Well studied in migratory populations of this species, owing to solid data on breeding rates, survival of fledged young and adults, dispersal distances (natal and between breeding attempts), and age at first breeding—the key variables influencing avian population dynamics. See Poole 1989a (Chapter 8) and Postupalsky 1989 for details; these built on earlier Osprey studies by Henny and Wight (1969) and Spitzer et al. (1983).
A few key findings regarding dispersal: females disperse farther than males between sites of fledging and first breeding, as is typical of birds, but only rarely do Ospreys of either sex breed >50 km from their natal sites; year-to-year fidelity to breeding locales appears to be even higher (few move >10–15 km). Together these findings suggest that (1) growth or decline of a population is determined largely by local reproductive and survival rates and (2) Ospreys are slow to colonize new areas.
Precise data on changes in population size in a few well-studied locales, along with data on annual survival of breeders and younger birds and breeding rates in these populations, have allowed researchers to link these parameters and determine breeding rates needed for population stability (Spitzer et al. 1983, Poole 1989a, Postupalsky 1989 and references therein). About 0.8–0.9 young/active nest appears to be cut-off point in this species; field data show that New England Osprey populations declined significantly in years (1950s and 1960s) when reproductive rates were lower than this (owing to DDT contamination), but grew quickly when rates improved above this level in the 1970s and 1980s (Table 8.4 in Poole 1989a). During same period, Michigan Osprey populations achieved stability at almost precisely the same breeding rate (Postupalsky 1989).
Rate at which new breeders are recruited to a population, however, is also a function of the age at which they start breeding, which appears to vary among populations, and perhaps (over time) within populations as well. Thus in a region like Chesapeake Bay, where Ospreys start breeding at a mean age of 5.7 yr, a breeding rate of 1.15 young/active nest appears needed to achieve population stability, assuming survival of adults and fidelity to breeding and natal sites is the same as in New England and Michigan; this estimate jumps to 1.30 young/active nest when mean age at first breeding is 6.7 yr (Poole 1989a).
One notes that nearly all studies of Osprey population dynamics have examined populations that were small relative to the resources (food, nest sites) available to them. Poole (1989b) argued that quality and availability of nest sites were key factors in the dynamics of Osprey populations studied during the 1980s: reproductive success was significantly higher among pairs nesting on artificial sites (owing mostly to stability of such sites); and populations with the largest number of empty sites available to them were those that grew most quickly. Findings during the 1990s suggest that it may be time to modify this assessment, however: (1) Gardiners I., where breeding numbers declined 20–30% during 1996–2001, apparently because fish in nearby waters were not consistently available (P. Spitzer unpubl.); (2) Martha’s Vineyard, Massachusetts, where number of active nests doubled every 4–5 yr during 1980s, leveling off and even dropping during the 1990s, despite empty nest sites in apparently desirable locations (ROB); (3) Finland, where breeding numbers have grown only about 1%/yr during past 2 decades, despite reproductive success equal to that of U.S. and Scottish populations growing at 5–10%/yr (Saurola 1995). Clearly we still have a lot to learn about Osprey population dynamics.
One also notes that nearly all studies of Osprey population dynamics have examined migratory populations. How populations of resident Ospreys breeding in the Neotropics might compare in population dynamics remains guesswork. One might expect, however: 1) lower mortality among both adults and younger birds, as these can avoid the hazards of migration; 2) higher age at first breeding, since there is potentially less turn-over among breeders and pairs can guard their nest sites for more of the year -- i. e., young may have to wait longer to become breeders; 3) lower breeding rates: clutch size is known to be smaller in Ospreys in south Florida and northwestern Mexico, compared to their northern counterparts (Poole 1982; Judge 1983).