Ecological drivers of global gradients in avian dispersal inferred from wing morphology

An organism’s ability to disperse influences many fundamental processes, from speciation and geographical range expansion to community assembly. However, the patterns and underlying drivers of variation in dispersal across species remain unclear, partly because standardised estimates of dispersal ability are rarely available. Here we present a global dataset of avian hand-wing index (HWI), an estimate of wing shape widely adopted as a proxy for dispersal ability in birds. We show that HWI is correlated with geography and ecology across 10,338 (>99%) species, increasing at higher latitudes and with migration, and decreasing with territoriality. After controlling for these effects, the strongest predictor of HWI is temperature variability (seasonality), with secondary effects of diet and habitat type. Finally, we also show that HWI is a strong predictor of geographical range size. Our analyses reveal a prominent latitudinal gradient in HWI shaped by a combination of environmental and behavioural factors, and also provide a global index of avian dispersal ability for use in community ecology, macroecology, and macroevolution.


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Policy information about availability of data All manuscripts must include a data availability statement. This statement should provide the following information, where applicable: -Accession codes, unique identifiers, or web links for publicly available datasets -A list of figures that have associated raw data -A description of any restrictions on data availability Field-specific reporting Please select the one below that is the best fit for your research. If you are not sure, read the appropriate sections before making your selection. Wing morphological measurements for all extant and recently-extinct species for which we were able to find museum specimens or capture live birds (n=10,338 species, >99% of all extant birds). Measurements were taken from adult individuals, where possible in at least 2 males and 2 females, to capture a representative species average. Range and migration data is publicly available from birdlife.org; climate data from worldclim.org; phylogenetic data from birdtree.org; and territoriality, habitat use, and additional migratory data from Tobias et al. 2016.
Morphological measurements were obtained for all available bird species. An average of 4.43 adult individuals were measured per species to provide reliable mean species values.
Morphological data were obtained using callipers and wing rulers from live caught individuals and preserved museum skins, by the nine authors and the 90 collaborators and assistants thanked in the Supplementary Notes. Ecological and social data were obtained from the literature, and biogeographic and climate data from BirdLife International, WorldClim, and other sources, by CS and JAT.
Timing: Data is based on museum collections spanning the last~200 years and on recent fieldwork. There was no systematic sampling based on museum specimen age, other than selecting specimens in good condition to better preserve the collections. Most measurements were taken 2011-2015, with some as early as the mid-1990s and as late as 2018.
Specimens labelled or identified in-hand as juvenile were excluded, as we were interested in intraspecific variation in adult morphology/behaviour and not in ontogeny. This exclusion was pre-determined.
Inter-measurer repeatability was measured by taking replicate measurements for 220 specimens for 146 species; measurer identity explained 0.01-0.5% of the variation in measured traits. Differences in measurements taken on museum specimens and live birds explained 0.002-0.35% of the variation in measured traits, across 4,018 specimens in 362 species. Analyses run on two alternative sets of migration scores produced minimal differences in results.

NA
No laboratory animals were used Most data are from museum specimens. Some data are included from wild-caught birds that were not harmed during data collection and subsequently released into the wild. In all cases, birds were caught by mist-netting, a passive, non-invasive technique which does not harm the individual birds.