Bird Migration

In a world fueled by current environmental uncertainty, there are very few constants.  Yet every autumn without fail, soaring streaks of color crisscross the sky in tell-tale “V” shapes.  Migrating birds, a dependable sight, indicate a shifting of seasons.  They come and go like clockwork, their wings flapping in harmony as flocks fly south in search of warmer weather. 

However, mounting evidence indicates that increasing temperatures across the globe are forcing birds to alter and shorten their migration routes, according to ornithologists.  Ecologists and ornithologists are noticing a change in birds’ migration patterns, a change which results in minimized reproduction opportunities, competition for territory and food, and decreased biodiversity.  They anticipate a warmer future in which bird species will cease to speciate and evolve effectively, ultimately leading to potential extinction.

 Climate change is more than just a passing phenomenon, according to ecology and evolutionary biology professor Tom Sherry of Tulane University.

 “Global climate change is real, it’s happening now, it’s caused by humans,” Sherry said.

 According to Sherry, scientists are using models that date back to the 1960s to predict climate change, and so far, their predictions seem to be accurate. Because birds migrate to escape cold weather, they adjust their migratory patterns accordingly, he said.

Migratory birds are grouped into two categories: neotropical migrants and casual migrants.  Neotropical migrants consistently travel long distances year after year; some even fly from Canada to South America, according to the U.S. Fish and Wildlife Service.  Because they face such a long trip, their patterns are not visibly affected by climate change.

 The concern lies with casual migrants, Sherry said.  This group includes common North American species such as robins. Greg Neise of the American Birding Association said casual migrants adjust their behavior according to their environment.

“These are birds that migrate only as far south as they have to,” Neise said.  “They’re opportunistic.”   

When winters are less severe, they do not travel as far.  However, as these species settle in new areas, they must compete with existing populations for food sources and breeding grounds.

Neise, who has been birding for forty years, observed quite a few oddities this past winter, which he said was unseasonably warm.  He noticed Sandhill and Whooping Cranes, usually seen in Florida or Tennessee, wintering in Illinois and Indiana.  The same is true of the Carolina Wren. 

“The wrens have pushed further north and in greater numbers,” Neise said.  “They’re usually a southern bird, but they’ve been all over northern Illinois this year.”

Cumulative research is necessary to yield concrete conclusions, said Dr. Keith Bildstein, director of Conservation Science at the Hawk Mountain Sanctuary, a wildlife sanctuary located in Kempton, Pennsylvania, that is open to the public year-round.

“To know if something has changed, you have to have monitored migration over a long period of time, and that’s done at Hawk Mountain Sanctuary,” Bildstein said.

Ornithologists keep track of species’ migratory whereabouts with a Christmas bird count, which provides tallies of bird species in specific areas around late December, he said.

Bildstein said he noticed declines in the numbers of Sharp-shinned Hawks in the early 1990s, but at first thought decreasing populations were to blame. However, he said continued monitoring proved otherwise. 

“The reason we’re seeing fewer Sharp-shinned Hawks is because they are not migrating as far as they used to, something we might predict with climate change,” Bildstein said.

 The hawks are staying in Connecticut and New York, rather than migrating to Pennsylvania and New Jersey because of warmer temperatures, he said.

This phenomenon is known as migration short-stopping, and has been directly traced to at least eight other bird species in both the United States and Europe, including the Marsh Harrier, the Common Buzzard, and the Short-Toed Eagle.Most people are not aware of the subtle alterations in migration patterns, however. 

Migration short-stopping is gradually reducing bird biodiversity.  Migratory behavior is a necessary part of species survival and actually enhances the chance of speciation, or the creation of new species, according to Bildstein.

When migrating, some birds inevitably get lost, make navigational mistakes, and end up in the wrong places, he said.  The misplaced birds either die immediately, fail to reproduce, or breed and evolve into new species adapted to living in the alternate locations. When birds migrate shorter distances, they have a lesser chance of miscalculating routes, and thus limited opportunities to speciate and evolve.