Peter M. Potts

The buffer effect and large-scale population regulation in migratory birds.

Buffer effects occur when sites vary in quality and fluctuations in population size are mirrored by large changes in animal numbers in poor-quality sites but only small changes in good-quality sites. Hence, the poor sites ‘buffer’ the good sites, a mechanism that can potentially drive population regulation if there are demographic costs of inhabiting poor sites. Here we show that for a migratory bird this process can apply on a country-wide scale with consequences for both survival and timing of arrival on the breeding grounds (an indicator of reproductive success). The Icelandic population of the black-tailed godwit, Limosa limosa islandica, wintering in Britain has increased fourfold since the 1970s (ref. 5) but rates of change within individual estuaries have varied from zero to sixfold increases. In accordance with the buffer effect, rates of increase are greater on estuaries with low initial numbers, and godwits on these sites have lower prey-intake rates, lower survival rates and arrive later in Iceland than godwits on sites with stable populations. The buffer effect can therefore be a major process influencing large-scale population regulation of migratory species.

Seasonal matching of habitat quality and fitness in a migratory bird

When species occupy habitats that vary in quality, choice of habitat can be critical in determining individual fitness. In most migratory species, juveniles migrate independently of their parents and must therefore choose both breeding and winter habitats. Using a unique dataset of marked black-tailed godwits (Limosa limosa islandica) tracked throughout their migratory range, combined with analyses of stable carbon isotope ratios, we show that those individuals that occupy higher quality breeding sites also use higher quality winter sites. This seasonal matching can severely inflate inequalities in individual fitness. This population has expanded over the last century into poorer quality breeding and winter habitats and, across the whole population; individual birds tend to occupy either novel or traditional sites in both seasons. Winter and breeding season habitat selection are thus strongly linked throughout this population; these links have profound implications for a wide range of population and evolutionary processes. As adult godwits are highly philopatric, the initial choice of winter habitat by juveniles will be critical in determining future survival, timing of migration and breeding success.

Why is timing of bird migration advancing when individuals are not

Recent advances in spring arrival dates have been reported in many migratory species but the mechanism driving these advances is unknown. As population declines are most widely reported in species that are not advancing migration, there is an urgent need to identify the mechanisms facilitating and constraining these advances. Individual plasticity in timing of migration in response to changing climatic conditions is commonly proposed to drive these advances but plasticity in individual migratory timings is rarely observed. For a shorebird population that has significantly advanced migration in recent decades, we show that individual arrival dates are highly consistent between years, but that the arrival dates of new recruits to the population are significantly earlier now than in previous years. Several mechanisms could drive advances in recruit arrival, none of which require individual plasticity or rapid evolution of migration timings. In particular, advances in nest-laying dates could result in advanced recruit arrival, if benefits of early hatching facilitate early subsequent spring migration. This mechanism could also explain why arrival dates of short-distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants.

Costs, benefits, and fitness consequences of different migratory strategies

The relative fitness of individuals across a population can shape distributions and drive population growth rates. Migratory species often winter over large geographic ranges, and individuals in different locations experience very different environmental conditions, including different migration costs, which can potentially create fitness inequalities. Here we used energetics models to quantify the trade-offs experienced by a migratory shorebird species at locations throughout the nonbreeding range, and the associated consequences for migratory performance, survival, and breeding habitat quality. Individuals experiencing more favorable winter conditions had higher survival rates, arrived on the breeding grounds earlier, and occupied better quality breeding areas, even when migration costs are substantially higher, than individuals from locations where the energy balance on the wintering grounds was less favorable. The energy costs and benefits of occupying different winter locations can therefore create fitness inequalities which can shape the distribution and population-wide demography of migratory species.

Sex-biases in distribution and resource use at different spatial scales in a migratory shorebird

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex-biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex-biased impacts, we use range-wide tracking of individually color-ringed Icelandic black-tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non-breeding range to within-site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large-scale sex differences in distribution during the non-breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small-scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within-site distribution and resource use means that local-scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds.

Sexing of Black-tailed Godwits Limosa limosa islandica: a comparison of behavioural, molecular, biometric and field-based techniques

Capsule Biometrics and plumage characteristics can both be used to reliably sex Black-tailed Godwits. Aims To develop methods of sexing Black-tailed Godwits and to validate their relative accuracy. Methods A sample of 84 Black-tailed Godwits was sexed by DNA analysis of feather samples. The biometric data and plumage characteristics of these birds were then used to develop protocols for sexing godwits in the field. Results A discriminant function analysis of biometric data correctly sexed 95% of the DNA-sexed reference sample. Of 808 birds caught throughout the range, 74% could be sexed with this method. Approximately 85% of the reference sample were correctly sexed on three plumage characteristics in the hand. Of 105 birds sexed by DNA or biometrics, 82% were sexed correctly on general impression and shape in the field. Conclusions For the many species with limited sexual dimorphism, a relatively small sample of accurately sexed birds can provide a means of testing and improving current morphological methods of sexing.

