Growth and Brood Reduction of Mid-atlantic Coast Ospreys

-Ospreys (Pandion haliaetus) exhibit both large geographic differences in growth rate and variable levels of brood reduction, which are assumed dependent on local prey abundance. We examined Ospreys nesting along the mid-Atlantic coast to determine how growth rate and brood reduction were influenced by an abundant, available prey base. Growth rate was substantially higher than previously reported and best described by a logistic growth curve (k = 0.173). We found little or no difference in growth related to year, to brood size, or in broods with and without nestling loss. Brood reduction occurred in this area, despite an apparently ample prey base, and was more common in larger broods. Higher incidence of brood reduction in large broods was a function of larger disparities in within-brood nestling size, probably caused by inadequate food provisioning by males. We believe large intercolony differences in Osprey growth are primarily a function of local food abundance and availability, whereas intracolony variation-reflected in higher rates of brood reduction-may be a function of a male's ability, experience, or motivation to provide food to nestlings. Received 12 March 1990, accepted 29 October 1990. INTRASPECIFIC variation in avian postnatal growth is influenced by an array of extrinsic and intrinsic factors (Ricklefs 1983: 58-63). Extrinsic factors, which include food quality and abundance, weather, and habitat quality, have previously explained differential nestling growth within a species (Lack and Lack 1951; Van Balen 1973; Bryant 1975, 1978; Dunn 1975; Ross 1980; Poole 1982). Intrinsic factors-which include components of a species' reproductive biology such as type of development (precocial, semiprecocial, or altricial), clutch size, hatching synchrony, and quality and amount of parental care-also affect growth (Ricklefs 1968; Klomp 1970; Howe 1976,1978; Ricklefs and Peters 1981). Because growth within a single species is constrained by these intrinsic factors, variation in extrinsic factors such as food supply, weather, and parental care should account for differences observed in growth rate. Growth rate and reproductive success of Ospreys (Pandion haliaetus) will vary with the amount of food supplied by adults to nestlings (Poole 1982). Because Ospreys hatch asynchronously, nestlings often compete with one another when prey is delivered at an insufficient I Present address: Department of Fisheries and Wildlife, Oregon Cooperative Wildlife Research Unit, Oregon State University, Corvallis, Oregon 97331-3803 USA. rate (Poole 1979, 1982; Jamieson et al. 1983; Hagan 1986). When sibling competition becomes severe, brood reduction-manifested through death of the youngest nestling-usually occurs in Ospreys as in many other species that hatch asynchronously (Ricklefs 1965, Meyburg 1974, Stinson 1980, Hahn 1981, Poole 1982, Hagan 1986). Although usually linked with reduced food supply (Newton 1977), sibling aggression has also been observed in Ospreys nesting near areas rich in available prey (Jamieson et al. 1983). To determine growth rate and factors that affect brood reduction in an area rich in suitable prey, we investigated growth rates of Ospreys in an area with abundant prey, the effects of brood size on growth, and the frequency of brood reduction and its effect on growth. We examined why brood reduction in Ospreys occurs in an area with abundant prey.