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Osprey

Pandion haliaetus

Order:
Accipitriformes
Family:
Pandionidae
Sections

Behavior

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Locomotion

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With relatively long, narrow wings, Ospreys have a steady, languid wingbeat that is somehwat shallow.

© Anita Regler, New York, United States, 13 April 2016
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Ospreys will engage in hovering flight, frequently during foraging but also occassionally as part of courtship and nest defense.

© Mary Harrell, Florida, United States, 24 April 2016

Ospreys are somewhat awkward on ground and rarely walk, although individuals can move delicately around small young in the nest, toes and claws closed tight. Usually flies with a 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; see Nature of Migration). Steady, languid wing-beat that is somewhat shallow; appears centered at elbow joint (Dunne et al. 1988). 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; recently flighted young occasionally found dead, tangled in vegetation, presumably after a failed landing (AFP).

Flight speeds measured by radar in Europe: Average flap-gliding speeds (13.7 m/s) and gliding speeds (12.5 m/s) are realistic values for migration (45–50 km/h; Bruderer and Boldt 2001). Migrating birds equipped with GPS units typically fly between 6.7 and 12.5 m/s (24.1 to 45 km/h) (ROB). One adult migrating south with strong tailwinds reached 22.4 m/s (80 km/h; OspreyTrax-Belle).

Self-Maintenance

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Ospreys bathe in salt and fresh water, generally at shallow edges. Wades into water and ducks head and breast, vigorously beating wings; emerges and shakes plumage, then preens. May perch and droop wings to dry afterward.

© Daniel & Joan Emlin, Florida, United States, 22 December 2015

From AFP and Cramp and Simmons 1980b. Active preener and bather. Bathes in salt and fresh water, generally at shallow edges. Wades into water and ducks head and breast, vigorously beating wings; emerges and shakes plumage, then preens. May perch and droop wings to dry afterward. Also preens dry plumage, especially after incubating/brooding at nest. Male often preens after delivering fish to mate. Primaries, rectrices preened individually, run through bill. Head-scratching direct. Shakes body plumage after preening to settle feathers. After diving for fish, emerges from water and shakes plumage vigorously by twisting body in flight, head to tail; distinctive behavior. After feeding, cleans bill and claws; may wipe bill on branch; may flick away fish scales adhering to beak with lateral head-shakes. Often drags feet in water after feeding, presumably to clean them (perhaps also cooling).

Generally roosts alone, but on overwintering 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 western 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 likely find a thermal advantage in doing so. During nesting, male generally prefers roost near nest, used for feeding and loafing (e.g., bare branch of a nearby tree, ground hummock); often quick to use artificial roosts if installed near nest (AFP).

Daily time budget

Varies widely; influenced by sex, stage of annual cycle, and location. Poole 1984 compared time devoted to foraging among males with unfledged young in South Florida and eastern Long Island, New York. On average, Florida males (n = 4) foraged 1.86 h/d (12 h daylight); New York males (n = 14), 2.41 h/d and 5.55 h/d at 2 different locations, the latter far from productive fishing areas (16 h daylight). Other aspects of time budgets less well quantified here, but 90–95% of remainder of day spent perched (resting, eating, preening), 5–10% in movement near the nest; e.g., bathing, nest defense (flight), etc. At the same time, females in these populations spent over 95% of day (and presumably all night) at the nest (brooding, guarding, and feeding young), 2–5% of the day off the nest (bathing, chasing intruders [nest defense], foraging [rare until young fledge]). In northeastern Nova Scotia, females spent much more time at nest than males did, incubated more (70 vs. 30%); male hunting time increased markedly after hatching, from 19 to 55% of daylight observation time (Jamieson et al. 1982).

Few data on time budgets during overwintering periods, when individuals are often mobile and hard to follow. Prevost 1982 provided rough estimates for Ospreys overwintering 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 remainder of 24-h period was spent resting. Not a strenuous day; compare with breeders, above. Substantial amounts of data providing minute-by-minute locations from GSM-GPS satellite tagged birds yet to be analyzed will add to our understanding of daily time budgets both on the breeding and wintering grounds.

