New research on wing shape has shown that lifstyle and environment is key to how adapted birds are to flying.
By studying the wings of some 45,000 birds from 10,000 species, researchers have shown that birds living at higher latitudes are better equipped to disperse and, thus, migrate.
The gradient in average dispersal ability – which is high at higher latitudes and low near the equator – was best estimated by whether birds were migratory or how they defended a territory. Both of these factors correlate highly with variability in temperature (seasonality), the study found. For example, species found at low latitudes tend to live in relatively stable and unchanging climates, which makes them rather sedentary, resulting in low levels of migration and a high year-round defence of their territories.
The results of the research could impact conservation – with tropical birds less able to cross gaps in habitat or to disperse over larger distances, they may die out more readily when habitat is destroyed or broken into fragments (such as through deforestation).
The new research, published in Nature Communications, provides the first comprehensive study of dispersal ability across an entire class of animals. Two extreme examples are Arctic Tern and Inaccessible Island Rail. The former flies from the Arctic to the Antarctic and back again each year, while the latter – the world's smallest flightless bird – never leaves its five-square-mile island. This variation in how much different animals move around is a key factor in understanding and conserving biodiversity.
However, tracking animals even over short distances is difficult and expensive, meaning there are still huge gaps in knowledge about animal movements and dispersal, particularly in the tropics where most species are very poorly known. To address this, a global team of researchers measured the wing shapes of 45,801 birds in museums and field sites around the world. These measurements – particularly a metric called the 'hand wing index', which reflects the elongation of the wing – help researchers work out how well different wings are adapted for flight.
From their measurements, the team created a map of the global variations in wing shape, showing that the best-adapted fliers are primarily found in high latitudes while birds adapted to more sedentary lifestyles are generally found in the tropics. The study’s lead author, Dr Catherine Sheard at the University of Bristol, said: "This geographic pattern is really striking. Given the role we know dispersal plays in evolutionary processes, from speciation to species interactions, we suspect this relationship between behaviour, the environment, and dispersal may be shaping other aspects of biodiversity."
Dr Joseph Tobias, senior author of the study, based at Imperial College London, added: "We hope our measures of wing shape for almost all bird species will have numerous practical applications, particularly in ecology and conservation biology."
Sheard, C, Neate-Clegg, M H C, Jones, S E I, Vincent, C, Bregman, T P, Claramunt, S & Tobias, J A. 2020. Ecological drivers of global gradients in avian dispersal inferred from wing morphology. Nature Communications, 11. DOI: https://doi.org/10.1038/s41467-020-16313-6