Long-distance Dispersal Patterns of Male Cerulean Warblers (Dendroica cerulea) Measured by Stable-hydrogen Isotopes Patrons de dispersion sur de longues distances de Parulines azurées mâles (Dendroica cerulea) mesurés à l'aide d'isotopes stables d'hydrogène

Despite the fundamental role played by long-distance dispersal in population dynamics, the mechanisms promoting or inhibiting dispersal by migratory songbirds are poorly understood. We used stable-hydrogen isotopes (δD) to evaluate several hypotheses related to long-distance dispersal in the Cerulean Warbler (Dendroica cerulea), a migratory songbird at the center of considerable conservation attention. Feather samples were collected from 103 males throughout the breeding range. We assumed feathers were molted in late summer on the breeding grounds and thus δD values provided an estimate of breeding or hatching location for the previous year. We used a likelihood-based assignment test to estimate the origin of birds the previous year and derived expected δD values for the entire Cerulean Warbler breeding range from precipitation-based maps. Using Bayes' Rule, we also incorporated a prior probability of breeding origin based on estimated rates of site fidelity and survival between breeding seasons. We found that long-distance breeding dispersal by adult male Cerulean Warblers was a fairly common occurrence with 20 of 71 (28.2%) individuals originating from a region other than the one they were sampled in. Surprisingly, long-distance natal dispersal was uncommon; only three of 32 (9.4%) second-year birds were estimated to have hatched in a region outside their capture region. Young males may be using a form of post-fledging prospecting or conspecific attraction when selecting their first breeding habitats. Populations on the breeding range periphery, such as Ontario and Tennessee, tended to receive fewer long- distance dispersers than did other regions but produced more dispersing individuals than did core regions, suggesting that these areas could act as important sources for other regions. The tendency of a region to produce dispersing individuals was not significantly related to its relative abundance, implying that population abundance is not a good indicator of population health for this species. Range-wide variation in long-distance dispersal patterns may contribute to variation in sensitivity to habitat loss and fragmentation, further complicating conservation and management efforts for the Cerulean Warbler. RESUME. En depit du fait que la dispersion sur de longues distances joue un role fondamental dans la dynamique des populations, les mecanismes qui la favorisent ou la freinent sont peu connus chez les passereaux migrateurs. Nous avons utilise des isotopes stables d'hydrogene ( δD) pour tester plusieurs hypotheses en rapport avec la dispersion sur de longues distances de la Paruline azuree, un passereau dont la conservation fait l'objet d'une grande attention. Des echantillons de plumes ont ete recoltes sur 103 mâles dans l'ensemble de l'aire de reproduction de l'espece. Nous presumons que la mue de ces plumes s'est produite a la fin de l'ete sur les sites de reproduction et que les valeurs δD fournissent, par consequent, une estimation du lieu de reproduction ou d'eclosion de l'annee precedente. Nous avons utilise un test d'attribution axe sur la vraisemblance pour estimer le lieu d'origine de l'annee precedente des oiseaux et avons calcule les valeurs δD probables pour l'ensemble de l'aire de reproduction de la Paruline azuree a