Seasonal variation in stopover site use: Catharus thrushes and vireos in northern Colombia

Abstract

Migratory routes and stopover sites used by migrant landbirds may vary among seasons and years. Understanding the degree of such variation is an important aspect of migrant ecology, especially in South America, where routes and sites are not well known. We studied stopover site use by two Vireo species and three Catharus species in the Sierra Nevada de Santa Marta in northern Colombia during a two-year period. We used capture totals corrected for mist-net effort and density estimates derived from variable distance transect observations performed at sites located at three different altitudes and thus habitats to describe variation in stopover site use. Four of the five study species showed significant seasonal variation in terms of presence, such that ecologically similar species were present during opposite migration periods; for example, Vireo flavoviridis was an autumn migrant, while Vireo olivaceus was commonest in spring. A similar pattern was observed for Catharus fuscescens and Catharus minimus. We detected differences in density with habitat/altitude, with C. minimus and Catharus ustulatus most abundant in pre-montane forest, while C. fuscescens abundance peaked in lowland forest. At the same altitude, Catharus species were more abundant in pre-montane forest than in shade coffee plantations. The abundance of C. ustulatus varied markedly between years during autumn migration, potentially in association with changes in the prevailing wind conditions. The temporal and spatial variation in stopover site use described here provides important insights into migratory strategies for Neotropical migrants within northern South America, and also regarding the multiple factors that may have shaped these strategies.