Flight altitude dynamics of migrating European nightjars across regions and seasons

Avian migrants may fly at a range of altitudes, but usually concentrate
near strata where a combination of flight conditions is favourable. The
aerial environment can have a large impact on the performance of the
migrant and is usually highly dynamic, making it beneficial for the bird
to regularly check the flight conditions at alternative altitudes. We
recorded the migrations between northern Europe and sub-Saharan Africa of
European nightjars Caprimulgus europaeus to explore their altitudinal
space use during spring and autumn flights to test whether their climbs
and descents were performed according to predictions from flight
mechanical theory. The nightjars commonly operated at ascent rates below
the theoretical maximum, and periods of descent were commonly undertaken
by active flight, and rarely by energetically cheaper gliding flight,
allowing the birds to sink at lower rates than possible during a best
glide scenario. Spring migration across all regions was associated with
more exploratory vertical flights involving major climbs, a higher degree
of vertical displacement within flights, and less time spent in level
flight, although flight altitude per se was only higher during the Sahara
crossing. This study demonstrates a surprisingly frequent use of slow
vertical displacements during migratory flights of a long-distance avian
migrant and show how these behaviours vary between seasons and regions,
presumably in response to different flight conditions. The results should
inspire future studies on the potential costs associated with frequent
altitude changes and their trade-offs against anticipated flight condition
improvements for aerial migrants.