maternal age as a new factor underlying variation in rates of ageing, and, given the delayed trans‐generational nature of this effect, poses the question as to proximate mechanisms linking age‐effects across generations.     

Larval carry‐over effects from ocean acidification persist in the natural environment

An extensive body of work suggests that altered marine carbonate chemistry can negatively influence marine invertebrates, but few studies have examined how effects are moderated and persist in the natural environment. A particularly important question is whether impacts initiated in early life might be exacerbated or attenuated over time in the presence or absence of other stressors in the field. We reared Olympia oyster (Ostrea lurida) larvae in laboratory cultures under control and elevated seawater pCO₂ concentrations, quantified settlement success and size at metamorphosis, then outplanted juveniles to Tomales Bay, California, in the mid intertidal zone where emersion and temperature stress were higher, and in the low intertidal zone where conditions were more benign. We tracked survival and growth of outplanted juveniles for 4 months, halfway to reproductive age. Survival to metamorphosis in the laboratory was strongly affected by larval exposure to elevated pCO₂ conditions. Survival of juvenile outplants was reduced dramatically at mid shore compared to low shore levels regardless of the pCO₂ level that oysters experienced as larvae. However, juveniles that were exposed to elevated pCO₂ as larvae grew less than control individuals, representing a larval carry‐over effect. Although juveniles grew less at mid shore than low shore levels, there was no evidence of an interaction between the larval carry‐over effect and shore level, suggesting little modulation of acidification impacts by emersion or temperature stress. Importantly, the carry‐over effects of larval exposure to ocean acidification remained unabated 4 months later with no evidence of compensatory growth, even under benign conditions. This latter result points to the potential for extended consequences of brief exposures to altered seawater chemistry with potential consequences for population dynamics.     

Restricted dispersal reduces the strength of spatial density dependence in a tropical bird population

Spatial processes could play an important role in density-dependent population regulation because the disproportionate use of poor quality habitats as population size increases is widespread in animal populations-the so-called buffer effect. While the buffer effect patterns and their demographic consequences have been described in a number of wild populations, much less is known about how dispersal affects distribution patterns and ultimately density dependence. Here, we investigated the role of dispersal in spatial density dependence using an extraordinarily detailed dataset from a reintroduced Mauritius kestrel (Falco punctatus) population with a territorial (despotic) breeding system. We show that recruitment rates varied significantly between territories, and that territory occupancy was related to its recruitment rate, both of which are consistent with the buffer effect theory. However, we also show that restricted dispersal affects the patterns of territory occupancy with the territories close to release sites being occupied sooner and for longer as the population has grown than the territories further away. As a result of these dispersal patterns, the strength of spatial density dependence is significantly reduced. We conclude that restricted dispersal can modify spatial density dependence in the wild, which has implications for the way population dynamics are likely to be impacted by environmental change.     

Temperature and prey density jointly influence trophic and non‐trophic interactions in multiple predator communities

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Pathways to fitness: carry‐over effects of late hatching and urbanisation on lifetime mating success

Life history theory and most empirical studies assume carry‐over effects of larval ­conditions to shape adult fitness through their impact on metamorphic traits (age and mass at metamorphosis). Yet, very few formal tests of this connection across metamorphosis exist, because this entails longitudinal studies from the egg stage and requires measuring fitness in (semi)natural conditions. In a longitudinal one‐year common‐garden rearing experiment consisting of an outdoor microcosm part for the larval stage and a large outdoor insectary part for the adult stage, we studied the effects of two factors related to time constraints in the larval stage (egg hatching period and urbanisation) on life history traits and lifetime mating success in the males of the damselfly Coenagrion puella. We reared early‐ and late‐hatched larvae from each of three rural and three urban populations from the egg stage throughout their adult life. Key findings were that both the hatching period and urbanisation shaped adult fitness, yet through different pathways. As expected, the more time‐constrained late‐hatched individuals accelerated their larval life history and this was associated with a lower lifetime mating success. A path analysis revealed this carry‐over effect was mediated by the changes in the two metamorphic traits (reduced age and lower mass at emergence). Notably, urban males had a 50% lower lifetime mating success, which was not mediated by age and mass at emergence, and possibly driven by their shorter lifespan. Our results point to long‐term carry‐over effects of the usually ignored natural variation in egg hatching dates, and further contribute to the limited evidence showing fitness costs of adjusting to an urban lifestyle.     

Individual repeatability in laying behaviour does not support the migratory carry‐over effect hypothesis of egg‐size dimorphism in Eudyptes penguins

Penguins of the genus Eudyptes are unique among birds in that their first‐laid A‐egg is 54–85% the mass of their second‐laid B‐egg. Although the degree of intra‐clutch egg‐size dimorphism varies greatly among the seven species of the genus, obligate brood reduction is typical of each, with most fledged chicks resulting from the larger B‐egg. Many authors have speculated upon why Eudyptes penguins have evolved and maintained a highly dimorphic 2‐egg clutch, and why it is the first‐laid egg that is so much smaller than the second, but only recently has a testable, proximate mechanism been proposed. In most species of Eudyptes penguins females appear to initiate egg‐formation at sea during return migration to breeding colonies. In macaroni penguins E. chrysolophus, females with a shorter pre‐laying interval ashore (and thus presumably greater overlap between migration and egg‐formation) lay more dimorphic eggs, suggesting a physiological conflict may constrain growth of the earlier‐initiated A‐egg. This migratory carry‐over effect hypothesis (MCEH) was tested in eastern rockhopper penguins E. chrysocome filholi on Campbell Island, New Zealand, by recording the arrival and lay dates, body sizes, and egg masses of transponder‐tagged females over two years. Females with longer pre‐laying intervals laid less dimorphic clutches, as predicted by the MCEH. However, repeated measures of individual females revealed that within‐individual variation in egg‐size dimorphism between years was unrelated to within‐individual variation in pre‐laying interval. Egg masses, and to a lesser extent egg‐size dimorphism, were highly repeatable traits related to body size and body mass. These results and a detailed consideration of the MCEH suggest that egg‐size dimorphism in Eudyptes penguins is unlikely to be caused by a migratory carry‐over effect.