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.