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Osprey

Pandion haliaetus

Order:
Accipitriformes
Family:
Pandionidae
Sections

Breeding

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Phenology

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Figure 3. Annual cycle of molt, breeding, and migration of Ospreys breeding in southern New England.

Phenology of other populations differ (see text). Thick lines show peak activity and thin lines show off-peak.

See Figure 3. Summarized for selected North American locations by Poole 1989a.

South Florida (population comprises year-round residents and full migrants; Martell et al. 2004): Residents laid eggs late November–January (one record of eggs laid in September; E. Forys, personal communication), with peak December to mid-January (Ogden 1977b, AFP); returning migrants on eggs January–early March (rarely). North Carolina: First arrivals late February, most breeders back by mid-March; eggs mid-March to mid-May (rare into early June), peak laying early to mid-April (Hagan and Walters 1990). Chesapeake Bay: First arrive at nests early to mid-March; eggs late March to mid-May, peak in April (Reese 1977), pairs established on upper tributaries earlier than on the Bay (P. Spitzer, personal communication). Southern New England: First arrival mid- to late March, late arrivals (generally young, pre-breeding or first-time breeders) through mid-May; eggs early April–early June, with peak in mid- to late April (Poole 1984). In east-central Labrador (54°N): Arrival early May; eggs mid-May to mid-June (Wetmore and Gillespie 1976), unfledged young reported in September on Churchill River (J. Pfeiffer, personal communication). Southeastern British Columbia: Return mid- to late April, eggs early May (peak) to late May (Steeger and Ydenberg 1993).

Thus, there is a latitudinal cline in breeding dates, at least in eastern U.S. populations, reflecting temperature, day length, and availability of prey. Ice a significant factor at northern locations; pairs may not breed in years of late ice-out (but see Henny 1977b). Altitude probably also delays laying, but few data. Mean incubation period (first egg laid to first egg hatched) at 33 nests in southeastern Massachusetts = 39 d (range 35–43); hatching to fledging = 50–60 d depending on location, weather, number of nest mates, and other factors (Poole 1984, Poole 1989a).

In migratory populations, males generally arrive a few days before females. Few published data for North America, but at Loch Garten, Scotland, male arrived first in 14 of 19 yr; mean difference between sexes 4.1 d (Cramp and Simmons 1980b). Pairs with established (intact) nests can lay as early as 7 d after arrival, with only minimal time devoted to nest building; pairs with no nest usually take longer (10–20+ d) to build before laying (AFP).

Older females lay significantly earlier than younger females; e.g., in southeastern Massachusetts, females 4–5 yr old laid about 2 wk later, on average, than females ≥ 10 yr old (Poole 1984, Poole 1989a). Likewise, pairs in this region that had nested together previously laid significantly earlier than pairs in which individuals were new to each other; mean duration of pair bond of pairs laying 1–8 d after first egg in colony was 1.7 yr ± 0.4 SE (n = 34); of pairs laying 17–24 d after first egg in colony, 0.6 yr ± 0.7 (n = 17; Poole 1984). Thus, early breeding is an advantage of long-term monogamy in this species, especially in areas with climatic extremes where the breeding season is short; compared to pairs that lay late, earlier pairs generally produce more young and young that survive longer; see Demography and Populations: measures of breeding activity.

Distribution of laying dates varies with latitude. In southeastern Massachusetts, nearly 80% of eggs in 102 nests sampled were laid in the first 3 wk after the first egg in the colony (Poole 1989a). In North Carolina, only 43% (n = 136 nests) were laid in a similar period, reflecting a longer breeding season at this southern locale (Hagan and Walters 1990). At subtropical latitudes (e.g., Baja California and southern Florida), laying season is more protracted: 9–10 wk (clutches initiated early January–early March; Judge 1983); southern Florida, late November through early March (Ogden 1977b). At least in Florida and perhaps in Baja California, this spread may be explained by non-migrants nesting early before migrants have returned north, providing two pulses of breeding.

Nest Site

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Active nest in a bald cypress (Taxodium distichum).

© Patty Maloney, Virginia, United States, 19 April 2016
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Ospreys can be tolerant of human activity, especially when nesting; this nest was built at the top of a conifer and adjacent to a construction site.

© Deanna Uphoff, Florida, United States, 18 February 2016
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Osprey at an artificial platform nest in the Wasatch Mountains.

© Sharyn Isom, Utah, United States, 1 May 2016
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Artificial platform for nesting Osprey.

© Russ Smiley, Connecticut, United States, 15 April 2016

Selection

Generally the male seeks the nest site, before the arrival of the female, but pairs do visit sites together. Male may start nest-building before pairing; female not known to do so. Rarely single females will settle at a nest site before pairing, especially if they have bred at the site previously (AFP).

