Weather conditions affect spring and autumn migration of Siberian leaf warblers

Weather effects on bird migration are well-studied among Passerines moving from Europe to Africa or within the American flyway systems. However, little is known about the weather impact on songbirds migrating along the East Asian flyway. Our study aims to describe the effects of various weather elements on the migration of four species of leaf warblers by using bird ringing data from a stopover site in Far East Russia. We determined the migration periods for each species and included maximum temperature, precipitation, air pressure, wind speed and wind direction in general linearized mixed models to predict the number of migrating birds. We found strong impacts of weather variables on the number of trapped warblers during spring and autumn migration. Preferred or avoided weather conditions were similar among the studied species. All species seem to migrate preferably during warm, calm days without precipitation. A positive effect of tail winds was only confirmed in autumn, but in spring, most birds were trapped during crosswinds (eastern or western winds). The studied species might exhibit a loop migration, leading to a more longitudinal (from east to west) migration pattern in our study area during spring. Relationships between weather variables and the number of migrating individuals were much stronger during autumn. We argue that birds during spring migration would continue migration under sub-optimal conditions, as a result of strong competition to arrive earliest on their breeding grounds.