DOI: 10.5061/dryad.31zcrjdk7
title: Data from: The avian lightweights: trans-Saharan migrants show lower lean body mass than short-/medium-distance migrants

description: 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.
description: During spring migration 2017–2019 and autumn migration 2017–2018,
 5,410 individuals of 22 Palearctic songbird species were routinely caught
 in the trapping garden of the Institute of Avian Research ‘Vogelwarte
 Helgoland’ on Helgoland (54°11’N, 07°55’E). Body mass
 (±0.1 g) using an electronic balance and maximum wing length (i.e. maximum
 chord, ±0.5 mm), as a measure of body size, were taken for each bird.
 Hereafter, we used the EchoMRI™ (EchoMRI Body Composition Analyser
 E26-262-BH, Zinsser Analytic GmbH, Frankfurt am Main, Germany) to directly
 measure for each bird (1) QMR fat mass, i.e. all fat molecules, and (2)
 QMR lean mass, i.e. the sum of all water-containing body parts. QMR fat
 mass was subtracted from the actual body mass to calculate the individual
 lean body mass. For each species, mean ± standard deviation was determined
 for actual body mass and lean body mass (g) as well as wing length
 (mm). We classified birds according to two main
 migration groups: (1) short-/medium-distance migrants (pre-Saharan
 migrants) and (2) long-distance migrants (trans-Saharan migrants). Mean
 migration distance was estimated between the centre of the breeding and
 wintering area for each species (rounded up to the nearest 50
 km).  
description: One special case is the <i>Oenanthe oenanthe</i> L.
 (Northern Wheatear) because two subspecies with significantly different
 migration distances pass Helgoland during migration. We generated the mean
 migratory distance for each subspecies (<i>leucorhoa</i> =
 7,750 km; nominate = 5,850 km) and then used the average distance of these
 two (= 6,800 km). Additionally, we were able to sample
 the local breeding population of the House Sparrow. Ít usually stays on
 the island year-round and can, therefore, be considered “resident”. As
 many “resident” and pre-Saharan species show partial migration, where
 transition between dispersal movements and true migration is flexible, and
 only one resident species was considered in this study, we included the
 House Sparrow with an average migration distance of 0 km in the group of
 pre-Saharan migrants.