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Pandion haliaetus

Osprey

  • Order: Accipitriformes
  • Family: Pandionidae
  • Polytypic 4 Subspecies

Authors: Poole, Alan F.

Life History

Food

Osprey in flight with fish, date and location unknown, © Jean M. HoogackerAlmost exclusively live fish; wide variety of species taken, both marine and fresh-water. Size of prey also varies greatly, but generally 8-14 inches (20 - 36 cm) in length. Because Ospreys can dive only about 0.5 - 1.0 m deep, they are restricted to surface-schooling fish, or to those in shallow water.  

In very general terms, comparisons of the ecology of fish taken by Ospreys over a broad expanse of their range showed that benthic-feeding fish (taken only in shallow waters) are the easiest to catch and that in the limnetic-zone piscivorous fish are harder to catch than non-piscivorous fish (Swenson 1979). Benthic feeders may have their attention focused down on food, rather than up where the Ospreys are, and limnetic piscivores are faster than their nonfish-eating relatives.

Very few data on diet from Central and South America.  In the southern Florida keys, spring/summer:  Speckled trout (Cynoscion nebulosus) (64%); striped mullet (Mugil cephalus) (27); sea catfish (Galeichthys felis) (Szaro 1978) -- probably typical for Ospreys in the Caribbean as these species are widespread there.  

In coastal mangroves of Santos and Cubatão, Brazil, key species in the diet of wintering Ospreys: mullets (Mugil spp.) and pompanos (Diapterus rhombeus), representing 77% and 18%, respectively, of 90 prey items recorded (Silva e Silva and Olmos 2002).

Other anecdotal observations from the Neotropics and Caribbean/Gulf waters: Ospreys wintering in the Sea of Cortez leave piles of skulls of coronet fish (Fistularia sp.) under feeding perches on Cardon cacti, and on the upper Texas coast, wintering Osprey (common) take many saltwater catfish (Gafftops sp.; R. A. Behrstock pers. comm.). E. Malaga (pers. comm.) observed Osprey taking Mugil cephalus from lagoons in Mejia along southwest coast of Peru. Some 20 observations of Ospreys taking red piranha (Pygocentrus caribe) in the Venezuelan llanos (C. Sharpe pers. comm.).

Behavior

northern migrant (carolinensis) Osprey perched, Bayamón, Puerto Rico, 05 May 2009, © Alberto LópezGenerally hunts on the wing over open water -- flaps or glides, usually 10–40 m high. When fishing offshore, may climb to 200 m to locate schools and then drop down to begin hunting (Prevost 1982). When spotting a fish, often hovers prior to diving, then plummets, legs extended forward just before plunging feet-first into water. Different angles of attack observed for different types of fish—long, shallow dives for fast-swimming fish near surface, steeper dives for slower fish found deeper in water (Prevost 1982).

Also hunts from perches when available; perch-hunting may be more frequent on wintering grounds, where individuals can tolerate a lower frequency of encounter with potential prey items (not feeding young or mate). In a particularly unusual example, an Osprey was observed catching flying fish from a perch in the rigging of a research vessel in Pacific Ocean, >1,000 km off west coast of Mexico (Rogers and Leatherwood 1981). 

Rarely walks; somewhat awkward on ground, although can move delicately around small young in nest, toes and claws closed tight. Usually flies with steady, rowing flight, as befits a species with narrow wings and relatively high wing-loading. In migration, readily crosses large water and desert barriers (e.g, Caribbean Sea, Sahara Desert). Soars high on thermals, particularly at midday near breeding colonies, but also in migration. Long wings preclude flight in all but open areas; not maneuverable; fledglings occasionally found dead, tangled in vegetation (AFP).

