As the planet warms, scientists are seeing evidence of earlier springs and later autumns all over the world. Snow is melting sooner, plants are flowering earlier—and now, researchers find that birds are changing their migration patterns, too.
A new study, just out this week in the journal The Condor: Ornithological Applications, is the latest in a growing body of research that suggests our feathered friends may change their behavior in response to the climate. It finds that some Pacific Coast species are migrating earlier in the spring and later in the fall than they used to. And these changes appear to be linked to warmer, wetter climate conditions.
The researchers analyzed 22 years of data on birds captured and banded at a migratory stopover point in Northern California. They focused on five different species: the Pacific-slope flycatcher, orange-crowned warbler and Wilson's warbler—all of which have short migrations up and down the coast—and the Swainson's thrush and yellow warbler, which have longer migrations between North and South America.
Changes in the birds' migrations varied from one species to the next. But the researchers found a general pattern of earlier spring migrations, changing by about 2.5 days per decade, and later fall migrations, moving forward by about three days per decade.
The researchers evaluated these changes alongside long-term data on three types of natural climate variations, including El Niño and other indices related to oceanic and atmospheric temperature and circulation patterns. These natural climate patterns can cause shifts from warmer to cooler or wetter to drier conditions (and vice versa) over time. They found that up to a third of the changes in migration timing, from one year to the next, could be explained by shifts in these climate cycles, with earlier spring and later fall migrations associated with warmer and wetter conditions. This was especially true for the short-migration species, while those with longer migrations were much less affected.
In this case, annual shifts in migration timing were compared with natural climate cycles. But the results reinforce several growing ideas about bird migrations in a warming world. First, they suggest that some species are sensitive to changes in temperature and precipitation—and although they're linked to natural climate patterns in this study, these same factors will continue to shift under future climate change.
And the study also reinforces other recent research indicating long-term changes in the timing of bird migrations, likely linked to ongoing climate change. A paper published a year ago in the Journal of Animal Ecology, for instance, examined data on more than 400 species across five continents and found that birds have, on average, adjusted their spring migration time by about two days per decade—close to the 2.5 days per decade indicated in this week's study—or one day for every degree Celsius of global warming.
In general, biologists are increasingly concerned with the effects of climate change on plant and animal species—how it might affect their ranges and distributions, their feeding and reproduction habits and their movements and migrations. In migratory birds, it's still unclear how changes in migration timing might affect whole populations or species.
On the one hand, arriving earlier in the spring and leaving later in the fall could extend a population's breeding season—potentially a boon for the birds. On the other hand, global warming will not affect all regions equally, and shifts in migratory timing could put birds out of sync with the food or habitat resources they usually depend on later on in the journey. Leaving too early in the spring, for instance, might mean they arrive at their destinations before the plants or insects they feed on in those locations are abundant enough to support them.
Then again, failure to adjust migration behavior in response to climate change—as indicated by the two long-migration species in this study—is also a concern for similar reasons. Birds that stick to their old migration patterns in a warming world might leave too late in the spring and arrive at their wintering grounds long after other species have already settled in and claimed all the food and space.
In a statement, study co-author Brett Sandercock of the Norwegian Institute for Nature Research pointed out that the long-term effects of these types of migratory shifts on bird populations "are difficult to predict, in part because our understanding of the migratory behavior of western songbirds remains incomplete."
So while the study reinforces the growing evidence that climate change can significantly affect bird behavior, it also underscores the fact that much more research and monitoring is needed before scientists understand what the long-term consequences will be.
Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.
