Migratory species can experience limiting factors at different locations and during different periods of their annual cycle. In migratory birds, these factors may even occur in different hemispheres. Therefore, identifying the distribution of populations throughout their annual cycle (i.e., migratory connectivity) can reveal the complex ecological and evolutionary relationships that link species and ecosystems across the globe and illuminate where and how limiting factors influence population trends. A growing body of literature continues to identify species that exhibit weak connectivity wherein individuals from distinct breeding areas co-occur during the nonbreeding period. A detailed account of a broadly distributed species exhibiting strong migratory connectivity in which nonbreeding isolation of populations is associated with differential population trends remains undescribed. Here, we present a range-wide assessment of the nonbreeding distribution and migratory connectivity of two broadly dispersed Nearctic-Neotropical migratory songbirds. We used geolocators to track the movements of 70 warblers from sites spanning their breeding distribution in eastern North America and identified links between breeding populations and nonbreeding areas. Unlike blue-winged warblers (), breeding populations of golden-winged warblers () exhibited strong migratory connectivity, which was associated with historical trends in breeding populations: stable for populations that winter in Central America and declining for those that winter in northern South America.

Aim We examined whether variation in species composition of breeding birds and resident butterflies in the Great Basin of North America depended on sampling grain (the smallest resolvable unit of study) and on the relative proximity of sampling units across the landscape. We also compared patterns between the two taxonomic groups with reference to their life-history characteristics.Location Data for our analyses were collected from 1996 to 2003 in three adjacent mountain ranges in the central Great Basin (Lander and Nye counties, Nevada, USA): the Shoshone Mountains, Toiyabe Range and Toquima Range.Methods Data on species composition for both taxonomic groups were collecting using standard inventory methods for birds and butterflies in temperate regions. Data were compiled at three sampling grains, sites (average 12 ha), canyons (average 74 ha) and mountain ranges. For each sampling grain in turn, we calculated similarity of species composition using the Jaccard index. First, we investigated whether mean similarity of species composition among the three ranges differed as a function of the grain size at which data were compiled. Secondly, we explored whether mean similarity of species composition was greater for canyons within the same mountain range than for canyons within different mountain ranges. Thirdly, we examined whether mean similarity of species composition at the site level was different for sites within the same canyon, sites within different canyons in the same mountain range, and sites within canyons in different mountain ranges. We used a Bayesian model to analyse these comparisons.Results For both taxonomic groups, mean similarity of species composition increased as the sampling grain increased. The effect of spatial grain was somewhat greater for birds than for butterflies, especially when the intermediate sampling grain was compared with the smallest sampling grain. Similarity of species composition of butterflies at each sampling grain was greater than similarity of species composition of birds at the same grain. Mean similarity of species composition of both birds and butterflies at the canyon level and site level was affected by relative proximity of sampling locations; beta diversity increased as the relative isolation of sampling locations increased.Main conclusions The sensitivity of beta diversity to sampling grain likely reflects the effect of local environmental heterogeneity: as sampling grain increases, biotic assemblages appear more homogeneous. Although breeding birds in our study system have larger home ranges than resident butterflies, birds may have more specialized resource requirements related to vegetation structure and composition, especially at small sampling scales. The degree of variation in species composition of both taxonomic groups suggests that spatially extensive sampling

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Widespread population declines of the Golden-winged Warbler (Vermivora chrysoptera) are thought to be due in part to hybridization with the expanding Blue-winged Warbler (V. pinus), which predictably replaces Golden-winged Warblers at breeding sites in which the two species come into contact. However, the mechanism by which this replacement occurs remains unresolved. Recent genetic work has indicated that, even in areas where the two species have been in contact for a short period, introgression of Blue-winged mitochondrial (mtDNA) and nuclear genes into Golden-winged individuals is common. To explore this process on a broader scale, we screened more than 750 individuals from nine U.S. states and three provinces to examine geographic patterns of mtDNA introgression. The only population in which all phenotypic Golden-winged Warblers had Golden-winged mtDNA haplotypes, and in which there are no breeding Blue-winged or hybrid individuals, was in the province of Manitoba, near the northwestern edge of the species' breeding distribution. The near ubiquity of mitochondrial introgression suggests that there are far fewer genetically pure populations of Goldenwinged Warblers than previously believed, a finding with important implications for this threatened species. RÉSUMÉ. On pense que le déclin à grande échelle des populations de Paruline à ailes dorées (Vermivora chrysoptera) est en partie attribuable à son hybridation avec la Paruline à ailes bleues (V. pinus), laquelle est en expansion et remplace, comme on peut s'y attendre, la Paruline à ailes dorées aux sites de nidification où les deux espèces entrent en contact. Toutefois, le processus par lequel survient ce remplacement demeure inconnu. Des études génétiques récentes ont montré que, même aux endroits où les deux espèces n'ont été en contact que sur une courte période, l'introgression des gènes mitochondriaux (ADNmt) et nucléaires de la Paruline à ailes bleues chez les individus de Paruline à ailes dorées est commune. Pour explorer ce processus à plus grande échelle, nous avons procédé au criblage de plus de 750 individus provenant de neufs États des États-Unis et de trois provinces afin d'examiner les variations géographiques de l'introgression mitochondriale. La seule population dans laquelle toutes les Parulines à ailes dorées phénotypiques présentaient des haplotypes d'ADNmt de Paruline à ailes dorées, et dans laquelle il n'y avait pas d'individus nicheurs de Paruline à ailes bleues ni d'hybrides, se trouvait au Manitoba, près de la limite nord-ouest de l'aire de reproduction de l'espèce. La quasiomniprésence de l'introgression mitochondriale permet de croire qu'il y a beaucoup moins de populations génétiquement pures de Paruline à ailes dorées que ce qu'on pensait, une découverte significative pour cette espèce menacée.