An international team of researchers underwent one of the most comprehensive analyses of bird wings in an effort to unravel patterns of dispersal. The results suggest that the enormous variation in bird shape and function can be linked to both environmental and social factors.
According to a previous study performed by the American Museum of Natural History, there are over 18,000 species of birds in the world.
Birds can be found on land, over oceans and fresh water, and in virtually every habitat, from the lowest deserts to the highest mountains.
However, the distribution of birds across the globe is uneven, with some regions experiencing far richer avian biodiversity than others. For instance, the neotropical realm (extending south from the Mexican desert into South America as far as the subantarctic zone) is home to ten times as many species of birds as the Oceanic realm (the islands of the Pacific Ocean).
There's also a great variation in the way bird species disperse, in the sense of the geographical distance that they cover. The Arctic tern (Sterna paradisaea) is famous for its long migratory journeys from pole to pole all the way from the Arctic to the Antarctic, while the inaccessible island rail (Atlantisia rogersi) never leaves the confinements of its five-square-mile habitat of Inaccessible Island in the Tristan Archipelago, south Atlantic.
Knowing how various species of birds move around is a key factor in understanding biodiversity at large and improving wildlife conservation outcomes. In this sense, measurements of wing shape -- most importantly the 'hand wing index' that reflects the elongation in the wing -- can inform biologists about how different bird species have adapted for long-distance flight.
Since 2012, Dr. Catherine Sheard of the University of Bristol's School of Earth Sciences has been combing through the collections of various natural history museums, painstakingly cataloging and measuring bird wings.
"I was privileged to be able to measure specimens collected by Darwin and Wallace, to hold specimens of extinct species and specimens representing the only sample known to science, and to personally handle far more avian biodiversity than I could ever see in several lifetimes of watching living birds," Sheard told ZME Science in an email.
"To set the scene for anyone who has never had the opportunity to go behind the scenes into a museum collection, they're dimly lit rooms, sometimes windowless, kept very chilly to best preserve the collections. Many older bird skins were treated in arsenic, so once you start touching birds, you have to be very careful not to touch your face. Some specimens are filthy; some of them smell. It's glorious!"
The massive collaboration that involved over 90 researchers and 64 natural history museums analyzed the hand wing index for 45,801 specimens corresponding to over 10,000 species of birds. This is the most comprehensive study of a dispersal-linked trait published thus far.
"We embarked on this study out of an interest in "dispersal", or the geographic distance an animal moves, which is a key process in ecology and evolution. Dispersal is very difficult to measure directly, and thus for most species we have little idea how much they move around. The shape of a bird's wing, however, provides a proxy for their flight ability, and thus for their dispersal; these measurements can be taken on museum study skins without damaging the specimen. Thus, we could build a database of a dispersal-linked trait for nearly all living and recently-extinct birds, enabling the study of wing shape, flight, and dispersal at an unprecedented scale," Sheard said.
The researchers found a strong latitudinal gradient in adaptations for avian flight. Namely, birds with more elongated wings have adapted for long-distance flight and are largely found in higher latitudes. Tropical birds have wings that are more adapted for a sedentary lifestyle.
This geographical gradient is primarily driven by three factors: temperature variability, territory defence, and migration.
"My personal biggest ah-ha moment was the tight link between wing shape and territory defense. The other two strongest correlations, migration and environmental variability, are both well-predicted in the literature -- while it was very satisfying to demonstrate it on an unprecedented scale, I don't think any of us found it particularly surprising. The ecological theory, though, linking dispersal and territoriality is much less extensive, and I was honestly expecting this relationship to be a by-product of some of the other hypotheses we were testing. Instead, though, social behaviour seems to be playing an important role in the evolution of long-distance flight," Sheard said.
It is quite possible that the relationship between the environment, dispersal, and behavior is driving other aspects of biology. This is something that would have to be investigated by new studies.
In the meantime, these findings will prove important in wildlife conservation -- something of critical importance in the context of ever-dwindling bird habitats and the acceleration of climate change.
"By studying the evolutionary drivers of wing shape and flight ability, we hope to be able to better predict species' responses to, for example, habitat fragmentation or shifting migratory routes," Sheard said.
"Furthermore, dispersal – or the geographic distance that an animal moves – is a key component of many ecological and environmental processes, from gene flow and speciation to geographical range expansion and community assembly. Here we've demonstrated that a prominent biogeographic pattern in dispersal ability is shaped by a combination of environmental and behavioural factors, suggesting potential links between dispersal and other latitudinal gradients and many avenues for future study."
Sheard and colleagues hope that their work will inspire other scientists to study their database and come up with new findings of their own.
"We've made our entire dataset available open access, and we look forward to seeing the different uses the ornithological community might have for metrics such as ours that can estimate dispersal across many different species," she added.
The findings were reported in the journal Nature Communications.