Diet
Ospreys (Pandion haliaetus) are of particular interest to researchers at the Virginia Institute of Marine Science because they are the only birds of prey that subsist almost exclusively on a diet of live fish—hence their common name of "fish hawks."
A 1985 study by researchers in the Department of Biology at the College of William and Mary showed that Atlantic menhaden (Brevortia tyrannus) is the major food source for ospreys in lower Chesapeake Bay, accounting for nearly 75% of the birds’ diet. White perch (Morone americana), Atlantic croaker (Micropogonias undulatus), oyster toadfish (Opsanus tau), and American eel (Anguilla rostrata) are also common prey.
The study found that the birds delivered 5.4 fish per day on average to the nest, with the average fish weighing just over a third of a pound. The length of the fish captured by the ospreys ranged from 4 to 43 centimeters (1.5 to 16 inches).
A 2009 study by researchers with W&M's Center for Conservation Biology (CCB) refined the results of the earlier study, showing that the diet of ospreys in Chesapeake Bay varies by salinity, with Atlantic menhaden and seatrouts (Cynoscion spp.) the most common prey item for breeding pairs within the saltier waters of the lower Bay, whereas gizzard shad (Dorosoma cepedianum) and catfish (Ictaluridae) dominated in the fresher waters of tributaries.
Their study also showed that fish from higher-salinity waters were on average 6% shorter, 34% lighter, and 40% lower in energy content than those from fresher waters upstream. They conclude that this difference in diet quality may help explain why osprey populations are recovering from their DDT-induced, 1970s decline more rapidly along the Bay's tributaries than they are in the saltier waters of the Bay's mainstem. A 2004 study by CCB researchers revealed that the mean doubling times of osprey populations ranged from 4.3 yr along the lower-salinity reaches of upper tributaries to more than 40 years in higher-salinity areas of the Bay proper.
Based on an estimated Chesapeake Bay population of 3,000 breeding pairs (in 1985), the authors of the 1985 study estimated that ospreys eat about 132,171 kg (291,387 pounds) of fish during the 52-day nestling period. They note that this "harvest" represents 0.004% of the annual Chesapeake Bay commercial harvest and likely has a minimal impact on local fisheries.