Agonistic Behavior

Best described among breeders in Corsica, but context not always clearly presented (Bretagnolle and Thibault 1993). No North American studies, but behavior does not appear to differ greatly (AFP). Revolves mostly around defending nest sites against other Ospreys. Nest sites are a key limiting factor for the species, so defense is critical and often intense, especially where individuals nest close together in colonies. But in hyper-dense colonies such as pre-DDT era Gardiners Island (ca. 300 pairs on a 13.4 km2 island) where nests were 10 m apart on the ground or Blue Cypress Lake, Florida (260 pairs around a 26 km2 lake) where 2 or 3 nests are in the same tree, aggression is often suppressed – it appears that individuals recognize neighbors and tolerate them, while continuing aggression toward non-neighbors (AFP).

Directed toward other Ospreys; 2 or 3 displays described and illustrated in Bretagnolle and Thibault 1993 (see also Cramp and Simmons 1980b). Vocalizations (see Sounds and Vocal Behavior: Vocalizations) often accompany these displays:

(1) Upright: Distinguished from resting by more vertical body axis, extended neck, wings held slightly opened; rarely male may erect head (crest)-feathers. Appears a prelude to more intense displays (see below); i.e., initial reaction to an approaching Osprey (mate or intruder). Thus signals increased alertness.

(2) Protection/defense: Body held horizontal, with wings (carpal joints) tight to body; as display intensifies (Defense), wings opened with carpal joint drooped. In highly intense state, tail may be raised and contracted. Guard Calls often given during Defense. Given mostly by female when an intruder flies over a nest. Male may Defend, especially early in breeding season and typically during copulation period, when sighting an intruder (perhaps only males). Thus display probably involves defense of mate as well as of nest and suggests remarkably well-developed recognition of neighbors vs. outside intruders (guard calls likely aid individual recognition).

(3) Nest-protecting: Seems an extension of (more intense) Defense. Characterized by wing-shaking, fanned tail, and horizontal position of body. Bretagnolle and Thibault 1993 suggested that wing-shaking emphasizes white border of back to an individual flying overhead. Given almost exclusively by females facing intrusion of another Osprey approaching nest, in flight. Often accompanied by Guard, Excited, or Screaming calls (gradation from low to high intensity). Not clear if display is given to known neighbors in dense colonies, but preliminary studies suggest not—that breeders distinguish neighbors from unknown intruders (AFP). Needs study.

Vigorous aerial chases often ensue if an intruding Osprey approaches a nest too closely. Defender may try to strike the intruder in the air with its claws, driving it away from the nest site. Individuals may be injured or even killed in such fights (ROB). This needs closer study in dense breeding colonies; intensity of defense undoubtedly differs among individuals.

Nestlings sometimes fight over food, particularly when food is limited; see Breeding: Parental Care.

Spacing

Territoriality

Like other fish-eating birds, generally defends only the nest site, not feeding territories. On wintering grounds, may be territorial at times; e.g., when hunting from perches in mangroves (Y. Prevost in Cramp and Simmons 1980b), but as mentioned above, individuals can congregate with no apparent aggression; needs further study. Spacing of nests varies greatly depending on region (food availability) and availability and type of nest support. Mean distance between Osprey nests in 4 regions of eastern North America varied with nest supports as follows: New York State (upland trees), 410 m (n = 17 nests); North Carolina (swamp trees), 170 m (n = 35); southeastern Massachusetts (artificial platforms in salt marshes), 140 m (n = 33); Nova Scotia (island ground nests), 50 m (n = 9; Table 8.6 in Poole 1989a). In boreal-forest habitats, nests are generally much farther apart, dozens of kilometers or more (e.g., 0.2–2.1 pairs/100 km2 in northern Quebec [Great Whale River basin; Bider and Bird 1983]; 1 pair/51.0 ha–2.6 km2 in New Brunswick; Stocek and Pearce 1983). Nest sites defended aggressively (see Agonistic Behavior) but generally only up to 50–100 m around nest. Can form dense colonies (see above); e.g., Gardiners Island in New York, Lakes Istokpoga and Blue Cypress in Florida, and in Baja California with nests as close to 10 m apart on the ground or up to 3 nests in the same tree.

Individual Distance

Generally solitary except when breeding. Breeding males tend to roost near mate or nest (always within sight); sometimes roost at edge of nest but more often away from nest. At artificial nesting platforms in southeastern Massachusetts salt marshes, males given the choice between an artificial perch at the nest versus one below on the ground (10–20 m away) nearly always chose the ground perch for feeding and loafing. Once young near fledging, the female parent generally moves away from the nest to a nearby perch (within sight). On wintering grounds, small flocks (6–10) may form at productive feeding areas, with individuals occasionally roosting just a few meters apart (Prevost 1982); in migration, larger flocks may form at key bottleneck sites.

