Spatio-Temporal Overlap between Local and Non-Local Reed WarblersAcrocephalus scirpaceusDuring the Autumn Migration Period

Abstract

During the autumn migration period, northern populations commonly reach southern stopover sites where they meet conspecifics that may still be breeding or at least still at their breeding sites. Establishing whether local breeding birds overlap temporally with migrants is essential to studying the strategies adopted by the former when they have to suddenly cope with conspecific influxes. Our objectives here were (1) to establish the phenology of local and transient birds within the study area and (2) to test whether biometrics can differentiate between these two groups. Local reed warblers were observed nesting until the end of July; these birds were recaptured up to late August (although very few later than mid-August), suggesting that at least some individuals depart from the area before most nonlocals arrive. Local breeding birds were found to have longer wings than non-locals. A discriminant function classified 73.4% of the non-local and local reed warblers correctly, indicating considerable morphological overlap between the two groups.