The spatial consistency of migratory route and stopover choice in
European nightjars quantified by nearest neighbour analysis on
multi-annual GPS tracks
Abstract
The degree to which avian migrants return to the same stationary sites
to mimic routes from previous years has received more and more attention
as the possibility of tracking small to medium avian migrants over
multiple annual cycles has increased. Repeated measurements of
individuals can potentially inform about their navigation and migration
strategies and to what extent the degree of variation observed within
and among individuals may reflect the selective potential in the
population. Here we perform a k-nearest neighbour analysis along with a
repeatability measure to distinguish events in the annual cycle with
intra-individual spatial convergence and to quantify the degree of
individual consistency and repeatability at those events. To demonstrate
the usefulness of our approach we analyse the annual space-use of
European nightjars (henceforth nightjars) Caprimulgus europaeus tracked
in multiple years between northern Europe and southern Africa. We found
that the nightjars consistently used the same breeding and wintering
sites but that individual route choice during migration were flexible,
but significantly repeatable relative to population level variation
during the Sahara-crossing. Thus, the nightjars followed
individual-specific flyways while allowing for variation of a few
hundred kilometres in the actual route in both autumn and spring. The
exception was a strong within-individual convergence down to a few tens
of kilometres recorded at the initiation of the trans-Saharan flight in
spring. Our results suggest that nightjars have incorporated an
individual-specific space-use within their annual cycle, but that they
allow for a state-dependent flexibility possibly driven by the
cost-benefit balance between the use of known stationary sites and an
economical route-choice.