We already know that climate change isn't making things easy for birds—it's led to a recent upswing in disease and starvation among avians all around the world. And now we can add another danger to that list: sibling rivalry in Zebra Finches. Normally this isn't a problem with these finches, as all the chicks are supposed to hatch at the same time. But in Australia, a steep rise in atmospheric temperatures is leading some Zebra Finches to emerge early and gain an elder-sibling advantage, according to recent research published in Royal Society Open Science. This slight edge allows bigger first-borns to get more food from their parents, lowering the odds that other chicks in the nest will survive. 

Because the speed of embryo development is tied to heat—more heat equals faster development—it’s up to adult Zebra Finches to carefully control the temperature of their eggs via incubation. The female will typically lay five eggs over the space of five days, so to make up for the early start that some eggs get, she and her mate will hold off on incubating the embryos (and triggering development) until all of them are snug in the nest.

Sometimes, though, parents have to leave the nest unattended, and when that happens, the eggs are at the complete mercy of their surrounding climate—a state known as “ambient incubation.” In most cases, ambient incubation doesn’t seem to affect finches—under normal conditions adults can keep their embryos cool by sitting on them and shading them. But add global warming to the mix and . . . well, you can see where this is going.

A Zebra Finch couple. Simon Griffith

To study the effects of scorching temperatures on Zebra Finch eggs, biologists from Macquarie University in Sydney set up camp in an area of southeastern Australia that was hit by abnormal heat waves (defined as five or more days in a row when temperatures hit 104 degrees Farenheit or hotter) in 2010 and 2012. They kept tabs on local finch nests during the heat waves, some of which were positioned in the shade, and some of which were positioned in the direct sun (the researchers also removed the roofs of these nests to amp up the effect). What they found was that in the sunny nests, which ran about 6 degrees warmer, eggs hatched in less time than the shaded nests—and the eggs within a single sunbaked nest hatched at different times. Since the trigger temperature for finch eggs to start developing is around 100 degrees, it’s likely that the intense sunlight caused the embryos to start incubating right away, says Simon Griffith, an ecologist at Macquarie University in Sydney and lead author of the study. That means that during the heat waves, the eggs that are laid first get a major head start—and previous research suggests that that means the younger chicks may not make it out of the nest. 

This is bad news for the Zebra Finches, since the Intergovernmental Panel on Climate Change predicts that even in the most optimistic global warming scenarios, we're bound to see an increase in prolonged heat waves all over the world. Scientists have already established a direct link between climate change and record-high temperatures in Australia, New Zealand, Asia, and Europe, and their projections show that it’s only going to get worse.

In the meantime, what can a Zebra Finch do to keep its chicks in synchrony? It’s possible that the birds will adapt, Griffith says, either by not breeding during heat waves, or by shifting more resources and testosterone to the younger eggs during incubation to help bring them up to speed. But they'll definitely have to catch on in time. 

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