Bird! Bird! Bird! (To Everything There is a Season).
With apologies to Pete Seeger (“Turn! Turn! Turn! (To Everything There Is a Season)”), I can’t help but think of that song at this time of year. Of course, the seasons are changing. With that comes many things — the shorter days, the cooler weather, the changing colors, little kids in scary costumes…and bird migrations.
The big picture reveals that Earth, as it rotates on its axis and travels around the sun, begins to receive more light (and incoming solar radiation) in the Southern Hemisphere as summer begins, while less incoming solar radiation allows the Northern Hemisphere to begin to cool as winter begins. As the days shorten, many species of birds migrate south for the warmer climes and longer days, ensuring (hopefully) uninterrupted food supplies and a continued growing season for the young hatched only a few months before.
Though the dangers of migration are outweighed by the advantages, the dangers are not trivial. Exhaustion, predation and even disease are always potential threats for the transiting birds. And that’s not all.
Cloudy skies can obscure celestial navigation aids such as the moon and stars. Fog can cause birds to descend to the ground and interrupt their journey. Cold air masses colliding with warm air masses can cause widespread thunderstorms and rain which will also cause birds to seek shelter on the ground (this can be disastrous if the birds are over large bodies of water). Airports are often areas of concern for birds — migrating, or not. Lights from communication towers and tall buildings can attract the birds resulting in collisions. Bright city lights can add too much light, bringing confusion to the internal navigation systems of the birds. In fact, only last weekend during the Tribute In Light ceremony at Ground Zero, the extreme brightness of the twin columns of light first drew thousands of migrating birds into the beams, then disrupted and bedazzled them enough so that when they ventured out of the beams they were attracted right back into them. Fortunately, experts on hand were ready for this and were able to turn off the lights so that the birds could carry on with their journey (the lights were turned off and on several times during the course of the ceremony). This video shows the birds effectively trapped in the beams of light.
Interestingly, one manmade hazard in particular that was originally thought to be a major disaster for migrating birds has turned out to be less of an issue: wind turbines. Studies and local turbine effects vary by region, but most areas are proving to not be a significant hazard to migrations. A study using radar found that less than 1% of birds in migratory flocks were close enough to the blades to risk collision (although 1% of thousands upon thousands of birds is still a very large number). Other studies are more critical of the turbines and experts can have questioning, concerned or supportive views.
Are you a bird watcher? As our days shorten you know this is the time for birds to begin heading south. The birds know, too.