Rapid changes in phenotype distribution during range expansion in a migratory bird

The capacity of species to track changing environmental conditions is a key component of population and range changes in response to environmental change. High levels of local adaptation may constrain expansion into new locations, while the relative fitness of dispersing individuals will influence subsequent population growth. However, opportunities to explore such processes are rare, particularly at scales relevant to species-based conservation strategies. Icelandic black-tailed godwits, Limosa limosa islandica, have expanded their range throughout Iceland over the last century. We show that current male morphology varies strongly in relation to the timing of colonization across Iceland, with small males being absent from recently occupied areas. Smaller males are also proportionately more abundant on habitats and sites with higher breeding success and relative abundance of females. This population-wide spatial structuring of male morphology is most likely to result from female preferences for small males and better-quality habitats increasing both small-male fitness and the dispersal probability of larger males into poorer-quality habitats. Such eco-evolutionary feedbacks may be a key driver of rates of population growth and range expansion and contraction.

Estimating population size in Black-tailed Godwits Limosa limosa islandica by colour-marking

Capsule Adult population size of the Icelandic race of Black-tailed Godwit was estimated at 37 500 individuals. Aims To estimate the population size of the Icelandic race of Black-tailed Godwit and infer numbers wintering in areas where both the Icelandic and nominate races overlap. Methods Adult population size was estimated using resightings of individually colour-ringed birds in spring staging flocks in Iceland. Winter distribution was assessed by comparing January counts to the population estimate, assuming 0.5 chicks per breeding pair to estimate the number of first-year birds and the resulting winter population size. Results The adult population size of the Icelandic race of Black-tailed Godwit was estimated at about 37 500 individuals (range 35 565–41 999) between 1999 and 2002. This gives an estimated winter population of about 47 000 individuals. From counts in wintering areas and the calculated population estimate, we estimate that about 15 000 Icelandic Black-tailed Godwits winter in areas where the races overlap (mostly in Portugal, Spain and Morocco) but the numbers in each country remain unknown. Conclusion For species which can be fitted with conspicuous colour rings and for which a high proportion of individuals can be observed at particular times of year, mark–recapture methods can be very useful techniques to estimate the number of individuals of different races of birds wintering in the same areas.

Influence of age and sex on winter site fidelity of sanderlings Calidris alba

Many migratory bird species show high levels of site fidelity to their wintering sites, which confers advantages due to prior knowledge, but may also limit the ability of the individual to move away from degrading sites or to detect alternative foraging opportunities. Winter site fidelity often varies among age groups, but sexual differences have seldom been recorded in birds. We studied a population of individually colour-marked sanderlings wintering in and around the Tejo estuary, a large estuarine wetland on the western coast of Portugal. For 160 individuals, sighted a total of 1,249 times between November 2009 and March 2013, we calculated the probability that they moved among five distinct wintering sites and how this probability is affected by distance between them. To compare site fidelity among age classes and sexes, as well as within the same winter and over multiple winters, we used a Site Fidelity Index (SFI). Birds were sexed using a discriminant function based on biometrics of a large set of molecularly sexed sanderlings (n = 990). The vast majority of birds were observed at one site only, and the probability of the few detected movements between sites was negatively correlated with the distance among each pair of sites. Hardly any movements were recorded over more than 15 km, suggesting small home ranges. SFI values indicated that juveniles were less site-faithful than adults which may reflect the accumulated knowledge and/or dominance of older animals. Among adults, females were significantly less site faithful than males. A sexual difference in winter site fidelity is unusual in shorebirds. SFI values show site-faithfulness is lower when multiple winters were considered, and most birds seem to chose a wintering site early in the season and use that site throughout the winter. Sanderlings show a very limited tendency to explore alternative wintering options, which might have implications for their survival when facing habitat change or loss (e.g., like severe beach erosion as can be the case at one of the study sites).

ENERGETIC TRADE-OFFS OF BLACK-TAILED GODWITS

Photo 1. Wintering flock of Icelandic Black-tailed Godwits roosting during the high-tide period. In winter, Icelandic godwits forage mostly in estuarine mudflats during the low tide and, in some sites, also forage on wet grasslands. The energy benefits vary greatly between different winter sites. Photo by Graham Catley. The study explored fitness inequalities of energetic trade-offs on Icelandic Black-tailed Godwits wintering across the range. The conditions experienced by Icelandic Godwits (Limosa limosa islandica) at wintering locations throughout their range vary greatly, with important consequences for individual fitness. The costs of thermoregulation and low food availability on some sites can result in a negative energy balance, which is associated with low survival, later arrival in Iceland, and use of poorer-quality breeding sites, even when migration distance and costs are half those of conspecifics from more favorable winter sites.