The interplay of wind and uplift facilitates over-water flight in facultative soaring birds

Flying over the open sea is energetically costly for terrestrial birds.
Despite this, over-water journeys of many birds, sometimes hundreds of
kilometers long, are uncovered by bio-logging technology. To understand
how these birds afford their flights over the open sea, we investigated
the role of atmospheric conditions, specifically wind and uplift, in
subsidizing over-water flight at the global scale. We first established
that ∆T, the temperature difference between sea surface and air, is a
meaningful proxy for uplift over water. Using this proxy, we showed that
the spatio-temporal patterns of sea-crossing in terrestrial migratory
birds is associated with favorable uplift conditions. We then analyzed
route selection over the open sea for five facultative soaring species,
representing all major migratory flyways. The birds maximized wind support
when selecting their sea-crossing routes and selected higher uplift when
suitable wind support was available. They also preferred routes with low
long-term uncertainty in wind conditions. Our findings suggest that, in
addition to wind, uplift may play a key role in the energy seascape for
bird migration that in turn determines strategies and associated costs for
birds crossing ecological barriers such as the open sea.