A study with thousands of birds that have been measured and weighed for 40 years shows that birds are getting smaller and smaller. At the same time, to compensate for the dwarfing, they are lengthening their wings. The study authors only find an explanation for this double phenomenon: the warming caused by climate change.
Since 1978, ornithologists of Field Museum Natural History (Chicago, USA) perform a somewhat macabre rescue operation. With the beginning of the spring and autumn migrations of millions of birds that fly over the city, they leave the museum and leave at the foot of the tallest skyscrapers. Dozens of birds that hit the dazzling windows of the buildings fall to the ground. Scientists can do little for them except classify them by species, sex, age, weigh them or measure the wingspan of their wings. They have already cataloged more than 100,000 copies of 52 migratory species.
Now, when reviewing a sample of 70,000 birds preserved in the museum, a group of researchers has noticed that the birds are changing. Thus, as published in Ecology Letters, all species have reduced their mass, with an average of 2.6%. All have also shortened the dimensions of the tarsus, the set of bones at the base of the leg, by 2.4%. In parallel, 40 of the species have enlarged their wings significantly, with an average of 1.3%.
The species would be responding to heat with a smaller size and larger wings to compensate for the metabolic cost of the reduction
"Body size is a key morphological feature that determines how one species interacts with others and the environment, so a change in size is key to the ecology of the species," recalls the University of Michigan ornithologist and co-author of I study Brian Weeks. "In migratory birds, which are adapted to carry out extremely demanding seasonal migrations from the physiological point of view, a reduction in body mass may be especially relevant, since it affects metabolic efficiency," he adds.
The authors of the study, in fact, relate these morphological changes to each other. "We believe that the magnitude of the reduction in body size is significant since it seems that it would have caused the increase in the length of the wings," says Weeks. By getting smaller, the migration would become harder and, to compensate, they would have enlarged their wings seeking greater efficiency in the flight. The next thing was to look for why.
"When we began to collect the data analyzed in this study, we were interested in aspects of interannual variations or from season to season in birds. The term climate change as a modern phenomenon was barely visible on the horizon," says Dave Willard, the first in collecting dead birds 40 years ago and now responsible emeritus of the ornithological collection of the museum and co-author of the study.
For the authors, climate change is the only thing that can explain the dwarfing of birds. The studied species breed north of Chicago, in the temperate forests and boreal region of Canada and spend the winter south of the city, in warmer areas of the United States or even below. To the north, the average regional temperature has varied to a degree. Although they analyzed other variables, such as the availability of resources, the one that best correlated with changes in mass and wings was that of temperatures.
"The increase in temperature in the breeding areas appears associated with the reduction in body size," Weeks recalls. This connection is not new, it was postulated in the 19th century and ecologists know it as Bergmann's rule. In its summary version, it says that, within the same species, populations or subspecies that live in warmer areas tend to be smaller than those in colder areas. The differences will have to do with a better adaptation to the thermal environment. The different thing is that the adaptation would be caused by human action.
However, the pattern observed in the US does not seem universal. In Spain, a group of scientists have been studying two populations of nightingales for 20 years. They nest in the south of Madrid after a long migration from their winter barracks, in the African Sahel. "Unlike what they have found in the US, we have registered a shortening of the wings and a slight increase in size," says the researcher at the Complutense University, Javier Pérez-Tris.
The Spanish ecologist presented the preliminary results of his study with the nightingales at the recent National Ornithology Congress of SEO / BirdLife. They have seen that more and more nightingales arrive after flying thousands of kilometers, a trip that should favor those with longer wings. The underlying processes or the connection between the different morphological features that are generating such opposite patterns are not clear, but, as Pérez-Tris says, "the global phenomenon is that birds are changing."
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