Ospreys breeding south of the USA tend to lay eggs in the boreal winter months: e.g., November to early February in Belize (peak laying late November to early January; AFP). During a 2 yr study in northwestern Mexico, onset of egg-laying ranged approximately 9-10 weeks from early January to early March (Judge 1983).
Wide variety of natural and artificial sites, well described in Bent 1937, Poole 1989a. Common features, generally: proximity to water, especially good feeding areas; openness, allowing easy access to nest; safety from ground predators, achieved by height or over-water location (islands; flooded trees, channel markers); sufficiently wide and stable base to accommodate the large nest. Habituates quickly and easily to nearby human activity (e.g., highways, houses).
In northwestern Mexico (Baja), where trees are scarce, often nests on various species of tall cacti (26% of 810 nests); also on cliffs (59%; Henny and Anderson 1979). Increasingly in this region, shifts to artificial sites: power poles, nesting platforms; e.g., near San Ignacio Lagoon, Baja, use of artificial nesting structures was 6.2% in 1992/1993, 26.4% in 2006 (Henny et al. 2008). In the Caribbean (coastal Belize), most nests in flat-topped mangroves; also on artificial structures like power poles, abandoned fishermen's shacks.
Nest a large mass of sticks, with sea grasses and flotsam and jetsam used for lining. Generally male brings bulk of material to nest, female arranges it once there. May break dead sticks off nearby trees in flight or (more often) snatch from ground. Most nest-building pre-laying and right after hatch; sporadic nest-building throughout nestling period and even after nest failure.
Nest sites generally reused year to year, often for a decade or longer. Of 68 nests monitored in Gulf of California in 1977, all but 9 reused in 1978; birds changing mates most likely to shift sites (Judge 1983). In Florida Bay, reuse about 70–80%; here most nests in natural sites (mangroves) and thus prone to decay (AFP).
Generally 3-4 eggs in northern populations, 2-3 in nonmigratory populations. In Baja California: mean = 2.63 ± 0.58 SD (n = 86), with 5.8% 1-egg, 24.4% 2-egg, and 69.7% 3-egg (Castellanos, unpubl.; Judge 1983); s. Florida, 1978 and 1979: 2.7 ± 0.6 SE (n = 22; Poole 1982a); Sonora, Mexico, 1992–1997: 72% 3 eggs, 25% 2-egg nests, a few 1- and 4-egg (n = 105; Cartron 2000). Data needed from Cuba and the Caribbean.
The eggs usually are creamy white in color, heavily blotched or marked with dark brown or reddish brown.
Both sexes incubate, but female generally does most; e.g., about 70% versus 30% (male) of daylight hours; female nearly always incubates at night. Male usually provides female with all food during this period; female takes fish to nearby perch and feeds there; male generally incubates while female feeds, but will initiate incubation independent of food transfers. Considerable variation among pairs in division of incubation labor, male sometimes doing the majority. Incubation often begins with first egg but sometimes sporadic until second is laid. Incubation period (first egg to first hatch) averages 37-39 d in northern populations (AFP).
Semiprecocial; down-covered; body mass about 50 g; weak in movements but can beg, briefly; brooded almost continually; fed small bits of fish by female parent (Poole 1989). In south Florida and in southern New England, significant age and size disadvantage for third-hatched chicks: 3.9 d ± 0.06 SE (n = 29) younger than first-hatched; second-hatched only 1.4 d ± 0.17 SE (n = 44) younger than first; survival to fledging 88% for second, 38% for third (Poole 1982).
Nestlings brooded almost continually by female up to age ca. 14 d; intermittently thereafter. Males provides food to female and young; female distributes food to young at the nest. By about 40 d of age, young begin to feed on their own, taking prey from male as he lands at nest or from female parent after she has fed. Age at first flight 50-55 d in northern populations, older (65 d) in a well-studied Baja population (Judge 1983). Latter likely to be more typical for Neotropical breeders, which appear to grow more slowly than their northern counterparts (shorter day-length [less foraging time] likely a key factor in this difference).
Brood size at fledging tends to be lower for Ospreys breeding in the Neotropics than for their counterparts farther north: e.g., Bowman at al. 1989, s. Florida (Florida Bay and Keys): young/active nest = 0.56 ± 0.8 SE (n = 34) in the bay, 1.21 ± 1.0 SE (n = 19) in the Keys, a significant difference apparently related to food availability; Florida Bay a more disturbed ecosystem with food less accessible than along Atlantic Coast, where Keys Ospreys forage. Likewise only 38% of active bay nests successful versus 63% of Keys nests. Poole (1982) likewise found poor reproduction in Florida Bay Ospreys during the early 1980s.
Sonora, Mexico, 1992–1997: mean annual productivity 0.67 fledged young/occupied nest (breeders plus nonbreeders), 0.83/active nest (breeders only; range 0.18–1.3 over 6 yr; Cartron 2000). Considerable local variation in success; years/areas with poor success had more late breeders than others, but overall no clear explanation for large annual variation in productivity. No difference in success of early versus late pairs, unusual for this species. Feeding rates not monitored. Overall more eggs than chicks lost, but chick death rates high in poor (low food?) years (47–73% loss).