Friday, April 18, 2014
By Herb Wilson
After a wet and cool June, the warm temperatures in July convince us that summer has finally arrived.
However, birds operate on a different calendar. Tree swallows and barn swallows can be seen by the tens and even hundreds perched on utility wires with migration on their minds. Least sandpipers, greater yellowlegs and short-billed dowitchers are appearing on coastal mudflats.
The fall migration has begun. Perhaps a less confusing term would be post-breeding migration but I think the phrase fall migration is here to stay.
Some ornithologists have estimated that five billion birds in North America migrate south every year. In today's column, we will consider the why and when of the fall migration.
To begin, we need to recognize two types of migrating birds.
First, we have species that breed locally but winter to our south. We can call these species migratory breeding birds.
Second, we can see species that breed to our north and winter to our south. We only see these birds, called passage migrants, during their migration to and from their breeding grounds. Various sandpipers and snow geese are examples of passage migrants through Maine.
You may wonder why tree swallows depart southward from Maine when the summer weather is just starting to become glorious. The answer is food.
The need to migrate is not impelled by temperature but rather by lack of food. Given sufficient food, birds are capable of tolerating marked extremes of temperature. The abundance of flying insects, on which the swallows depend, is beginning to decline. The reduction in food necessitates an August departure for most of our swallows.
Cuckoos, warblers and vireos rely heavily on caterpillars and other insects, which feed on the leaves of trees and shrubs. The abundance of these insects is sufficient to allow leaf-gleaning birds to stay in Maine well into September.
After the first killing frosts of autumn, leaf-eating insect abundance declines markedly and our warblers and vireos are forced to migrate south. Except for yellow-rumped warblers and palm warblers, most of our warblers will depart by the beginning of October.
A number of our migratory breeding birds are seed-eaters. Seeds from herbaceous vegetation can be found through the fall until a snow cover accumulates.
White-throated sparrows, chipping sparrows and rose-breasted grosbeaks can linger well into October.
The movement of passage migrants begins in July with the arrival of post-breeding shorebirds. Birds breeding above the Arctic Circle have a narrow window of opportunity for breeding. Insect and fruit in the Arctic is amazingly abundant during the time of the midnight sun, but rapidly shortening days and cooling temperatures take their toll on food availability for birds.
The semipalmated sandpiper, a species I have studied on its migration, provides a typical example of the migration of sandpipers.
Semipalmated sandpipers arrive on their Arctic breeding grounds in late May or early June. Both the female and the male incubate the eggs and tend the young.
Before the young sandpipers can fly, the females will begin their fall migration. They are followed a week or so later by the males. The young are left on the tundra, barely capable of flying but obviously able to find food and avoid predators.
They begin their migration about a month after their parents have departed. The juveniles inherit the instinct to migrate and find the way to their South American or Caribbean wintering grounds without the benefit of a guide.
I encourage you to get out this summer and to enjoy the fall migration.
The post-breeding migration is much more protracted than the spring migration. Spring migration is characterized by an urgency to get back to the breeding area and secure a good territory and mate.
The fall migration is more leisurely, lasting into November when the last of our sparrows and hawks depart.
Herb Wilson teaches ornithology at Colby College and perches in South China. He welcomes reader comments and questions at