Data from: The avian lightweights: trans-Saharan migrants show lower lean body mass than short-/medium-distance migrants

Avian trans-Saharan migrants travelling long distances and crossing
ecological barriers experience different constraints in terms of time,
energy and safety than short-/medium-distance migrants without
barrier-crossings. As such, natural selection shapes the aerodynamic
properties of these groups differently. Yet, to the best of our knowledge,
we lack information on whether natural selection has contributed to
reducing energetic flight costs through generally lower body mass in
trans-Saharan migrants. To fill parts of this gap, we investigated this
eco-morphological pattern in 5,410 individuals of 22 Palearctic songbird
species ranging from short-/medium-distance to trans-Saharan migrants. We
used individual size-independent scaled lean body mass values based on
wing length as a measure of body size and, for the first time, precisely
determined lean body mass values by direct measurements via quantitative
magnetic resonance technology. Scaled lean body mass for a given body size
was significantly higher in short-/medium-distance migrants than in
trans-Saharan migrants. While scaled lean body mass significantly
decreased with increasing migration distance in short-/medium-distance
migrants, no such effect was found in trans-Saharan migrants. Our results
thus show an eco-morphological pattern relating species’ lean body mass
not only to migration distance but also to migration group. This suggests
that selective effects of the presence/absence of ecological barriers
and/or of a threshold level for migration distance on migrant birds may be
more important than the linear continuum of migration distance per se.