Climate change discourages second families

Birds out of sync with local baby food supply aren’t nesting a second time

Birds have empty nest issues too. And with the climate changing, some nests in the Netherlands are becoming more likely to stay empty, researchers say.

In the Netherlands, a bird called a great tit is much less likely to start raising a second clutch of eggs during breeding season than the species was years ago. IMAGES: Jan Visser

A young bird of the great tit species in the Netherlands depends mostly on caterpillars of the winter moth species supplied by parents. But climate change has shifted the burst of caterpillar abundance out of sync with the birds’ breeding season. Jan Visser

Back in the 1950s, little birds called great tits (Parus major) at four Dutch study sites were more likely than not to start a second family during the breeding season as soon as the first chicks left the nest, says Arild Husby of the University of Edinburgh. Now maybe five percent of those birds start a second nest during the same season.

Taking a closer look at temperature, caterpillar and nesting data from those sites, climate change seems like the cause for this decline in second families, Husby and his colleagues report online February 24 in Proceedings of the Royal Society B.

Rising temperatures have driven caterpillar numbers to peak earlier in the spring, the researchers found, and birds need those caterpillars to feed the chicks. Yet great tits aren’t starting their first nests early enough to compensate for the shift.

Any birds trying to raise second families are so far out of sync that virtually all the chicks starve. So great tits hardly seem to be trying to raise doomed second families these days, Husby and his colleagues report.

Plenty of studies have linked changes in timing of breeding to faster warming in springtime, Husby says. The new work documents another effect — the number of egg clutches might be changing too.

“These birds are going to have a harder time maintaining population size in some areas because they don’t have the same options to breed they had before,” says ornithologist Thomas Bancroft at the Washington, D.C., office of the National Audubon Society.

Great tits occur widely across Europe, making them a favorite of ornithologists studying ecological trends. The populations Husby and his colleagues worked with live in habitats ranging from a woodland to an island, and researchers at the Netherlands Institute of Ecology in Heteren have monitored nesting and survival in these populations since the early 1950s. “It’s a fantastic dataset to worth with,” Husby says.

A tendency for second nesting had been trending somewhat downward during the early years of the study, but declines sped up starting in the 1970s. Since then, temperatures have increased some 1.7 degrees Celsius in the region, Husby says. During that period great tits at one of the most studied of the four sites, a central Netherlands woodland, have advanced the laying dates for their first clutch 5.4 days on average.

That’s not enough, as far as food supplies go, Husby says. From 1973 to 2003, peak hatching in caterpillars of the winter moth species advanced about 22 days. These caterpillars provide the bulk of the baby food for the great tits.

There’s no need to explain the drop in second nests by invoking some kind of rapid evolution, says avian ecologist David W. Winkler of Cornell University. “It’s much more likely to be a response to local food supplies.”

A study of barn swallows in Denmark failed to find a change in second clutches, but the swallows aren’t as dependent on a particular insect. Whether other bird species are having similar troubles, Husby hesitates to say because most other researchers haven’t focused on changes in avian empty nest syndrome.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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