Estimated Status of Osprey Populations in the United States

Abstract

The status of ospreys (Pandion haliaetus carolinensis) in seven states, as detexmined by conaparison of recently observed rates of production to rates required to maintain a stable population, is discussed. It is estimated that 1.2b1.30 young must be produced per active nest tincluding unsuccessful nests) to maintain a stable population. The decline in numbers of ospreys was not uniform throughout the United States. The population at Florida Bay, Florida, seemed to be stable as opposed to the estimated 1214 percent (minimal) annual decline in numbers during the 1960s of localized populations in Connecticut, Maine, Michigan, and Wisconsin. For the same time period, populations in MarylaIld and Minnesota were estimated to be declining at a 2-3 percent rate annually. The purpose of this paper is to summarize data on osprey reproductive success derived from several studies conducted since 1952 on the breeding biology of the osprey, and to determine the magnitudes of declines in osprey populations by comparing observed rates of production to calculated rates of production necessary to maintain stable populations. Several nesting studies ( Ames and Mersereau 1964, Reese 196S, Kury 1966, Dunstan 1968, Berger and Mueller 1969, Postupalsky 1969) to assess the productivity of ospreys apparently were prompted by recent declines in breeding populations. In order to compare and interpret results of these studies, it was necessary to recalculate data on productivity of ospreys, and to express these data in terms of numbers of young fledged per active nest ( including unsuccessful nests). Use of this parameter permitted direct comparison of observed production to production required for a stable population estimated by employing the model developed by Henny et al. ( In press ) . The senior author's research was supported by Grant No. 14-16-008-922 from the Department of the Interior, Bureau of Sport Fisheries and Wildlife. We are deeply in1 Technical Paper No. 2686. Oregon Agricultural Experiment Station, Corvallis. 214 debted to B. J. Verts, H. M. Wight, W. B. Robertson, Jr., and W. H. Stickel, who critically read and offered advice in the preparation of the manuscript. An estimate of production necessary to maintain population stability was available for only one osprey population. Henny and Wight (1969), using combined recoveries from birds shot and found dead, determined survival rates for immature and adult ospreys, and calculated that an annual production of 1.2b1.30 young per female of breeding age was necessary to maintain stability of a New York-New Jersey population of ospreys. Ospreys usually lay three eggs per clutch (Bent 1937). Support for the reliability of the estimate of production may be derived from a stable population of ospreys at Florida Bay, Florida ( 141 active nests in 1968 and 138 in 1969) with a current production of 1.22 young per nest ( Table 1 ). ( For more details of this study see Ogden 1969. ) Because of the apparent reliability, and because no other data were available, these estimates of production necessary to maintain stability were applied to other osprey populations. Comparison of the 1.2g1.30 estimate with actual fledging rates per active nest ( Table 1 ) demonstrate the degree and variability of deteriorating osprey reproduction. Apparently Ames and This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:16:07 UTC All use subject to http://about.jstor.org/terms ESTIMATED NO. ACTIVE NO. ( MINIMAL ) NESTS FLEDGED PERCENT ANNUAL RATE (ALL YEARS YEAR OF PER ACTIVE NESTS DECLINE SOURCE OF STATE SUMMED ) STUDY NEST SUCCESSFUL ( PERCENT ) NESTING STUDY Florida 83 196S69 1.22 70 stable This paper Minnesota 161 196668 1.03 65 2-3 Dunstan 19;68 Marylandb 136 196465 1.03 54 2-3 Reese 1965 Wisconsin 128a l952-59 O.98 53 34 Berger and Mueller 1969 Wisconsin 67 1960-65 0.39 30 12-13 Berger and Mueller 1969 Michigan 162 19%-67 0.39 23 1b13 Postupalsky 1969 Maine 8 1964 0.38 25c 1K13 Kury 19fi6 Connecffcut 157 196043 0.29 23d 1>14 Ames and Mersereau 1964 Connecticut 3