At sunrise, an interested observer might see several thousand blackbirds leaving their roost, usually inside a low wetland area. The whole thing takes about five minutes. The birds will be flying in a narrow band like a fast-moving stream. About a half-hour before sunset the same event takes place in reverse, as a steady stream of blackbirds moves back to the roost.

Soon after adult blackbirds stop caring for the young, the creatures begin flocking together. Each night in summer until they move farther south later in fall, huge congregations of common grackles, red-winged blackbirds and brown-headed cowbirds, all members of the blackbird family, will fly from their feeding grounds to their roosts, creating the largest and most commonly observed groups of land birds in North America. Assemblies of red-wings peak in about mid-October. Grackle flocks peak a bit later. Slowly, as the season progresses, the enormous flights drift south. Most of these blackbirds winter in the Southern states. Very few attempt to winter in southern Minnesota.

It's easy to see the evolutionary reasons for this migration. Take mobile animals and a seasonally fluctuating food supply, and the natural consequence is migration. Long ago, and even today, the birds that moved in the right direction were the ones to survive. Joining the blackbirds as they winter in Southern states are house wrens, eastern bluebirds, yellow-rumped warblers and wood ducks.

Jim Gilbert's Nature Notes are heard on WCCO Radio at 7:15 a.m. Sundays. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota.