The Old Farmer’s Almanac says “If crows fly in pairs, expect fine weather; a crow flying alone is a sign of foul weather.” There is no evidence for that, but we have all noticed birds perching on power lines as a storm approaches. That’s because low barometric pressure is a reflection of the low density of air molecules that makes it harder to fly. In an old article in a local bird newsletter someone reported a flock of swallows on a string of five parallel power lines that reminded him of notes on a musical staff. He proceeded to get his mandolin out and played the score, which turned out to be “God Bless America.” Probably not, but bird behavior can be used to predict bad weather because they perch more as a low pressure center (cold front) approaches. Lower air pressure, indicating the onset of bad weather, simply makes it more energetically expensive for birds to fly. In one study, captive White-throated Sparrows were experimentally subjected to different pressure regimes. Under high pressure the birds awakened in the morning and began to preen themselves in preparation for the day’s activities. When the pressure was lowered, the birds awoke and immediately began to feed, expecting bad weather challenges.
There is no question that the migratory behavior of birds is affected by weather. The impetus to migrate is not the weather, although storms can delay departure and winds can slow or speed up the passage and even blow birds off course. Hurricane Sandy in 2012 produced some unusual sightings such as Northern Gannets in New York Harbor and a Pomarine Jaeger off Cape May, New Jersey. In 2005 a study in Quebec found that the local Chimney Swift population fell by 50 percent after Hurricane Wilma blew the birds off course, some as far as Western Europe.
A snowstorm interrupted a spectacular migration of Lapland Longspurs in northern Iowa and southern Minnesota on the night of March 13-14, 1904. It was not particularly cold and the wind was calm, but it was snowing heavily. The birds fell from the sky and landed everywhere – in towns, on the roads, on roofs, and especially around streetlights. On the surface of two small lakes in the area an estimated 750,000 birds were found floating. The total estimate for this disaster was a million and a half dead Lapland Longspurs which were caught in unusual weather.
During spring migration, weather on the south end of a low pressure center encourages birds to move northward as the prevailing wind is from the south. In the fall, many migratory birds move south behind a cold front along with helpful but stiff north winds and cold temperatures. But behind that cold front are a rising barometer and high pressure center. When the high pressure center arrives the winds become light and variable and the birds take a breather to rest and feed. Since migrating birds tend to stay flanked by cold and warm fronts, they move in waves and sometimes spectacular appearances of swarms of birds can be seen in places like Grand Isle, Louisiana, a barrier island at the northern edge of the Gulf of Mexico that is reputed to have the highest density of birds in the U.S. during a spring migratory landfall.
The above was extracted from my latest book: Beaks, Bones, and Birdsongs.