Environmental conditions in winter and their ecological and evolutionary consequences for American redstarts (Setophaga ruticilla)

I used both observational and experimental approaches to assess the causes of nonbreeding habitat quality and to evaluate their ecological and evolutionary consequences for a Neotropical-Nearctic migratory bird, the American redstart (Setophaga ruticilla). Relative to control birds overwintering in second-growth scrub, redstarts experimentally upgraded from scrub to mangrove forest incorporated mangrove stable-carbon isotope signatures, maintained mass over winter, departed earlier on spring migration, and had higher apparent annual survival. Significantly higher arthropod biomass on upgrade territories implicated food availability as a proximate mechanism of habitat quality. Food availability, body condition, and spring departure schedules also depended on nonbreeding season rainfall. Food availability in mangrove was higher than in scrub in three of four years, allowing birds in this habitat to maintain superior body condition and depart earlier on spring migration. Abundant rainfall in a single year led to abnormally high food availability in scrub and early departure of birds in both habitats, suggesting both the amount and timing of rainfall influenced nonbreeding performance. Habitat occupancy and annual variation in rainfall had significant consequences for natal dispersal and selection through annual survival. Stable-hydrogen isotope ratios (δD) in feathers of immature birds captured again as adults indicated that habitat use in the first nonbreeding season interacted with spring phenology on temperate breeding grounds to influence the distance traveled on the first spring migration and direction of natal dispersal. In contrast, adults showed site fidelity between breeding seasons, suggesting nonbreeding conditions did not affect breeding dispersal and that migration distance becomes fixed later in life. Patterns of δD also revealed directional selection for short migration distance to southern breeding areas, a pattern that was nearly twice as strong in scrub compared to mangrove. During dry winters, redstarts experienced stabilizing selection on departure dates and directional selection for short migration. In years of high rainfall, birds experienced correlational selection favoring late departure when in good body condition. Thus, occupancy of moist habitats and years of high rainfall relaxed selection against late departure and longer migration. Collectively, these findings emphasize the need to understand how events throughout the annual cycle interact to shape fundamental biological processes

Challenging claims in the study of migratory birds and climate change

Recent shifts in phenology in response to climate change are well established but often poorly understood. Many animals integrate climate change across a spatially and temporally dispersed annual life cycle, and effects are modulated by ecological interactions, evolutionary change and endogenous control mechanisms. Here we assess and discuss key statements emerging from the rapidly developing study of changing spring phenology in migratory birds. These well-studied organisms have been instrumental for understanding climate-change effects, but research is developing rapidly and there is a need to attack the big issues rather than risking affirmative science. Although we agree poorly on the support for most claims, agreement regarding the knowledge basis enables consensus regarding broad patterns and likely causes. Empirical data needed for disentangling mechanisms are still scarce, and consequences at a population level and on community composition remain unclear. With increasing knowledge, the overall support (‘consensus view’) for a claim increased and between-researcher variability in support (‘expert opinions') decreased, indicating the importance of assessing and communicating the knowledge basis. A proper integration across biological disciplines seems essential for the field's transition from affirming patterns to understanding mechanisms and making robust predictions regarding future consequences of shifting phenologies

Patterns in departure phenology and mass gain on African non-breeding territories prior to the Sahara crossing for a long-distance migrant

This work was supported by the British Ornithologists’ Union, the Linnean Society, the A.P. Leventis Foundation and Chris Goodwin.Afro-Palaearctic migrants are declining to a greater degree than other European species, suggesting that processes occurring in Africa or on migration may be driving these trends. Constraints in food availability on the wintering grounds may contribute to the declines but little is known about when and where these resource constraints may occur. Sufficient resources are particularly important prior to spring migration, when migrants must cross the Sahara desert. We examined mass gain and departure phenology in a long-distance Palaearctic passerine migrant to determine the degree to which pre-migratory fattening occurs in their long-term non-breeding territories in the Guinea Savannah region of Africa. We monitored 75 Whinchats Saxicola rubetra for departure from their non-breeding territories in one spring, and analysed mass data of 377 Whinchats collected over three non-breeding seasons plus 141 migrating Whinchats caught in April over eight years, all within the same few square kilometres of anthropogenically-modified Guinea Savannah in central Nigeria. Whinchats left their winter territories throughout April, with males departing on average eight days earlier than females. However, there was no evidence that time of departure from territory was linked to age, body size or mass at capture. Whinchats departed their territories with a predicted mass of 16.8 ± 0.3 g, which is much less than the ~24 g required for the average Whinchat to cross the Sahara directly. Comparing departure dates with arrival dates in southern Europe shows a discrepancy of at least two weeks, suggesting that many Whinchats spend considerable time on pre-migratory fuelling outside of their territory prior to crossing the Sahara. Over-wintering birds gained mass slowly during February and March (0.03 gd-1 34 ), and non-territorial or migrating birds at a much higher rate in April (at least 0.23 gd-1 35 ), with up to 20% of migrating Whinchats in April potentially having sufficient fuel loads to cross the Sahara directly from central Nigeria. Our results suggest that most Whinchats leave their winter territories to fatten up locally or, possibly, by staging further north. Resource constraints are therefore likely to be particularly focussed in West Africa during mid-April and possibly at staging areas before the crossing of the Sahara desert.PostprintPeer reviewe

