Ornithologists estimate that migratory birds of many species have about a 50% chance of living through a year. That year requires two migrations and two stationary periods for nesting and for overwintering in some reasonably moderate climate. We believe that much of the annual mortality occurs during the stressful migrations.
One of the few studies that examined mortality over the course of a year of a migratory focused on black-throated blue warblers. The population studied nested in the Hubbard Brook Experimental Forest in New Hampshire and overwintered in Jamaica. A year in the life of one of these warblers consists of a three-month stationary period in New Hampshire during the nesting season, a six-month stationary period in Jamaica during the winter and three months devoted to the two migrations
The researchers found that the survival rate of the summer period was 99% and winter survival rate was 93%. Survivorship in the migratory periods was only 50%. This result means that 85% of the annual mortality in these warblers occurred during the three months of migration. Weekly mortality during migration was about 15 times higher than weekly mortality in the stationary periods.
What are the factors increasing mortality during migration? First off, a bird migration demands a herculean effort. A flying bird increases its normal metabolic rate five times or even more in some birds. That effort requires a tremendous amount of energy. A significant number of migrating birds perish because of starvation. Scarce food poses a real threat for birds that are living on the edge as they migrate.
Bad weather certainly claims the lives of some migrating birds but it is difficult to get a handle on the magnitude of this effect. Birds are pretty good at evaluating the current weather before embarking on a migratory leg. If inclement weather arrives during a flight, birds generally abort their flight and seek shelter. We know that songbirds hunker down effectively during hurricanes and other storms.
Predation by raptors is minimal for nocturnal migrants while they are flying. However, exhausted birds end their migratory leg around dawn and raptors are often lurking by. A migrant may not have the energy to avoid that sharp-shinned hawk.
Sadly, many migrating birds die because of human alterations to the landscape. Migrating birds passing over cities become disoriented by lights and get lost in the urban canyons. The birds circle endlessly, exhausting themselves, and often fatally colliding with windows or the buildings themselves.
Toronto has been a leader in trying to reduce this mortality with their Lights Out Toronto program. The lights in municipal buildings go out after work and on weekends to reduce confusing light for birds.
Communication towers, required to have lights to make them visible to aircraft, can cause large kills of birds. Birds seem to mistake the tower lights for the moon and proceed to fly in circles, often colliding with the tower. In western Kansas, 10,000 Lapland longspurs were found dead beneath a 400-foot tower after one fateful night. An even taller tower in Florida had as many as 4,000 carcasses of 62 bird species on the ground below in the early morning.An alarming number of migrating birds are dropping dead across southwestern states including New Mexico, Arizona, Colorado and Nebraska. Wildlife biologists estimate hundreds of thousands of birds have died.
Observers report that birds on the ground are lethargic and often fearless when approached by humans. Many of these birds are flycatchers and swallows, which feed on insects caught on the wing. Resident birds like great-tailed grackles and white-winged doves do not seem to be affected.
The rampant western fires may be the reason for the die-offs. Perhaps the smoke is affecting the migrants. The fires may be forcing the birds to alter their migratory route over areas where insects are scarce. A cold snap throughout the southwest may have grounded flying insects, inducing starvation of the insectivorous birds. It’s clearly a disaster.