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Of the approximately 716 bird species that breed in North America, 386 (54%) are considered Nearctic–Neotropical migrants by the U.S. Fish and Wildlife Service. In the past 50 yr, scores of these migratory species, including some once considered common, have declined dramatically. Nearctic–Neotropical migrants normally spend 6–8 months in tropical habitats, making the identification, availability, and management of Neotropical habitats critical issues for their conservation. Yet, for most species, complete and nuanced information about their use of tropical habitats and the relative effects of breeding vs. wintering conditions on survival, productivity, and population trends is not available, though many studies point to Neotropical overwintering habitats as being a strong driver of population change. Particularly important for long-distance Nearctic–Neotropical migrants is an understanding of how “carry-over effects” arise and influence population trends when conditions on wintering grounds and tropical stopover areas affect subsequent reproductive performance on breeding grounds. For example, why some species show strong carry-over effects from tropical habitats while others do not is not fully understood. In recent years, many studies have offered insights into these issues by taking advantage of new scientific methods and technological innovations. In this review, we explore threats facing North American breeding birds that migrate to the Neotropics, summarize knowledge of habitat selection and use on the wintering grounds, describe how conditions at one point in the annual cycle may manifest in subsequent seasons or life history stages, and discuss conservation concerns such as climate change and the potential for phenological mismatch.

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Wilson's warbler comprises three subspecies separated into two geographic groups: C. p. pusilla that breeds in eastern North America; and C. p. pileolata and C. p. chryseola that breed in western North America. Given the differences between the groups in genetics, morphology, habitat use, and population decline, we tested for ecological niche similarity in both their breeding and wintering distribution using niche modeling based on temperature and precipitation data. We first conducted an inter-prediction approach considering the percent of summer and winter localities of one group that are predicted by the potential distribution of the alternate group. We also applied a null model approach that compares self-predictions and pseudoreplicates of each group to indicate similarity, divergence, or indeterminate niche overlap. Finally, we compared ecological distances between and within groups using the Gower similarity equation. We found that the western group had an ecological niche of broader climatic conditions, while the eastern group had a narrower ecological niche. The interprediction approach showed that, for both summering and wintering ranges, ecological niche models of the western group predicted ∼50% of the observed distribution of the eastern group, whereas eastern group models predicted  18% of the western group distribution. The null model approach found that similarity in ecological niches was indeterminate, possibly due to the large area occupied by the two groups; but it suggests a more restricted set of climatic conditions of the eastern group distribution. However, the Gower coefficients demonstrated that the ecological distance between the two geographic groups was larger than the ecological distance within groups, indicating distinct ecological niches. Overall, our results support the hypothesis that the eastern and western groups of Wilson's warbler are two cryptic species; this should be taken into consideration for future analyses, particularly with respect to vulnerability categorization and conservation efforts.

During partial moults birds replace a variable number or percentage of old feathers. This quantity, known as moult extent, has been a primary variable used in comparative studies. However, different spatial configurations of feather replacement may result from an equal number of renewed feathers. Few studies have addressed spatial aspects of moult, which may vary among species, among individuals of the same species and between episodes at the individual level. We present a novel approach to quantify the spatial configuration of a wing‐moult episode, hereafter referred to as moult topography, which comprises two elements, namely extent and vector, the latter condensing the spatial configuration of the replaced feathers on the wing plane. We apply this method to investigate preformative (post‐juvenile) wing‐feather moult pattern in the Spot‐breasted Wren Pheugopedius maculipectus and the White‐breasted Wood‐Wren Henicorhina leucosticta. We specified a null model of wing‐moult topography by which feather replacement follows a discrete anterior–posterior (vertical) axis between tracts and a discrete proximal–distal (horizontal) axis within tracts, and whereby wing feathers from a new tract are replaced only if all the feathers from the previous (anterior) tract have been replaced. Our sample of Spot‐breasted Wrens showed a strict single pattern of replacement that did not differ significantly from the null model. Our sample of White‐breasted Wood‐Wrens, however, differed significantly from the null model, showing prioritization of proximal wing feathers closer to the body. These differences might have biological relevance, for example in mate selection or in response to different environmental stressors, and might reveal the influence of these factors on the evolution of moult strategies. Overall, moult topography provides a new approach to future ecological and evolutionary studies of moult.