Identification of Putative Wintering Areas and Ecological Determinants of Population Dynamics of Common House-Martin (Delichon urbicum) and Common Swift (Apus apus) Breeding in Northern Italy

Abstract

ABSTRACT. To identify the causes of population decline in migratory birds, researchers must determine the relative influence of environmental changes on population dynamics while the birds are on breeding grounds, wintering grounds, and en route between the two. This is problematic when the wintering areas of specific populations are unknown. Here, we first identified the putative wintering areas of Common House-Martin (Delichon urbicum) and Common Swift (Apus apus) populations breeding in northern Italy as those areas, within the wintering ranges of these species, where the winter Normalized Difference Vegetation Index (NDVI), which may affect winter survival, best predicted annual variation in population indices observed in the breeding grounds in 1992-2009. In these analyses, we controlled for the potentially confounding effects of rainfall in the breeding grounds during the previous year, which may affect reproductive success; the North Atlantic Oscillation Index (NAO), which may account for climatic conditions faced by birds during migration; and the linear and squared term of year, which account for nonlinear population trends. The areas thus identified ranged from Guinea to Nigeria for the Common House-Martin, and were located in southern Ghana for the Common Swift. We then regressed annual population indices on mean NDVI values in the putative wintering areas and on the other variables, and used Bayesian model averaging (BMA) and hierarchical partitioning (HP) of variance to assess their relative contribution to population dynamics. We re-ran all the analyses using NDVI values at different spatial scales, and consistently found that our population of Common House-Martin was primarily affected by spring rainfall (43%-47.7% explained variance) and NDVI (24%-26.9%), while the Common Swift population was primarily affected by the NDVI (22.7%-34.8%). Although these results must be further validated, currently they are the only hypotheses about the wintering grounds of the Italian populations of these species, as no Common House-Martin and Common Swift ringed in Italy have been recovered in their wintering ranges. RÉSUMÉ. Afin de cerner les causes de déclin des oiseaux migrateurs, les scientifiques doivent déterminer l'influence relative des changements environnementaux sur la dynamique des populations, tant sur les aires de reproduction et d'hivernage que sur les haltes migratoires. Cette démarche est particulièrement problématique lorsque les aires d'hivernage de populations spécifiques sont inconnues. Dans la présente étude, nous avons d'abord déterminé les aires d'hivernage présumées des populations d'Hirondelle de fenêtre (Delichon urbicum) et de Martinet noir (Apus apus) qui se reproduisent dans le nord de l'Italie; ces aires présumées correspondent aux endroits, dans l'ensemble de l'aire d'hivernage de ces espèces, où l'indice de végétation par différence normalisée (IVDN) -qui affecte peut-être la survie hivernaleexpliquait le mieux la variation annuelle des indices de population observés sur les aires...

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