Generally roosts alone, but on wintering grounds small flocks (6–10) sometimes form near productive feeding areas (e.g., Prevost [1982] noted up to 10 perched on a beached log in west Africa). Prefers to roost in open areas (e.g., bare branches of tall trees; large logs on mudflats; channel markers) but will roost in trees with leaves if sufficiently open. Also roosts on ground, especially in cold, windy weather when individuals undoubtedly find a thermal advantage in doing so. Male generally has preferred roost near nest, used for feeding and loafing; e.g., bare branch of a nearby tree, ground hummock, etc.; quick to use artificial roosts if installed near nest (AFP).

Few data on winter time budgets; wintering individuals generally mobile and hard to follow. Prevost (1982) provides rough estimates for Ospreys wintering in west Africa: foraging time to meet daily requirements varied by habitat and month, but ranged from 16 to 145 min/24 h, averaging about 30 min; 12 h spent sleeping (roosting); about 0.5 h flying not related to hunting; and rest of 24-h period resting. Not a strenuous day; compare with breeders, above.

Agonistic behavior revolves mostly around defending nest sites/nests against other Ospreys. Nest sites a key limiting factor for the species, so defense is critical and often intense, especially where individuals nest close together in colonies. In general, the denser the colonies, the more frequent and intense the aggression (AFP).  Calling and displays appear to be key components of low intensity agonistic behavior: wing-shaking, fanned tail, and horizontal position of body (Bretagnolle and Thibault (1993).

Vigorous aerial chases often ensue if intruding Osprey approaches nest too closely. Defender may try to strike intruder in air with claws, driving it away from nest site. Anecdotal evidence suggests that (very rarely) individuals may be injured or even killed in such fights (AFP). Needs closer study in dense breeding colonies; intensity of defense undoubtedly differs among individuals.

Nestlings sometimes fight over food, particularly when food is limited; this often leads to dominance hierarchies among nestmates, and becomes a key component of brood reduction when food supplies are low -- smaller, late-hatched young starve to death because their older siblings prevent them from getting food when the female parent distributes it.

Territoriality

northern migrant (carolinensis) Osprey in flight, Mejía, Islay, Arequipa, Peru, 15 May 2008, © Tor Egil HøgsåsNot strictly territorial, as in other fish-eating birds; fish are a mobile resource, hard to defend, making efficient defense of feeding sites unfeasible.  

Will defend immediate nest site, sometimes reaching out as far as 0.5 km, but in many situations nests "colonially," with several nests in one tree (e.g., inland Florida; Lake Istokpoga, M. McMillian). On wintering grounds, individuals are known to congregate where fish are abundant [e.g., 4-5 birds on one log; Prevost 1982, Senegambia], but are generally more spread out, averaging 0.7 Ospreys/km in coastal mangroves in southeastern Brazil (Silva and Olmos 2002).  A large, shallow, fresh-water reservoir in southern Mexico (Oaxaca) hosted roughly 80 Ospreys each winter, 1981–1983 (Barradas 1984).

Sexual Behavior

Almost always monogamous, very rarely polygynous; latter likely driven by scarcity of nest sites.  A few males will try to defend > 1 nest site.  

Courtship displays well documented by Bretagnolle and Thibault (1993). Aerial Sky-Dance Display of male may begin and end at nest site; more often as male flies back to nest from foraging. Generally seen during courtship period and early in incubation, but nonbreeders display throughout breeding season. In this dramatic display flight (sometimes called “fish-flight”), male dangles legs (often clasping a fish or nesting material) and proceeds in slow, undulating flight over nest site, usually high overhead (up to 300 m or more) giving Screaming Calls repeatedly (see Sounds: Vocalizations). Display and calling may continue for ≥10 min; male sometimes losing altitude all the while, descending slowly in undulating staircase fashion to nest site.

Pairs appear to form at nest site. Females fed almost exclusively by their mates prior to laying (courtship feeding), starting a long period of dependency that lasts until young fledge or the pair fails in its breeding attempt. Females solicit food from their mates by begging (Solicitation Call); in solicitation display, body axis is horizontal, crest-feathers slightly erected, and wings held close to body (Bretagnolle and Thibault 1993).

