Early Birds: Songbird Migration and Climate Change

eastern bluebirds

Research indicates that climate change affects the migration behavior of songbirds. (Photo credit: iStockphoto.com)

The first green seedlings erupt from the cold, brown earth; pink blossoms unfold on tree branches; and insect populations burgeon. As our globe warms, those who live in the northern parts of North America and Europe do not generally complain about the earlier and earlier arrival of spring. But what does an early spring mean to the migratory birds that inhabit those ecosystems? It appears to be changing their rhythms. Over the past half century, abundant data has revealed a picture of changes in bird behavior that corresponds to changes in climate. One thing is clear: migratory birds of North America and Europe are arriving earlier at their northern nesting grounds each year. Some species seem to be adapting to the climate change, while others may be at risk.

Allen Hurlbert and a team of researchers at the University of North Carolina conducted a recent study of bird migration and climate change. Although covering only the past ten years, the dataset used in the study was robust—it consisted of more than 48 million bird observations taken of 18 different species. The data was gathered by about 35,000 citizens as part of a citizen science project called eBird. The website, created by the Cornell Lab of Ornithology and the National Audubon Society in 2002, allows amateur birdwatchers to log into the site and submit their sightings. Experts review any unusual observations. Hurlbert and his team then correlated recorded temperatures with first sightings of bird species at their nesting grounds. They found that birds have shifted their migration on average nearly a day for every degree Celsius the climate has warmed. Some species arrived as many as three to six days early for each rise in degree. Other species adhere to their historic schedules.

According to Hurlbert, the change in birds’ migration patterns could affect the health of bird populations, as the timing of their arrival is critical. Species that arrive early as well as those that maintain their historic migration patterns are both at risk. Birds that arrive at their northern breeding grounds too early may suffer if the threat of severe winter weather has not yet completely passed. Birds that arrive too late may miss the peak in the spring insect populations and be outcompeted by earlier arriving birds.

These and other researchers have found that birds with the longest migration routes may not adapt as quickly to climate change. Those, like the great crested flycatcher, which winters in South America, are the slowest to change. The triggers for migration come from environmental conditions, and the birds in South America have no way of knowing when spring is arriving in the northern United States. On the other hand, birds with shorter migration routes may have an advantage. For example, an early spring in Massachusetts usually coincides with an early spring in North Carolina, so birds wintering there may be receiving early cues that it is time to pack up and head north.

In another study, researchers found that a different flycatcher species, the pied flycatcher, has started migrating earlier in the spring due to climate change, but still reaches its destination on time. The study found that the earlier departure date appears to be related to birth time—that birds born earlier also migrate earlier. However, bad weather in North Africa, which is along the pied flycatcher’s route from West Africa to Europe, slows the birds’ advance and they end up reaching Sweden and The Netherlands at their usual times. This ability to adjust its schedule en-route may turn out to be an advantage for slower migrating species such as the pied flycatcher.

In addition to adjusting departure and arrival dates, several migratory bird species are changing their winter destinations. More than half of over 300 bird species monitored by the Audubon Society have shifted their winter ranges roughly 35 miles farther north from where they were 40 years ago. Again, the researchers point to climate change. The average temperature in January has climbed by nearly 3 °C in the U.S. during that time. The birds are most likely following shifting ranges in available food and nesting sites.

All these studies seem to confirm the same thing: that global climate change is affecting bird migration. The next step that scientists are working toward is understanding the mechanisms behind the changes. More research is needed to determine whether there is flexibility in birds’ inherited migration instincts or if the instincts themselves are changing. In other words, it is not clear whether the changes in migration behaviors are evolutionary responses to the changes in temperature. It is also not known what role a host of other factors play in modulating bird migration, such as wind direction, wind speed, and the condition of the birds themselves.

Finally, researchers would like to have enough information to make predictions about the future of specific bird populations. If temperatures continue to increase, how will migratory birds and other animals adapt or fail to adapt? This information may not only prevent extinctions, but help us understand the bigger picture of how our planet is changing. As indicator species, birds are affected by subtle tweaks in temperature. But ultimately, we all may need to adapt as the climate, and thus the ecosystems around us, change.

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