Abstract
Clues used by migrant birds to select sites for stopover are much less known than their reasons for leaving. Habitat characteristics and geographical location may affect the decision to use an island as a stopover site in different ways for different species. Thus, abundance and composition of migrants may be expected to differ between islands. Using standardized ringing from 9 western Mediterranean islands we evaluate drivers of abundance of trans-Saharan migrant passerines, specifically the role of species continental abundance, island characteristics and geographical location. Although continental abundance is a main driver of migrant composition on all islands migrant composition differs between them. Redundancy analysis and species response models revealed that the main drivers were distance to the nearest land toward the south, which has a positive effect on the number of migrants of most species, and island area, which appears as an important cue used for selecting a stopover island. Species whose abundance is positively related to island area have more pointed wings while species affected by distance to land toward the south have relatively more rounded wings. This suggests a hypothesis on the mechanism that may generate differences in passerine migrant composition between islands based on better efficiency of more pointed wings for long-distance flight.
Highlights
Clues used by migrant birds to select sites for stopover are much less known than their reasons for leaving
Several studies have identified some important variables for mainland stopover sites, such as habitat characteristics, in particular forest c over[8,13,14,15], and meteorological factors, such as rain[16], but such factors are far less understood for island stopover sites
Wing morphology is related to the efficiency and range of flight[18,19] so that flight performance related to wing shape could influence travel time or distance when crossing wide barriers[20] and is expected to affect species responses to island characteristics
Summary
Clues used by migrant birds to select sites for stopover are much less known than their reasons for leaving. If habitat characteristics affect the decision to use a stopover site in different ways for different species, it may be expected that abundance and composition of migrants will differ between islands. The average distribution of these fluctuating variables depends on other island features that are more stable over time Some of these features are related to the geographical location of the islands[17], which determines its location relative to the main migration flyways, their distance to the mainland source of migrants, prevailing winds and, to a large extent, their climate. The interaction between the particular characteristics of an island, the specific habitat selection rules of the different migrant species and the physical condition of individuals should determine the probability of the decision to land (Fig. 1). Stable and changing island features interact to determine the abundance and composition of the migrants that stop on a given island each day along a migratory passage, and the number of migrants landing on an island is highly variable on a daily s cale[23]
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