Migratory connectivity and barrier-crossing flights of Vermivora warblers are associated with synoptic weather conditions.

Migratory birds often navigate inhospitable barriers (e.g. oceans, deserts) during migration. Barrier crossings are frequently associated with increased rates of mortality and likely impose selective pressures on migratory species that shape their behaviour and distribution. Therefore, understanding how weather conditions influence the behaviour of migratory birds at a major barrier can provide insight into the adaptive evolution of long-distance migrations involving barrier crossings and how changing climatic conditions might affect migratory species in the future. We used light-level geolocator data from 89 individual Vermivora warblers to identify the weather conditions associated with individuals initiating barrier-crossing flights across the Gulf of Mexico (i.e. 'trans-Gulf flights') during both autumn and spring migrations from 2013 to 2017. Weather conditions associated with the initiation trans-Gulf flights differed between autumn and spring. In autumn, the initiation of trans-Gulf flights was positively associated with favourable wind conditions and temperature but negatively associated with relative humidity and 24-h change in barometric pressure. During spring migration, the initiation of trans-Gulf flights was negatively associated with surface-level relative humidity and barometric pressure but not associated with wind conditions. We found that the frequency of days with weather conditions associated with a high-predicted probability of Vermivora warblers initiating trans-Gulf flights varied geographically (range 0%-58% of days). Distinct breeding populations of golden-winged warblers (V. chrysoptera) with strong migratory connectivity between breeding and non-breeding regions exhibited weak migratory connectivity and overlapped extensively during migration immediately prior to initiating trans-Gulf flights. Breeding populations of blue-winged warblers (V. cyanoptera) exhibited weak migratory connectivity and co-occurred during both autumn and spring migrations and during the non-breeding period. The weak migratory connectivity that we observed in Vermivora warblers prior to crossing the Gulf of Mexico may be shaped by shared evolutionary responses to consistent synoptic weather conditions in the region. Predicted future climate conditions including increased humidity and more frequent and/or severe storms may decrease the favourability of conditions associated with initiating trans-Gulf flights during spring migration for Vermivora warblers, which could negatively affect populations.