Sexual Behavior

Mating system and sex ratio. Generally monogamous; rarely polygynous. In southeastern Massachusetts, 3 of 190 nests monitored 1980–1986 held polygynous trios (1 male mated simultaneously with 2 females; Poole 1989a). Each of these cases involved nest sites in close proximity, so a male could easily defend 2 sites, undoubtedly a key factor in promoting polygyny. One female (secondary) was always given less attention than the other (primary). Secondary females settled later than primary females and were fed less by the attending male. Males copulated with both, but secondary females never hatched eggs, probably because, being inadequately fed, they had to leave the nest for extended periods to forage for themselves.

In Wyoming, 1 polygynous trio studied closely (Berry and Henny 1995). The male copulated with both females, incubated much more at the primary than the secondary nest, and fed young only at the primary nest; nonetheless, the secondary female fledged 1 young (vs. 3 fledged at primary nest). The secondary female appeared to do all the fishing for her brood; made only short flights away from the nest.

Rare records of 2 females at 1 nest. Bent 1937b recorded 1 clutch of 5 eggs and 1 brood of 7 young; in Scotland, 2 females tried (unsuccessfully) to incubate at 1 nest (Cramp and Simmons 1980b). Not clear what factors promote this; scarcity of nest sites may contribute.

One nest in South Carolina had 2 males provisioning 1 female and cooperatively defending the nest (B. Price, personal communication).

Sex ratio. No solid data for adults. Schaadt and Bird 1993 found 20 male and 12 female nestlings (confirmed by karyotypic analysis) in 19 nests studied in northeastern Nova Scotia, a surprising deviation from parity. Needs further study. In the southern Red Sea, 87% of 76 nests had apparent males as first hatchlings, a finding that needs confirmation in other populations (Fisher et al. 2001). In southeastern Massachusetts, sex ratios at hatching were significantly biased toward males in 2 of 3 years; slight (but not significant) trend toward females in the 3rd year (Y. Kapetanakos, unpublished data). Factors influencing such changes in hatchling sex ratios remain unclear.

Pair bond

Courtship displays. Well documented by (Bretagnolle and Thibault 1993). Aerial Sky-Dance Display of male described in detail in Cramp and Simmons 1980b. May begin and end at the 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 the 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 the nest site, usually high overhead (up to 300 m or more) giving Screaming Calls repeatedly (see Sounds and Vocal Behavior: Vocalizations). Undulations are shallow (10–20 m) with the bird rising steeply at the end of each and hovering. Display and calling may continue for ≥ 10 min; male sometimes losing altitude all the while, descending slowly in an undulating staircase fashion to the nest site.

Male (rarely female) may engage in hovering flight without undulations; flaps wings vigorously while dangling feet and giving Screaming Call. Appears about to fall, even moving backward. Seen during courtship period and in nest defense, suggesting this display has both courtship and territorial functions.

Pairs appear to form at the nest site. In migratory populations, males generally arrive at nest sites before females do, although females will defend sites on their own, especially if they have bred at a site previously. 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).

Poole 1985a measured rates of courtship feeding at nests in southeastern Massachusetts to determine how these affected breeding success and mate fidelity. Older males (≥ 5 yr old) fed more fish to their mates than did males < 5 yr old, mostly because they shared their catch more equitably with mates. Rates of feeding had little impact on breeding success (see Breeding), although well-fed females were less likely to solicit food from males not their mate or to copulate with outside males. Courtship-feeding was 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 brood growth rate. 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 is quick to chase intruding males from the nest site (AFP).

When one member of a pair fails to return from migration, a struggle may ensue to fill the vacancy, sometimes lasting long enough to preclude successful breeding that season. At one nest in New Hampshire, 2 or 3 males were copulating with the returning female and spending more time fighting among themselves than delivering food or nest material. As the female laid eggs, the males would kick them out of the nest (I. MacLeod, personal communication).

