The Why Files The Why Files --

Miracle of winged migration

Global warming: Are migration patterns a-changin'?
Could bird migration tell us something about climate change? Perhaps, and fittingly, data collected by pioneering environmentalist Aldo Leopold have been used to show changes in bird migration patterns since the 1930s, when Leopold, as a hobby, recorded spring arrival dates for various species in Wisconsin.

"There is a pretty clear-cut pattern," says University of Wisconsin-Madison wildlife ecologist Stanley Temple, who has worked with Leopold's data. "Some birds are shifting the timing of their migration in response to climate change, but others clearly are not."

Temple points to two closely related species, the robin, a short-distance migrant, and the wood thrush, which migrates to Central and South America. The wood thrush arrives on May 6, just one day earlier than in Leopold's time, but robin arrivals have moved up from March 26 to March 5 -- 21 days in just 60 years. Plenty of robins now even spend the winter in Wisconsin.

Why the difference? "They are using different cues to time their migrations," Temple explains. "Short-distance migrants are much more influenced by the temperature regimes in North America, while the timing of migration for long-distance migrants is almost all due to the photoperiod [day length]."

Springtime behavior has changed in other ways, Temple adds. "Many birds start defending territory and initiating nesting two to three weeks earlier than in the past." Between 1935 and 1945, cardinals started their first territorial song, on average, Feb. 24. Sixty years later, the average date is Feb. 1.

The robin is not the only bird that once migrated but now may stick around during the winter. "Leopold used to consider Canada geese a sure sign of spring," Temple adds. "Now if you take a walk at Christmas in Wisconsin, you are likely to encounter hundreds of Canada geese."

A dozen geese swim on a freezing lake, clouds of water vapor rising around them.

With temperatures near zero degrees during a winter sunrise, a flock of geese and an occasional swan swim in the steamy, yet-to-be-frozen waters of Lake Mendota near Picnic Point. Many Canada gees now spend the winter in the northern United States, without even bothering to migrate. Photo by: Jeff Miller, courtesy UW-Madison

So what?
But why would arrival dates matter so much? Because, as Leopold taught, everything is connected in ecology. "You end up setting the stage for a bird community that gets out of synch," Temple says. For example, some birds are considered "secondary cavity nesters," because they cannot bore out their own nesting holes, and so must rely on holes previously drilled by woodpeckers. "There is intense competition for nesting cavities," Temple says, "and a big part is getting there first and taking possession of the cavity."

And who is hogging those essential nest holes? The short-distance migrants that now arrive in Wisconsin much earlier. "The long-distance migrants are losing out, because by the time they return, the cavities are already taken," Temple says.

One example is the great crested flycatcher, a long-distance migrant that still arrives around May 1, much as during Leopold's time. But by then, short-distance migrants, including the European starling, have already arrived, on their accelerated, global-warming schedule. "Great crested flycatchers are declining in Wisconsin," Temple says. "We are not sure of the reasons, but this has to be one of the causes."

Small bird with yellow breast sits on a wooden post against a blue sky filled with fluffy clouds.

The great crested flycatcher is a secondary cavity nester that could lose out if competing birds monopolize the nesting cavities. Courtesy: US Fish & Wildlife Service

Neighbor trouble ahead?
Other problems are sure to arise because warming also alters the timing of plants, and thus the timing of insects associated with the plants. That, in turn, affects the birds that prey on those insects. "If the insects they depend on are now present three weeks earlier than the birds, migrants could miss out on a source of food they once relied on," Temple says.

It's a footnote, but an interesting one, that two of Aldo Leopold's children, Nina and Carl, worked on the 1999 study that linked global warming to bird migration (see #4 in the bibliography).

Temple says the migration data show that global warming has already affected animal behavior. "We can show that the changes we are observing correlate with changes in temperature. If changes in the timing of seasonal events of birds were random, we would expect an almost equal number of species to be delayed as advanced, but the overwhelming majority of changes concern species that are advancing. Hardly any have been delayed."

Migrate over to our bibliography.


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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