Rapid population decline in migratory shorebirds relying on Yellow Sea tidal mudflats as stopover sites

Migratory animals are threatened by human-induced global change. However, little is known about how stopover habitat, essential for refuelling during migration, affects the population dynamics of migratory species. Using 20 years of continent-wide citizen science data, we assess population trends of ten shorebird taxa that refuel on Yellow Sea tidal mudflats, a threatened ecosystem that has shrunk by >65% in recent decades. Seven of the taxa declined at rates of up to 8% per year. Taxa with the greatest reliance on the Yellow Sea as a stopover site showed the greatest declines, whereas those that stop primarily in other regions had slowly declining or stable populations. Decline rate was unaffected by shared evolutionary history among taxa and was not predicted by migration distance, breeding range size, non-breeding location, generation time or body size. These results suggest that changes in stopover habitat can severely limit migratory populations

Optimal Conservation of Migratory Species

Background. Migratory animals comprise a significant portion of biodiversity worldwide with annual investment for their conservation exceeding several billion dollars. Designing effective conservation plans presents enormous challenges. Migratory species are influenced by multiple events across land and sea-regions that are often separated by thousands of kilometres and span international borders. To date, conservation strategies for migratory species fail to take into account how migratory animals are spatially connected between different periods of the annual cycle (i.e. migratory connectivity) bringing into question the utility and efficiency of current conservation efforts. Methodology/Principal Findings. Here, we report the first framework for determining an optimal conservation strategy for a migratory species. Employing a decision theoretic approach using dynamic optimization, we address the problem of how to allocate resources for habitat conservation for a Neotropical-Nearctic migratory bird, the American redstart Setophaga ruticilla, whose winter habitat is under threat. Our first conservation strategy used the acquisition of winter habitat based on land cost, relative bird density, and the rate of habitat loss to maximize the abundance of birds on the wintering grounds. Our second strategy maximized bird abundance across the entire range of the species by adding the constraint of maintaining a minimum percentage of birds within each breeding region in North America using information on migratory connectivity as estimated from stable-hydrogen isotopes in feathers. We show that failure to take into account migratory connectivity may doom some regional populations to extinction, whereas including information on migratory connectivity results in the protection of the species across its entire range. Conclusions/Significance. We demonstrate that conservation strategies for migratory animals depend critically upon two factors: knowledge of migratory connectivity and the correct statement of the conservation problem. Our framework can be used to identify efficient conservation strategies for migratory taxa worldwide, including insects, birds, mammals, and marine organisms

Patterns and processes in shorebird survival rates: a global review

Changes in demographic rates underpin changes in population size, and understanding demographic rates can greatly aid the design and development of strategies to maintain populations in the face of environmental changes. However, acquiring estimates of demographic parameters at relevant spatial scales is difficult. Measures of annual survival rates can be particularly challenging to obtain because large-scale, long-term tracking of individuals is difficult and the resulting data contain many inherent biases. In recent years, advances in both tracking and analytical techniques have meant that, for some taxonomic groups, sufficient numbers of survival estimates are available to allow variation within and among species to be explored. Here we review published estimates of annual adult survival rates in shorebird species across the globe, and construct models to explore the phylogenetic, geographical, seasonal and sex-based variation in survival rates. Models of 295 survival estimates from 56 species show that survival rates calculated from recoveries of dead individuals or from return rates of marked individuals are significantly lower than estimates from mark–recapture models. Survival rates also vary across flyways, largely as a consequence of differences in the genera that have been studied and the analytical methods used, with published estimates from the Americas and from smaller shorebirds (Actitis, Calidris and Charadrius spp.) tending to be underestimated. By incorporating the analytical method used to generate each estimate within a mixed model framework, we provide method-corrected species-specific and genus-specific adult annual survival estimates for 52 species of 15 genera

Environmental factors shaping the distribution of common wintering waterbirds in a lake ecosystem with developed shoreline

In this study, we tested whether the spatial distribution of waterbirds is influenced by shoreline urbanization or other habitat characteristics. We conducted monthly censuses along shoreline sections of a continental lake (Lake Balaton, Hungary) to assess the abundance of 11 common species that use this lake as a feeding and staging area during migration and winter. We estimated the degree of urbanization of the same shoreline sections and also measured other habitat characteristics (water depth, extent of reed cover, biomass of zebra mussels, distances to waste dumps and to other wetlands). We applied linear models and model averaging to identify habitat variables with high relative importance for predicting bird distributions. Bird abundance and urbanization were strongly related only in one species. Other habitat variables exhibited stronger relationships with bird distribution: (1) diving ducks and coots preferred shoreline sections with high zebra mussel biomass, (2) gulls preferred sites close to waste dumps, and (3) the abundances of several species were higher on shoreline sections close to other wetlands. Our findings suggest that the distribution of waterbirds on Lake Balaton is largely independent of shoreline urbanization and influenced by food availability and connectivity between wetlands