Raptor Res. 38(1) :47-54 2004 The Raptor ResearchFoundation, Inc. STATUS AND DISTRIBUTION OF BREEDING OSPREYS IN THE CHESAPEAKE BAY: 1995-96 BRYAN D. WATTS, 1 MITCHELL A. BYRD, AND MARIAN U. WATTS Center for Conservation Biology, College of Williamand Mary, Williamsburg, VA 23187 U.S.A. ABSTRACT.--We surveyed the tidal portions of the Chesapeake Bay for nesting Ospreys(Pandion hal- iaetus) during the breeding seasons of 1995-96. The population wasestimated to contain 3473 + 75 (SE) breedingpairs. The population hasmore than doubledsince the comprehensive survey conducted in 1973. During this recovery, there has been considerable spatialvariation in the rates of population growth. Mean doubling times for well-definedsubregions varied from a low of 4.3 yr to more than 40 yr. In general,growth rateshave been highestin the tidal fresh and upper estuarine areas, where pairs occurredin 1973. Based on the pattern and magnitudeof the recovery, it seems that the Chesa- peake Bay population experienceda greater decline during the post World War II era than wasprevi- ously believed. Nesting substrate use by Chesapeake Bay Ospreys has shifted since 1973. The use of trees for nesting has declined from 31.7-7.2% in 23 yr. Channelmarkers accounted for 53.5% of all nest structures in the current study. Platforms established specifically for Ospreys supported 12.1% of pairs. The proliferation and diversification of artificial substrates throughout the Chesapeake Bay has been one of the most important factors contributing to recent population expansion. KEY WORDS: Osprey; Pandion haliaetus; Chesapeake Bay;nest substrate;, population increase. ESTATUS Y DISTRIBUCI6N DE •GUILAS PESCADORAS DURANTE SU REPRODUCCI6N EN LA BAHIA DE CHESAPEAKE: 1995-96 RzSUMEN.--Estudiamos las porciones mar•ales de la bahia de Chesapeake en buscade •guilas pesca- doras (Pandionhaliaetus) que estuvieran anidando durante las estaciones reproductivas de 1995-96. Se estim6 que la poblaci6n contenia 3473 ___ 75 parejas en reproducci6n. La poblaci6n ha aumentado mas del doble desde que el completo estudio se11ev6 a caboen 1973. Durante esta recuperaci6n, ha habido una considerable variaci6n temporal en las tasas de crecimiento poblacional.La media de los tiempos de duplicaci6n para sub regionesbien definidasvari6 desde un promedio bajo de 4.3 aftos a uno de mas de 40 aftos. En general, las tasas de crecimiento mas altas han ocurrido en las zonas de marea fresca yen las•reas altas de los estuarios, donde se presentaron pocas parejas en 1973.Con base en el patr6n y magnitud de la recuperaci6n, parece que la poblaci6n de la bahia de Chesapeake experiment6 un declivemfts grande de 1oque se creia, durante la post guerra de la segunda guerra mundial. E1uso del sustrato de anidaci6n pot las •guilas de la bahia de Chesapeake ha cambiadodesde 1973. E1 uso de •rboles para anidar ha diminuido de 31.7% a 7.2% en 23 aftos. Los marcadores de canalesdan cuenta del 53.5% de todaslas estructuras de anidaci6n en el presenteestudio.Las plataformas estable- cidas especificamente para las•tguilas albergaron 12.1% de lasparejas. La proliferaci6n y diversificaci6n de sustratos artificiales a lo largo de la bahia de Chesapeake ha sidouno de losfactores mfts importantes que ha contribuido en la reciente expansi6nde la poblaci6n. [Traducci6n de C•sar Mftrquez] The Chesapeake Bay supports one of the largest Osprey(Pandion haliaetus) breedingpopulations in the world (Henny 1983). As with many similarpop- ulations, Ospreysin the ChesapeakeBay experi- enced dramatic declines in the post World War II era due to reproductive suppression (Truitt 1969, Kennedy 1971, 1977,Wiemeyer1971) induced by 1 E-mail address: bdwatt@wm.edu environmental contaminants (Via 1975,Wiemeyer et al. 1975). The Chesapeake Bay population ap- peared to have reacheda low point by 1973 when Henny et al. (1974) estimatedits size to be 1450 breeding pairs. Since that time, both reproductive performance (Reese, 1975,Henny 1977) and over- all population size (Spitzer 1989, Westall 1990, Houghton and Rymon 1994) have shownremark- able recoveries. 47