Drivers of migrant passerine composition at stopover islands in the western Mediterranean

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.

Stopover Ecology of the European Turtle Dove (Streptopelia turtur), a Threatened Migratory Bird Species, after the Crossing of an Extended Ecological Barrier

“…There is evidence that stopover sites and habitats are selected or avoided during migration [45][46][47][48]. The relatively large number of depleted birds observed in the eastern flyway after the sea crossing during the spring migration [21] indicates that at least a considerable portion of these birds choose their stopover sites, not based on a selection process but rather as a necessity.…”

Stopover Ecology of Passerine Birds on an Islet in the Bay of Biscay in Autumn Reveals a Pattern Similar to Their Use of Small Mediterranean Islands in Spring

“…It is relatively well known that, at least under certain meteorological circumstances, such as on days with strong southerly winds, many birds arrive exhausted at the southern end of the Bay of Biscay, stopping at the first piece of land they find. The landing of birds on islands during migration is a well-known phenomenon elsewhere, such as in the Mediterranean, where these small pieces of land in the vast sea constitute migratory stopover points for numerous species (Pilastro et al , 1998; Rubolini et al , 2005; Gargallo et al , 2011; López-Iborra et al , 2022). Often, and especially on smaller islands, these stopovers last no more than a few hours and their only function is for resting and recovery from physical exhaustion (Gargallo et al , 2011), acting as a ‘rescue boat’ (Sarà et al , 2023).…”