Summary: The purple martin, Progne subis, is the largest member of the swallow family and is a migratory bird that spends spring and summer nesting throughout most of the United States and southern Canada. Being colonial birds, they like to inhabit man-made houses. They are extremely popular and there are national organizations dedicated to their conservation. Veterinarians in the United States will benefit from having a basic understanding of the purple martin and its behavior, as we may be occasionally called upon to consult with enthusiasts who are having problems with their colony.
The purple martin, Progne subis, is the largest member of the swallow family, averaging 17-20 cm in length. It resembles other members of the swallows, having a slender body, long wings, and a wide beak. The tail is forked, but not as deeply, as other swallows. The purple martin migrates to North America during the spring, and spends the summer here breeding. During our winter season, it flies back to South America. Sao Paulo, Brazil is a major wintering area for millions of these birds.
The purple martin flies by alternating short glides with rapid flapping. They are superb fliers, and catch flying insects in flight. They have the ability to dive to their birdhouse door, spreading the wings to slow the dive just before entering the house.
Purple martins are sexually dimorphic. Males are a shiny blue-black, hens are smaller, lighter in color, and have a spotted pale gray throat, breast and abdomen. Juveniles resemble the adult in coloration, and males will not obtain their adult coloration until their second breeding season.
Martins arrive in the Deep South before the cold days of winter are over. They begin seeking nest sites during the first two weeks of February. Martins arrive singly or in small groups. Males arrive first, usually a few days before the hens, to scout for potential nesting sites. Because of their early arrival, martins may be caught in cold weather, and if the cold weather lasts for several days, insect activity may be severely reduced, and large numbers of martins may starve to death, since they feed only on insects. Martins continue to arrive through May.
Within three weeks of the young fledging in late July, the southward migration normally begins. By early August, most of the purple martins are beginning to congregate in large flocks in preparation for migration. Adult males are the first to leave, followed shortly thereafter by the adult hens and immature birds.
Martins usually nest in colonies, although in isolated areas, some pairs nest alone. Historically, purple martins nested in cliffs, hollow trees and abandoned woodpecker holes. Today, most nest in man-made nesting sites. Males that find a suitable breeding site may fight to keep out intruding males. Hens will do some window-shopping before deciding on a nesting site and male. Both the male and female contribute to nest construction.
Many martins return to the same nesting site as long as they live. They may even select the same room in a birdhouse, if possible. Man-made birdhouses and condominiums are the most common and preferred nesting sites. They prefer nesting in houses away from trees. The more apartments in a house, the more attractive the house is to martins. They build a nest of straw, leaves and mud. Green leaves are used to line the nest.
There are few natural predators of purple martins. Starlings and sparrows may take over martin houses, preventing the martins from breeding. Another problem is the decreasing numbers of natural nesting sites. They are vulnerable to weather. In cold weather, there may not be enough insects to sustain them, and in hot weather, young birds may die in their houses before fledging. Mites and other parasites may kill some young birds. Cats, raccoons, owls and snakes may catch and eat birds. Pesticides kill birds by eradicating insects that they consume.
Sparrows may establish dominance in man-made houses before the first martins arrive. Sparrow nests are made of straw and will fill the entire compartment. Sparrow eggs are white and speckled. All of their nesting material should be removed and destroyed to encourage purple martins to establish.
Martins are exclusively insectivores. On cool, cloudy days, martins feed low to the ground where insects congregate. During warmer periods, martins may forage 30-40 meters in the air. On warm summer mornings, martins may be seen 300 meters high, catching insects. Adults may feed young birds in the nest over 100 insects per day. Although they do consume mosquitoes, they have a reputation for controlling the mosquito problem in areas they inhabit, however, this is an exaggeration.
Oviposition and incubation usually begin in mid-April. Martins lay from three to five solid white eggs, weighing three to five grams each. The incubation period is about 14 days. The hen spends 70-80% of daylight hours incubating the eggs. The male usually helps the hen incubate the eggs. Martin chicks grow rapidly over the next two weeks, then the weight gain slows dramatically. Both parents feed the young. Martin chicks remain in the nest for a long period, typically 28-35 days. After fledging, the young may return to the nest to be fed by the parents.
The main reason people fail to attract martins is because the houses are placed incorrectly. Martins need lots of space to maneuver freely around the house. For this reason, houses should be placed in the center of the most open spot available. This should be a minimum of thirty feet from any trees, standing structures or overhead wires.
Birds seem to prefer several small martin houses instead of one large apartment-style colony. Twelve to twenty four rooms are preferable to a house with hundreds of rooms. Martin houses should be opened just after the first sightings of the martins in the spring. Improperly designed houses may have rooms that are too small, which may result in nestlings overheating and dying during hot summer days.
Nesting rooms should be about six inches by six inches by six inches to prevent overcrowding of nestlings. Good ventilation and drainage are necessary. The entrance to the nest should be 2 ½ inches in diameter and one inch above the base. Landing shelves should be very narrow to discourage other birds from using the rooms.
Houses should be 10-20 feet above the ground on a pole. No tall bushes or vines should be near the base of the pole. The house should be near water, if possible. The neatest thing about purple martins is that they allow inspection of their houses without causing any disruption to the breeding season. The house on a pole may be connected to a pulley system, and the house may be raised and lowered like a flag. The adult martins will circle around the lowered house or perch on a wire, waiting for the house to be raised back into position. With this system, it is possible to keep accurate records on number of eggs laid, number hatched, number of chicks that fledge, and number of compartments utilized.
Once martins have left for the winter migration, the house should be thoroughly cleaned.
It is very easy to attract purple martins, and this can be a very enlightening family hobby. By joining the Purple Martin Conservation Association, you can actively conduct research and contribute to the database of purple martin breeding information.
Copyright © 2006 Margaret A. Wissman, D.V.M., D.A.B.V.P.
All Rights Reserved
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