Relative Importance of Nesting Habitat and Measures of Connectivity in Predicting the Occurrence of a Forest Songbird in Fragmented Landscapes

Theoretical and empirical studies suggest that well-connected networks of forest habitat facilitate animal movement and contribute to species’ persistence and thereby the maintenance of biodiversity. Many structural and functional connectivity metrics have been proposed, e.g., distance to nearest neighboring patch or graph-based measures, but the relative importance of these measures in contrast to nesting habitat at fine spatial scales is not well established. With graph-based measures of connectivity, Euclidean distances between forest patches can be directly related to the preferred gap crossing distances of a bird (functional connectivity). We determined the relative predictive power of nesting habitat, forest cover, and structural or functional connectivity measures in describing the breeding distribution of Hooded Warblers (Setophaga citrina) over two successive breeding seasons in a region highly fragmented by agriculture in southern Ontario. Logistic regression models of nesting occurrence patterns were compared using Akaike’s information criterion and relative effect sizes were compared using odds ratios. Our results provide support for the expectation that nest-site characteristics are indeed related to the breeding distribution of S. citrina. However, models based on nesting habitat alone were 4.7 times less likely than a model including functional connectivity as a predictor for the breeding distribution of S. citrina. Models of nest occurrence in relation to surrounding forest cover had lower model likelihoods than models that included graph-based functional connectivity, but these measures were highly confounded. Graph-based measures of connectivity explained more variation in nest occurrence than structural measures of forest connectivity, in both 2004 and 2005. These results suggest that S. citrina selected nesting areas that were functionally connected at their preferred gap crossing distances, but nesting habitat was a critically important predictor of nest occurrence patterns. RESUME. Des recherches theoriques et empiriques laissent supposer que des reseaux de milieux forestiers bien connectes permettent le deplacement de la faune et contribuent a la perennite d’une espece et, par le fait meme, au maintien de la biodiversite. De nombreuses mesures de connectivite structurelle et fonctionnelle ont ete avancees, par exemple la distance a l’ilot voisin le plus proche ou encore, des mesures fondees sur des graphiques, mais l’importance relative de ces indices comparativement a l’habitat de nidification a des echelles spatiales fines n’est pas etablie clairement. A partir de mesures de la connectivite fondees sur des graphiques, la distance euclidienne entre les ilots forestiers peut etre directement reliee a la distance preferee de traversee d’une trouee par un oiseau (connectivite fonctionnelle). Nous avons determine la capacite predictive relative de l’habitat de nidification, du couvert forestier et des mesures de connectivite structurelle et fonctionnelle pour decrire la repartition de nidification de la Paruline a capuchon (Setophaga citrina) au cours de deux saisons de nidification successives dans une region tres fragmentee par l’agriculture, dans le sud de l’Ontario. Des modeles de regression logistique des tendances dans l’occurrence de nidification ont ete compares au moyen du critere d’information d’Akaike, et l’amplitude des effets relatifs a ete evaluee a l’aide du risque relatif approche (odds ratio en anglais). Nos resultats corroborent les suppositions selon lesquelles les caracteristiques du site de nidification sont vraiment reliees a la repartition de nidification de S. citrina. Toutefois, les modeles fondes sur l’habitat de nidification uniquement etaient 4,7 fois moins performants qu’un modele qui comportait la connectivite fonctionnelle comme variable predictive de la repartition de nidification de S. citrina. Les modeles fondes sur l’occurrence des nids en fonction du couvert forestier environnant etaient moins vraisemblables que les modeles qui incluaient la connectivite University of Toronto, Department of Ecology and Evolutionary Biology, University of Toronto, Ontario Program Manager Bird Studies Canada c/o Environment Canada, Wildlife and Landscape Science Directorate,