Cause and effect are hard to pin down with scientific rigor, but the facts are that global warming is happening and birds are shrinking. At least, the dead birds plummeting to the sidewalks of Chicago after crashing into buildings over nearly 40 years shrank. But weirdly and unexpectedly, their wings are getting longer.
And how was this discovered? Instead of being relegated to the nearest dump, the deceased avians were collected and measured, and the results of the work — led by the University of Michigan — were reported Wednesday in Ecology Letters.
The data consists of over 70,000 dead birds belonging to 52 different species removed from the streets by Field Museum personnel and volunteers during the spring and autumn migration seasons from 1978 to 2016. The scientists made multiple body measurements of each corpse to identify trends in body size and shape.
All had shrunk in size during those 38 years — an extraordinary uniformity of result given that different species ought to react to change in different ways. There were statistically significant declines in 49 of the 52 species, the biologists say.
They had anticipated some shrinkage as temperatures rise, but were shocked at the consistency. “I was incredibly surprised that all of these species are responding in such similar ways,” said study lead author Brian Weeks, an assistant professor at the University of Michigan School for Environment and Sustainability.
Why? In fact, climbing temperature is expected to cause animals in general to shrink.
“Within species, there is a tendency for individuals to be smaller in the warmer parts of a species’ range; this is called Bergmann’s rule,” Weeks explained to Haaretz. “This has led to the hypothesis that as the world warms, animals may get smaller.”
The classic explanation is that the bigger the animal, the smaller their skin-to-body-volume ratios. Thus, the larger they are, the easier it is to stay warm. Conversely, this is thought to be why massive animals like rhinos, hippos and elephants, for example, largely if not completely lost their fur: to preclude overheating.
But this classic reasoning wouldn’t apply to migratory birds, which just flap off when the weather gets chilly. So why did they shrink?
Hard to say. Apparently it’s because the breeding grounds are getting hotter, Weeks says.
Extending the wings
The consistency of the phenomenon is food for thought. Nature is a messy beast and the parameters involved are legion. And though there had been evidence of animals shrinking with rising temperature, biological data are typically noisy.
“Responses don’t tend to be this consistent among species,” Weeks says. “That is common in ecological and evolutionary studies — the natural world is a complicated place that tends to be characterized by highly idiosyncratic responses among species. However, in our study there were near-universal declines in body size across the species … and to find such a universal change was remarkable.”
Meanwhile, over those 38 years, wing length increased significantly in 40 species. That had not been anticipated.
“Wing length and body size are positively correlated, so we thought that as body size got smaller, wing length would also get shorter,” he says. “The increase in wing length represents a change in body shape that may have important ecological consequences.”
Asked why he thinks the wings grew longer as the birds grew smaller, Weeks says he thinks it’s a metabolic issue. The smaller the bird, the harder it is to migrate long distances because of the reduced metabolic efficiency that comes with reductions in body size, he explains: “Increases in wing length can improve flight efficiency. So essentially, what we think is happening is that climate change is shrinking the birds. And then in order to maintain migration while smaller, they need to be more efficient fliers and are achieving that increase in flight efficiency through an increase in wing length.”
Supporting that theory, the species whose foot bones (tarsus) shrank the fastest also showed the most rapid gains in wing length, Weeks says. The more they shrink, the more efficient they need their flight to be.
While North American migratory birds cannot be a proxy for all birds, the consistency of the response supports the idea that birds in general may shrink as temperatures increase, Weeks says.
Going by the response to this story in the newsroom, however, people seem less concerned with the plight of the plover and more concerned about chickens.
Worry not, dear carnivore. Chickens are captive and have been bred and morphologically manipulated over millennia to the point where any climate change-induced metamorphosis would be moot. Weeks agrees that their findings likely only apply to natural systems. “In particular, the change in wing length appears to be driven by natural selection on migration and so would not be happening in poultry,” he postulates.
Who says it’s the temperature causing the birds to shrink as opposed to some other direct cause? Multiple lines of evidence suggest a causal relationship, the researchers say. The strongest evidence is that embedded within the long-term trends of declining body size and rising temperature, they identified numerous short-term fluctuations in body size and temperature that seem to be synchronized.
“Periods of rapid warming are followed really closely by periods of decline in body size, and vice versa,” Weeks elaborates.
The team can’t predict what will happen to avian morphology as climate change accelerates. Wing length comes under many, many different selection pressures. Weeks notes that longer wings aren’t just more efficient: they may pose a hazard (“increased predation risk”).
“As such, birds may not be able to increase in wing length indefinitely, and we don’t know what will happen once they start to be limited in their ability to increase in wing length in order to compensate for body size reductions,” he says.
If you habitually stroll the streets of North America’s cities and have seen dead birds plummeting from the skies, you wouldn’t have noticed the size difference with the naked eye. The changes in avian body size and shape are subtle, the team says — at most, a couple of grams difference in body mass and a few millimeters in wing length. But that could change as our climate does.