Various aspects of Osprey biology, Pandion h. haliaetus, were studied from museum specimens and in the field. Size, breast markings and crown markings were found insufficient to discriminate populations, but underwing coverts were sufficient. Four subspecies were recognized, corresponding to the Palearctic, North America, the Bahamas and Australasia. However Ospreys can be divided into two groups, a Holarctic group, consisting of Palearctic, North American and Bahaman Ospreys, and the Australasian Ospreys. Ecology was studied in Senegambia during two visits in 1977-80. Over 800 Ospreys were estimated to winter there, mostly at river mouths and in mangrove. Marked Ospreys returned to the same area in consecutive years, and stayed within that area during winter. Dispersion along the coast was random except at a few localities where the birds concentrated. In mangrove, birds were regularly spaced at high tide but went to the coast or more open mangrove at low tide. Along the coast, Fish Eagles, Haliaeetus yocifer, and Ospreys seemed independently distributed, but in mangrove Ospreys avoided hunting in sight of a Fish Eagle, while along rivers and lakes eagles might have excluded Ospreys. The diet consisted mostly of Mugilidae, various Clupeidae, and, during part of winter, of Exocoetidae. Average fish size was generally 200-300 g, but fish were smaller in areas well protected from the open sea. Dive success and search time per capture varied between sites, partly because they were significantly correlated with fish size. Immatures 6 months old were less successful at catching fish than older birds. Foraging efficiency varied from 2.9 to 10 kcal/minute of foraging. Most Ospreys were found in mangrove even though foraging efficiency was lower there. This was most likely because hunting from a perch in mangrove was less demanding than hunting from flight elsewhere. Moult was studied from captured Ospreys and museum skins. A few Ospreys were caught more than once so that the development of the moult pattern of the primaries could be traced. Limits to the growth rate of feathers are emphasized as a major reason for the evolution of the Staffelmauser pattern of moult in large birds. Ospreys do not breed in the tropics, except in Australasia, even though the habitat seemed ecologically suited. In particular, foraging efficiency was as high in Senegambia as on the north-temperate breeding grounds. It is suggested that migrants Ospreys are physiologically inhibited from breeding in the tropics because daylength is too short, while south of the tropic of Capricorn, where migrants might be stimulated to breed, but six months out of phase, they are too scarce to start a permanent breeding population. It is suggested that non-migratory Ospreys have not spread their range south because of the presence of large numbers of migrants in the tropics.