Reproductive Ecology of Ospreys and Peregrine Falcons

Because ospreys (Pandion haliaetus) nesting along Delaware Bay have retained low productivity and little population growth since the mid-1970s, during 1987 and 1988 we compared reproductive dynamics of Bay ospreys with a recovering colony nesting along the Atlantic Coast. Productivity of Delaware Bay ospreys remains low because 48% of nests initiated along the Bay failed, compared to only 23% of nests along the Atlantic Coast. Only 49% of eggs laid in the Bay colony hatched, compared to 66% along the Atlantic, and 25% of those young successfully hatched near the Bay were preyed upon by great horned owls (Bubo virginianus). Higher frequence of unhatched eggs and thinner eggshells indicate that Bay ospreys may still be affected by environmental contaminants present in the Delaware estuary. Many factors may affect differential rates of growth within a species, including food supply, weather, and parental ability. We examined growth rate and brood reduction of ospreys nesting in an area supporting abundant, available prey along the Mid-Atlantic Coast. Growth of ospreys from this area was best explained by a logistic growth curve (k = 0.173). We found little difference in growth related to year, clutch size, and brood reduction, although brood reduction occurred most often in larger broods, probably because of large disparities in within-brood size of nestlings. We believe large geographic differences in osprey growth are a function of food availability, while smaller differences -- reflected in brood reduction -- may be a function of a male\u27s age, experience, or ability to supply food. Reestablishment of peregrine falcons (Falco peregrinus) as breeding birds in the eastern United States has been a highlight in endangered species management and recovery. To assess this reestablished population in New Jersey, we examined numbers, reproductive success, and eggshell thinning of peregrines during 1979-1988. From several pairs studied intensively during 1987 and 1988, we describe regional differences in reproductive success, nestling behaviors, and prey of these falcons. Although productivity compares favorably with stable populations, eggshells of New Jersey peregrines are thinning with time. This decrease in statewide-eggshell thinning and low hatching success of eggs along Delaware Bay indicates continued exposure to environmental contaminants, probably from their migratory prey