Copulation. Copulation behavior is described in detail by Birkhead and Lessels 1988, based on an exceptionally complete data set from Loch Garten, Scotland. Pairs copulated frequently, on average 160 times/clutch (range: 88–338), but only 39% of these resulted in cloacal contact. Pairs averaged 59 successful copulations/clutch, starting 14 d before and peaking few days before the start of egg-laying. Pairs copulated most often in early morning, at the same time as egg-laying. No association noted between courtship feeding and copulation; thus no evidence females traded copulations for food. Most copulations took place at the nest or nearby. Males maximized the time they spent at the nest with their mate just before and during egg-laying, suggesting a male protects his paternity by frequent copulation and by guarding his mate when she is most fertile.

Duration and maintenance of pair bond. No published information for North America; needs study. Separating divorce from mortality is difficult. Preliminary findings from color-banded populations in Michigan (S. Postupalsky, personal communication) and southeastern Massachusetts (AFP) suggest that 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 (Fernandez and Fernandez 1977). See Schmidt and Wahl 2001 for preliminary data from Europe.

Inbreeding. One case (of 157 breeding individuals) noted in Michigan; full siblings bred together for 7 yr; breeding success was equal to other pairs in the population (Postupalsky 1989a).

Extra-pair copulations. No published information for North America. In Scotland, rare (17 attempts in 6 of 170 nest visits by non-mate males; only 2 attempts successful), but nests were far apart (> 5 km) when the study was conducted (Birkhead and Lessels 1988). Higher density of nests, not surprisingly, appears to promote this behavior (AFP), although data are lacking from nesting colonies. This, plus paternity studies, a potentially fruitful area for research.

Social and Interspecific Behavior

Degree Of Sociality

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

Osprey breeding colonies were reported to facilitate transfer of information about where fish can be found; individuals key in on successful foragers returning to the colony with a schooling fish species; males returning with non-schooling fish were ignored (Greene 1987). However, other studies have not found this to be true (e.g., Hagan and Walters 1990; see discussion in Diet and Foraging: Feeding). 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 overwintering grounds in western Africa, dispersion was 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.

In migration, often alone, but sometimes (at coastal hawk-watch sites) in loose groups, with perhaps as many as a dozen (or more) individuals strung out along the same flight path (Dunne et al. 1988). During fall migration in Cuba, flocks observed at hawk-watch sites ranged between 2–52 individuals and one flock of 92 individuals was observed at a dam in central Cuba (Rodríguez-Santana et al. 2014).

Nonpredatory Interspecific Interactions

Numerous birds use Osprey nests for breeding, either nesting within the structure of the nest or taking over the nest itself. In the Great Lakes basin, for example, cavity-nesters such as Common Grackle (Quiscalus quiscula), Tree Swallow (Tachycineta bicolor), European Starling (Sturnus vulgaris), and House Sparrow (Passer domesticus) build nests in cavities within Osprey nests; 13 of 90 nests occupied by Ospreys in this region were so colonized by these smaller birds (Ewins 1997). Ospreys generally ignore such commensals, and vice versa. Ewins and Cousineau 1994summarized records of more than 12 species of birds known to nest within Osprey nests or structures supporting them.

Other, larger birds sometimes take over unoccupied Osprey nests. These species often nest earlier than Ospreys and so usurp nests with ease, before Ospreys return from migration (no records for nonmigratory populations). In the Great Lakes basin, 7 species bred in nests formerly occupied by Ospreys: Great Blue Heron (Ardea herodias), Canada Goose (Branta canadensis), Bald Eagle, Red-tailed Hawk (Buteo jamaicensis), Great Horned Owl (Bubo virginianus), Herring Gull (Larus argentatus), and Common Raven (Corvus corax; Ewins et al. 1994a). Such early nesting by large species usually disrupts Osprey breeding for the season. In the case of geese, displaced Ospreys either bred at a nearby site, if available, or skipped breeding altogether for that year, taking back the site once the geese hatched young and departed—but too late to breed.

A variety of other bird species are reported to nest on offshore duck blinds occupied by nesting Ospreys in Chesapeake Bay: several species of ducks, Green Herons (Butorides virescens), Barn Swallows (Hirundo rustica), Barn Owls (Tyto alba), and Common Grackles (Reese 1977) . Generally these species nested below the Ospreys, in parts of the blind not used by them.

