Stable atmospheric conditions underlie a steady pace of nocturnal bird migration in the tropics

Seasonal migrations between temperate and tropical latitudes have evolved repeatedly across the tree of life, but we know little about how tropical environments have shaped this life history strategy. In this first aeroecological radar study in South America, we tested whether tropical atmospheric stability leads to a more continuous pace of nocturnal bird migration across a season than the episodic pulses of migration in temperate regions. We estimated and compared the volume, flight directions and heights of birds detected by weather radars and acoustic sensors in three regions of Colombia across four years and assessed the effects of atmospheric conditions. We found that stable tropical winds have distinct effects on migration timing depending on geographic context. Within the Andes mountains, supportive winds explain the altitudes at which birds fly, but not the volume of birds aloft on a given night, resulting in lower nightly variation in migration intensity. However, lapses in prevailing headwinds east of the Andes drove higher variation in migration intensity similar to temperate latitudes. With a reduced relationship between forecastable weather and the magnitude of nightly migration, predicting high volume migration events to target for conservation initiatives may be more challenging in tropical climates.