Spring shift throws songsters off their timing

A silent spring was the apocalyptic scenario that biologist Rachel Carson warned a half century ago would result from the cavalier use of pesticides such as DDT. Now scientists warn that nine species of North American songbirds could be silenced by the creeping threat of climate change.

The birds, which migrate south to tropical climes each year to avoid the harsh winter conditions and return north in spring to mate and raise offspring, are failing to adapt to the earlier onset of spring, according to research published in Nature Scientific Reports.

“If anything could adapt to climate change, you’d think that birds that migrate thousands of miles could,” says lead author Stephen Mayor, a postdoctoral researcher with the Florida Museum of Natural History at the University of Florida. “It’s much easier for them to move in response to climate conditions than salamanders, for example, or trees. But because every species relates to another, one of our fears is that climate change can disrupt these relationships between organisms such that their critical life events are not timed optimally, putting them at risk.”

Of 48 bird species studied by Mayor and his colleagues, the nine species not coping well with the shifting seasons are: great crested flycatchers, indigo buntings, scarlet tanagers, rose-breasted grosbeaks, eastern wood-pewees, yellow-billed cuckoos, northern parulas, blue-winged warblers and Townsend’s warblers.

The birds’ flight north from Central and South America is cued by the seasonal shift in the hours of daylight, timing unaffected by increasing global temperatures. The warming climate, however, is triggering changes in the timing of plant growth. Some plants, dependent on “chilling” cycles to activate their spring bloom for example, are flowering later than normal; others are “greening-up” earlier. These shifts in plant cycles are leading, in turn, to changes in the timing of attendant insect booms, on which the birds feed, 

The researchers found “green-up” in North America’s eastern temperate forests was tending to run ahead of bird arrivals, while in western forests it was running behind, leaving the birds either too late or too early for optimum conditions at their breeding grounds.

The growing mismatch meant fewer birds were likely to survive, reproduce and return the following year, Mayor says. “These are birds people are used to seeing and hearing in their backyards. They’re part of the American landscape, part of our psyche. To imagine a future where they’re much less common would be a real loss.”