Where fish are plentiful, feeding flocks (see above) may include other species. In Florida Bay, where bottle-nosed dolphin (Tursiops truncatus) drive mullet (Mugilidae) and other fish into shallow waters, Ospreys join flocks of Brown Pelicans (Pelecanus occidentalis) and gulls (Larus spp.) in diving on the accessible prey (Poole 1989a). Many smaller birds, like Red-winged Blackbird (Agelaius phoeniceus) mob/attack Ospreys in defending their nests—misplaced aggression since birds are not food for Ospreys (Poole 1989a).

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Immature Bald Eagle (Haliaeetus leucocephalus) in close pursuit of an Osprey carrying a fish. Bald Eagles regularly pirate fish from Ospreys.

© Richard Merrigan, Florida, United States, 14 March 2016

Kleptoparasitism: Bald Eagles are well-known pirates of Ospreys, robbing them of fish, especially those recently caught (Bent 1937b, Ogden 1975). Very rarely such chases lead to attacks and death of the Osprey (Macdonald and Seymour 1994). The number of Ospreys brought to a Florida rehabilitation center with broken bones has been increasing as the density of Bald Eagles nesting in the area has increased (R. Munguia, personal communication).

Predation

Few data. Bald Eagles are known predators of nestlings (Flemming and Bancroft 1990, Liston 1997) and (rarely) of adults (Macdonald and Seymour 1994). In southern Florida, nesting Ospreys temporarily abandoned islands (keys) that supported active nests of Bald Eagles, although the 2 species can share nesting keys (Ogden 1975). Ospreys generally give Alarm Calls when Bald Eagles appear in flight near their nests (AFP). Seen from the air, the plumage of Osprey nestlings appears to provide excellent camouflage against the nest, suggesting that diurnal avian predators such as Bald Eagles have been a significant selective force on Ospreys.

Great Horned Owl a significant predator, especially in the northeastern U.S.; probably takes mostly nestlings but known to kill adults (Cold 1993). Of 101 nestlings lost from a southeastern Massachusetts colony in 5 years, at least 20% were likely killed and eaten by Great Horned Owls, but this is difficult to document; young simply disappeared from nests at night, with a few adult Ospreys found headless below nests (AFP). Also a likely source of mortality for Ospreys on migration as suggested by satellite telemetry data (ROB).

Raccoons (Procyon lotor) are another suspected predator at nests (eggs and small nestlings), but again it is difficult to document predation by this nocturnal mammal. Raccoons appear to be a powerful selective force on nest placement in North American Ospreys, however; e.g., Chesapeake Bay Ospreys historically nested mostly in trees around the edge of the bay, sites where they were vulnerable to climbing predators such as raccoons, although raccoon populations were apparently low there historically. Since the 1960s–1970s, nearly all Chesapeake nests have been built over water on channel markers, duck-hunting blinds, and small islets—sites safe from raccoons and other mammalian predators (Reese 1969, Henny et al. 1974). Here (as elsewhere) raccoon populations apparently increased greatly after the 1950s, boosted by access to human garbage and by decreased hunting of the species. More study is needed of how raccoons and other predators affect nesting Ospreys.

Nile crocodiles (Crocodylus niloticus) are known to kill Ospreys in Senegal, western Africa (Prevost 1977); caiman (Caiman sp. and Melanosuchus niger) may be important predators of Ospreys on overwintering grounds in South America. Ospreys are vulnerable when bathing and roosting at the water’s edge.

Breeding Ospreys react to predators in 2 ways: with Alarm Calls (when the potential predator is at a distance; see Sounds: vocalizations, above); at the nest, by diving at the intruder and occasionally striking it with their claws. There is considerable individual variation in intensity of nest defense. At a large nesting colony in southeastern Massachusetts, only about 5–10% of breeders vigorously defended nests against researchers checking nest contents, regularly striking the person unless an effort was made to fend the bird off. The other 90–95% of breeders simply circled overhead, giving Alarm Calls and occasionally swooping with shallow dives toward the intruder. Females are more vigorous in nest defense than males. At night, of course, Ospreys are at a disadvantage, undoubtedly why raccoons and Bubo owls are effective predators of the species.

Baffles and other nest guards help to deter predators; these are used especially at artificial nest sites; see Conservation and Management: Management.

Recommended Citation

Bierregaard, Richard O., Alan F. Poole, Mark S. Martell, Peter Pyle and Michael A. Patten. 2016. Osprey (Pandion haliaetus), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA. https://doi.org/10.2173/bna.683