Data from: Wind-associated detours promote seasonal migratory connectivity in a flapping flying long-distance avian migrant

1. It is essential to gain knowledge about the causes and extent of
migratory connectivity between stationary periods of migrants to further
the understanding of processes affecting populations, and to allow
efficient implementation of conservation efforts throughout the annual
cycle. Avian migrants likely use optimal routes with respect to mode of
locomotion, orientation and migration strategy, influenced by external
factors such as wind and topography. In self-powered flapping flying birds
any increases in fuel loads are associated with added flight costs.
Energy-minimizing migrants are therefore predicted to trade-off extended
detours against reduced travel across ecological barriers with no or
limited foraging opportunities. 2. Here we quantify the extent of detours
taken by different populations of European nightjars Caprimulgus
europaeus, to test our predictions that they used routes beneficial
according to energetic principles and evaluate the effect of route shape
on seasonal migratory connectivity. 3. We combined data on birds tracked
from breeding sites along a longitudinal gradient from England to Sweden.
We analysed the migratory connectivity between breeding and main
non-breeding sites, and en route stopover sites just south of the Sahara
desert. We quantified each track’s route extension relative to the direct
route between breeding and wintering sites, respectively, and contrasted
it to the potential detour derived from the barrier reduction along the
track while accounting for potential wind effects. 4. Nightjars extended
their tracks from the direct route between breeding and main non-breeding
sites as they crossed the Mediterranean Sea – Sahara desert, the major
ecological barrier in the Palaearctic-African migration system. These
clockwise detours were small for birds from eastern sites but increased
from east to west breeding longitude. Routes of the tracked birds were
associated with partial reduction of the barrier-crossing resulting in a
trade-off between route extension and barrier reduction, as expected in an
energy-minimizing migrant. 5. This study demonstrates how the costs of
barrier crossings in prevailing winds can disrupt migratory routes towards
slightly different goals, and thereby promote migratory connectivity. This
is an important link between individual migration strategies in
association with an ecological barrier, and both spatially and demographic
population patterns.