Birding
Why are some birds getting smaller, even as their wings get longer? Climate change, peregrine recovery, adaptations are some factors believed to be behind the changes
Why are some birds getting smaller, even as their wings get longer? Climate change, increased predation and adaptations to both. What does it mean?
It’s bad enough that we have to contend with shrinking bird populations, but shrinking birds as well?! A study published in Ecology Letters in December 2019 has discovered that many migratory bird species in North America are getting smaller in body size, even as their wings are getting longer. A team of scientists, led by conservation ornithologist Brian Weeks at the University of Michigan, analyzed more than 70,000 specimens collected over 40 years by Chicago’s renowned Field Museum. Many had been killed after striking building windows in the city during spring and fall migration, so they represented an excellent sampling of healthy populations over time. (continued)
Weeks and his colleagues found that between 1978 and 2016, body size dropped significantly in 49 of 52 bird species examined. This was based on measuring the lower leg bone, generally considered the most precise measure of body size variation within species. Interestingly, they also found wing length had increased significantly in 40 species.
Is climate change behind these changes? For the most part, studies on climate change have focused on alterations to timely events, such as asynchronous timing between food availability and raising young, or behaviour changes, like examining northward shifts in range due to warming trends. Birds evolving smaller body sizes in the face of warming temperatures makes sense when one considers Bergmann’s rule, a general biological concept stating that species of animals living in colder climates tend to have larger bodies than their counterparts in warmer southern regions. According to the rule, animals with large bodies generally have lower surface areas relative to mass than those with smaller bodies and, therefore, proportionally do not radiate as much heat. So, while migratory birds may be evolving smaller bodies to give off heat in a warming climate, this could also mean a lessened ability to store body fat and thus less energy available to make the long journey. Could evolving longer wings somehow compensate for it?
Ron Ydenberg, a professor at Simon Fraser University in British Columbia, would say no, I expect. In his study measuring wingspans of two species of long-distance migratory sandpipers that breed in the Arctic, Ydenberg discovered that the wingspan of the western sandpiper has been lengthening, while the wingspan of its eastern equivalent, the semipalmated sandpiper, has been getting shorter. He postulates these wing length changes might be attributable to… peregrine falcons. Or rather, changes in the two shorebird species’ encounters with these winged predators during migration.
Peregrines have made a huge comeback since the banning of organochlorine chemicals like DDT, and they are a major hunter of both these shorebirds. Ydenberg suggests the western sandpipers are evolving longer wings for energyefficient long-distance flight, enabling them to fly faster to avoid falcons during migration, while the eastern sandpipers’ shorter, rounder wings would help with acceleration and agility to elude attacking peregrines.
My (educated) guess is that these startling findings will lead to a flock of follow-up studies on bird species all over the world. We will be hearing a lot more about this, I expect, both findings and theories, before we get to the long and the short of IT.