Migration

Nature Watch
September 14, 2007

By Susan Benson
Director of Education
Cable Natural History Museum

We’re still in the thick of the season for bird migration. Though you may see many species “flocking up” as their departure time grows near, when do they actually take off?

The vast majority of birds migrate at night, so we usually don’t notice when they leave. For nighttime travelers like warblers, tanagers, vireos, orioles, kinglets, thrushes, many sparrows, catbirds, shorebirds, owls, herons, egrets, and waterfowl, study after study shows that migration is initiated 30 minutes to an hour after sunset.

Peak migration is usually from about 10 p.m. to midnight; the number of birds diminishes greatly four to six hours after sunset. Often in the hours before dawn, there are few to no birds aloft.

So why migrate at night? One hypothesis has to do with food and energy, and suggests that birds need to feed during the day before undertaking long flights. That makes sense since foraging at night for insects, seeds and berries would certainly be more difficult. However, some birds do forage and migrate at night, so there are exceptions.

The other explanation also has to do with energy. Nighttime migrants engage in powered flight—that is, they flap their wings rather than soar and glide in order to propel themselves. The nighttime atmosphere is cooler and less turbulent than during the day, so it’s easier and more energy efficient for birds to fly at night.

Powered migrants also generate enormous amounts of heat during flight; cooler night weather reduces loss of water that is used for stabilizing their body temperature. During long non-stop flights, the loss of water may in fact be more limiting than the loss of fat.

All that said, however, there are birds that migrate during the day. Some songbird species are true daytime migrants, like red-winged blackbirds, brown-headed cowbirds, grackles, finches, crows, and blue jays. Most of these species migrate much shorter distances than nocturnal migrants, so they tend to venture south with a series of short flights.

Soaring birds like hawks, pelicans, cranes, and swallows almost always migrate during the day. They depend on thermal updrafts to give them lift, and these warm funnels of wind are most available during the day.

Down in the forests and fields, one of our common wild neighbors is going through a traditional fall transformation. During the summer while his antlers grow, the male white tail deer socializes as part of a bachelor group. But as days shorten, his testosterone levels increase, his antlers harden and the velvet supplying blood to his antlers dries to a ragged sheath that falls off. He rubs off the remains, and from that time on he is a loner, focused on the mating season and using his keen senses to detect danger, rivals and receptive does.

The buck advertises his presence by leaving physical scrapes and signs. He applies scents from various parts of his body to broadcast his presence to other deer. Saliva, urine, and secretions from pre-orbital glands in front of his eyes, forehead glands at the base of his antlers, and tarsal glands on the insides of his hind legs all send aromatic messages to other deer.

Become a phenologist! Celebrate the autumnal equinox this September 23—that day when hours of day and night are equal and we officially enter the fall season. (“Equinox” means “equal night.”) Stay up a little later and bid farewell to departing migrant birds.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.