Courtship-feeding also studied in British Columbia (2 yr; Green and Krebs 1995), with the following conclusions: (1) Pairs that laid eggs had higher courtship-feeding rates (CFR) than those that did not lay. (2) Male CFR correlated negatively with duration of courtship period. (3) No evidence that females traded copulations for food; only 63 of 385 observed copulations were associated with food. (4) Male provisioning rates were predictable; i.e., CFR correlated with both male food delivery rate to nest when chicks were 1–2 wk old and with mean growth rate of the brood. Thus females may use male CFR to judge suitability of a mate. Mean CFRs in British Columbia 52.2 kJ/h ± 11.0 SE (n = 10).

In addition to feeding their mates, males guard them before and during egg-laying. A guarding male follows his mate closely wherever she flies and assiduously chases intruding males from nest site (AFP).

Preliminary findings from 2 color-banded U.S. populations (Michigan [S. Postupalsky pers. comm.], southeastern Massachusetts [AFP]) suggest mate fidelity is high, about 60–70%/yr; loss of nest sites may lower this figure since attachment to site appears strong and may contribute to mate fidelity. 

Social and interspecific behavior

Generally more solitary than social. Breeding colonies promote social behavior; e.g., group soaring on warm days when thermals form; groups (3–10+) soar high over colony, often giving slow Guard Calls (AFP). This behavior needs study; males certainly participate, not clear if females do, too. Not clear why individuals soar; perhaps a territorial function.

Greene (1987) suggested that Osprey breeding colonies facilitate transfer of information about where fish can be found; individuals key in on successful foragers returning to colony. Other studies have not found this to be true, however (e.g., Hagan and Walters 1990). In addition, on foraging grounds, group foraging may enhance foraging efficiency (Flemming et al. 1992) or may simply be a response to abundant, concentrated prey.

On wintering grounds in west Africa, dispersion well studied by Prevost (1977); key findings: (1) Dispersion seemed random along coasts of sandy beaches and between river mouths, but regularly spaced farther inland in closed mangrove forest, where individuals often hunted from perches; speculation that regular spacing was socially determined. (2) Tidal cycles affected dispersion; individuals more concentrated at low tide, when inland foragers moved to coast. (3) Largest, densest groupings (up to 20 individuals) at river mouths and coastal ponds.

Predation

Few/no data from the Neotropics.

Bald Eagles (Haliaeetus leucocephalus) known predators of nestlings and (rarely) of adults. In south Florida, nesting Ospreys temporarily abandoned islands (keys) that supported active nests of Bald Eagles, although the 2 species can share nesting keys (Ogden 1975). Seen from the air, plumage of Osprey nestlings provides superb camouflage against the nest, suggesting that diurnal avian predators such as Bald Eagles have been a significant selective force on Ospreys.

Crocodiles (Crocodylus niloticus) known to kill Ospreys in Senegal, west Africa (Prevost 1977); Crocodylus sp. may be important predators of Ospreys on wintering grounds in South America. Ospreys vulnerable when bathing and roosting at water’s edge.

Predation could be a key selective force explaining why Ospreys do not breed in the tropics.  On cayes along the coast of Belize, for example, boa constrictors are often abundant arboreal predators that are known to take nestling birds (AFP).  For species that nest in large colonies, these snakes are easily saturated, and thus have minimal impact during the nesting season.  For a bird like the Osprey, however, that nests solitarily, snakes could have a much bigger impact, cleaning out a nest over a period of a week or two. 

Reproduction

Breeding dates

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).  

Nest site/nest

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).

Clutch size

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.

Incubation

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). 

Nestlings/young

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).

Populations and Demography

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).  

Recommended Citation

Poole, Alan F.. 2009. Osprey (Pandion haliaetus), Neotropical Birds Online (T. S. Schulenberg, Editor). Ithaca: Cornell Lab of Ornithology; retrieved from Neotropical Birds Online: http://neotropical.birds.cornell.edu/portal/species/overview?p_p_spp=119196