Osprey Pandion haliaetus

  • Order: Accipitriformes
  • Family: Pandionidae
  • Polytypic: 4 subspecies
  • Authors: Alan F. Poole
Sections

Behavior

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

Information needed. Contribute

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.

Recommended Citation

Poole, A. F. (2009). Osprey (Pandion haliaetus), version 1.0. In Neotropical Birds Online (T. S. Schulenberg, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/nb.osprey.01