Breeding Bird Atlas 1 Species Accounts
Number of Broods
According to conservative population estimates from about 1900, there were 60 to 80 pairs of Ospreys in southeastern Massachusetts and eastern Rhode Island. Numbers reached a peak in the early 1940s. After World War II, eastern Osprey populations began a serious, well-documented decline, largely due to reproductive failure induced by organochlorine pesticides such as DDT in the environment. By 1964, only 11 pairs remained in the state. Once these pesticides were banned in 1972, the Osprey began to recover, aided by a program of erecting special poles with platforms to increase suitable nest sites. This latter factor has had a significant impact on the Massachusetts Osprey population, with over 95 percent of the pairs in the state since 1980 using these artificial sites. The result has been an increase in the number of nests in which young are reared as well as in the number of young fledged per nest (Poole 1989).
During the Atlas period, most confirmed nesting records were in Bristol and Dukes counties, with a few confirmations in Plymouth and Barnstable counties. In 1981, there were 45 nests in the Commonwealth. The first recent Nantucket nesting occurred in 1983. By 1985, one hundred forty-five young were produced at 88 active nests. While numbers have climbed appreciably, the range expansion has been much more limited because most young Ospreys return to breed close to their natal areas. It is hoped that eventually the Osprey will recolonize the entire coast and perhaps some inland sites.
Resident Ospreys arrive in the state from late March to early April, with males returning slightly earlier than the females. Males claim their old nest sites or select a location if they are first-time breeders. Favored natural nest sites are tall, living or dead trees in open marshy areas, but Ospreys also adapt readily to structures such as buoys and radio towers in addition to the nest poles. Although the nest sites are variable, they are always located near shallow fresh- or saltwater bodies, where the birds will have a ready supply of fish of various species. Ospreys are not truly colonial, but in favorable areas several pairs may nest amicably in close proximity.
Ospreys pair for life but will choose new mates if one partner fails to return. Courtship is often brief, involving dramatic aerial displays. The female does the nest construction, and the male gathers the necessary sticks, grass, seaweed, and sod. New materials may be added each year, and some nests measure 4 feet across by 3 feet deep, with a shallow central cup. Usually three eggs are laid, but two- or four-egg clutches are not rare. Some pairs will re-lay if their eggs are lost, but in Massachusetts late nesters and renesters generally have completed their clutches by the last week of May. Incubation, which lasts four to five weeks, commences with the first egg and is accomplished mostly by the female. She leaves the nest for brief periods to exercise and eat the food provided by her mate, and the latter may relieve her on the eggs during these times. Peak hatching dates are from late May to early June.
The female remains on the nest brooding and guarding the young chicks while the male supplies food for the entire family. Both sexes, but especially the females, will defend the nest from intruders, including humans, although they can do little to prevent the nocturnal raids of Raccoons and Great Horned Owls. When the young are seven weeks old, they spend a great deal of time exercising in preparation for their first flight, which occurs at 51 to 59 days of age. For the next several weeks, they use the nest as a home base during the day and as a roost at night. During this period, they learn to capture their own food, as parental care wanes. After this, the nest is abandoned, although the juveniles remain in the general vicinity. They are sexually mature at three years of age but may not breed until they are five years old.
Fledging success varies according to nest site quality and food supply. In the Westport-Dartmouth area (1981 to 1985) 145 attempts at breeding resulted in the following fledging success: zero young (45 nests), one young (30 nests), two young (34 nests), three young (33 nests), four young (3 nests) (Poole).
Adults depart from Massachusetts in August, with the young following a month later. Migrants continue to pass through during October. Ospreys winter from Florida and the Gulf Coast to South America.