The aeroecology of atmospheric convergence zones: the case of pallid swifts

Abstract

Trans‐Saharan migratory bird species encounter large scale seasonal atmospheric convergence zones, where opposing monsoon and continental air masses meet. These macro‐scale atmospheric conditions determine local weather, influence migratory and foraging behaviour and seasonal bird survival rates. Here, we investigate the flight behaviour of pallid swifts Apus pallidus, a small aerial insectivore, in relation to non‐breeding season atmospheric conditions using state‐of‐the‐art GPS logged data. Our analysis shows two novel diurnal flight patterns which suggest that pallid swift prey on insects concentrated along frontal convergence zones, in particular the continental Inter‐Tropical Convergence Zone (ITCZ) and a coastal sea‐breeze front. Resource use seems not only contingent on the abundance of insects, but also favourable atmospheric conditions. Persistence of swifts in wintering feeding grounds might therefore depend on the prevailing atmospheric conditions and their concentrating effects on insects rather than solely the vegetation state and co‐dependent insect populations. Migration events within, to and from, the non‐breeding season foraging locations might not only be guided by a decline in vegetation as common metric for prey availability, but also by shifting wind directions and their concentrating effects.