Climate change and maladaptive wing shortening in a long-distance migratory bird

Contemporary phenotypic trends associated with global change are widely
documented, but whether such trends always denote trait optimization under
changed conditions remains obscure. Natural selection has shaped the wings
of long-distance migratory birds to minimize the costs of transport, and
new optimal wing shapes could be promoted by migration patterns altered
due to global change. Alternatively, wing shape could vary as a correlated
response to selection on other traits favoured in a changing environment,
eventually moving away from the optimal shape for migration and increasing
transport costs. Data from 20 years of monitoring of two Common
Nightingale (Luscinia megarhynchos) populations breeding in central Spain,
where environmental conditions for breeding have deteriorated during the
last decades due to increased summer drought, show that birds have reduced
wing length relative to body size over the period 1995-2014. However,
long-winged nightingales survived their first round-trip migration better,
and the shorter the average wing length of individuals, the stronger the
survival-associated natural selection favouring longer wings. Maladaptive
short wings may have arisen because the mortality costs of migration are
outweighed by reproductive benefits accrued by short-winged nightingales
in these populations. Assuming that the phenotypic integration of
morphological and reproductive adaptations of migratory birds has a
genetic basis, we hypothesize that the maladaptive trend towards shorter
wings may be a correlated response to selection for moderate breeding
investment in drying habitat. Our results provide evidence that
contemporary phenotypic change may deviate average trait values from their
optima, thereby increasing our understanding of the ecological constraints
underpinning adaptation to rapid global change.