As polar vortexes loom and food grows seasonally scarce, there’s more enduring bad news about the plight of birds. On the heels of a study that found North American birds in the throes of a steep population decline, new research concludes that this continent’s songbirds have been steadily shrinking in size over a span of nearly four decades.

Fewer birds, smaller birds. Don’t just give Polly a cracker. Give her the whole box.

What does this mean for the species that beams at the sound of a warbler’s warble during a morning walk through the park, or gushes over the sight of a blue jay perched on the bough of a silver maple? Should Homo sapiens be worried about a bird apocalypse? Should we fret that, if it can happen to them, it can happen to us?

First, a look at the science. University of Michigan scientists working with Field Museum researchers studied thousands of birds belonging to 52 species that crashed into high-rises in Chicago between 1978 and 2016, and were preserved at the museum’s bird collection. Scientists took several measurements, including each bird’s weight and wing length.

The measurements, reported this month by The Wall Street Journal, showed that, over that 38-year span, body size dropped significantly, while wings became longer. Why? Climate change. Which makes sense: Over these decades, temperatures rose nearly 2 degrees Fahrenheit in the birds’ summer breeding grounds north of Chicago. That corresponded with a drop in body weight of up to a couple of grams, and a rise of a few millimeters in wing length.

The causal link between climate change and bird size has to do with the scientific tenet that warm-blooded species often have larger bodies in places that are colder, and smaller bodies in warmer locales. A smaller, more compact body releases heat faster. Hence, birds are getting smaller because their world is getting hotter.

The scale of body mass loss is small enough that we don’t have to worry about cardinals and robins shrinking to the point of being smaller than the insects they now rely on for a snack. It is, however, another red flag about the reach climate change has, and the need for mankind to stop turning up the temperature.

Many species have been adversely affected by global warming, and man’s reluctance to move fast enough to protect the planet. Populations of polar bears are dropping because of melting sea ice. Coral reefs around the world are in decline due to climate change. The adverse impact is never abruptly cataclysmic; it’s almost always gradual, and virtually imperceptible in real time.

But it’s there. It’s happening. And it’s a warble of warning that we should heed.

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