During the summer breeding season, single (and occasionally paired) birds may prospect sites, apparently for the next season (AFP). This is especially true for failed breeders, who spend considerable time visiting new sites but includes young birds who have yet to establish their first nest (ROB). At Lake Norman, North Carolina, Ospreys, perhaps failed breeders (see below Alternate/Non-breeding nests) occasionally built new, complete nests in August, just prior to migrating south (G. Vaughan, personal communication). Satellite data have shown that adults whose nests fail sometimes move regularly between their nests and alternate fishing areas up to 125 km away (ROB and MSM), returning every 3-5 d to maintain a claim on their nest. In migratory populations, late-arriving birds (usually young) may spend 1–2 mo, or longer, trying to secure a nest site. And prospecting pairs (pairs without nests) are occasionally seen at the same location year after year, apparently having chosen a spot that seems appropriate to them, even though they do not have a nest there (AFP). Such pairs generally take to artificial nest sites readily, when those are built at the chosen location.

In cases where one bird does not return to a nest, the surviving individual (male or female) may witness a scrum of candidates trying to fill the vacancy. Often the dispute may last long enough that no young are raised. Males in one such situation were seen to eject eggs from the nest shortly after the female laid them (I. MacLeod, personal communication).

Microhabitat; Site Characteristics

Wide variety of natural and artificial sites, well described in Bent 1937b, Cramp and Simmons 1980b, Palmer 1988e, Edwards and Collopy 1988, Poole 1989a, Ewins 1996. 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, parking lots, athletic fields; see Conservation and Management: Effects of Human Activity). See Table 10.1 in Poole 1989a for summary of regional differences in nesting structures used by U.S. Osprey populations.

Natural sites include trees, cliffs (including large pinnacles and stacks), large shoreline boulders, and, on predator-free islands, the ground. In trees (live or dead), usually nests on or near top, or lower down, where a large branch forms crotch that can support a nest. On islands in Florida Bay (southern Florida), most pairs nest atop flat-topped mangroves (especially Rhizophora mangle; Ogden 1977b, AFP). In coastal Carolinas, cypress (Taxodium sp.) swamps provide similar, flat-topped nesting trees in abundance, usually growing in water (Henny and Noltemeier 1975, Hagan and Walters 1990). Similarly, cypress rings lakes in north-central Florida where Ospreys chose nest trees that were higher and in more open areas than randomly selected trees (Edwards and Collopy 1988). In northwestern Mexico (Baja California peninsula and the eastern shore of the Gulf of California), where trees are scarce, often nests on various species of tall cacti (26% of 810 nests); also on cliffs (59%; Henny and Anderson 1979). In western states (Idaho, California, Oregon), most pairs (80–95%) nest in trees; historically, northeastern California (Tule Lake), 250–300 pairs nested in tall ponderosa pines (Pinus ponderosa) and junipers, 12–30 m from ground (Henny 1988a).

Ground nesting may occur where mammalian predators are absent: e.g., Gardiners Island, New York, where historically approximately 300 pairs nested on the ground (open fields or beach) or in low, windswept trees; also northwestern Mexico (Baja California), where significant ground-nesting concentrations have formed on islands in coastal lagoons (Henny and Anderson 1979, Danemann and Guzman-Poo 1992).

Quickly takes advantage of artificial nesting sites, even in areas of very dense human populations; significant and increasing use of such sites in many regions since 1980s is often linked to Osprey population growth. Artificial sites include duck-hunting blinds in shallow coastal waters; channel markers in harbors and busy waterways; towers for radio, cell phone, and utility lines; and platforms erected exclusively for the species. The shift to artificial nesting sites has been dramatic in many regions, with 90–95% of pairs choosing to nest at such sites; predation, loss of trees, and development of shorelines have been driving forces behind the change. In Chesapeake Bay, dramatic shift from most all nesting in trees (pre-1950) to 90% of 3,500 nests on artificial structures by the mid-1990s (Watts and Paxton 2007); increase in predator populations (raccoons) may have driven this shift, along with loss of shoreline trees and an increase in artificial structures such as buoys and channel markers (Reese 1969; see Behavior: Predation). In Ontario, 100% of 42 nestings during 1874–1944 were in trees; by 1991, only 47% of 179 nestings were in in trees, with 52% on artificial structures (e.g., power poles and platforms); roughly similar findings for Ospreys nesting along Lake Huron (Ewins et al. 1995a). Likewise, a nesting population along the Willamette River in western Oregon, grew from 13 pairs in 1976 (all in trees) to 78 pairs in 1993 (most on artificial sites;Henny and Kaiser 1996). In 2010, 95% of the nests between New York City and northeastern Massachusetts were on human-made structures (Bierregaard et al. 2014a). In Florida, an estimated 45% of all cell towers were occupied by Ospreys in 2012 (Gryta and Monga 2012). Takes advantage of other opportunities for nesting; e.g., reservoirs often flood trees, which provides abundant standing dead trees for nesting. This has been particularly important for western U.S. populations; in west-central Idaho, for example, an 11,000-ha reservoir supported about 50 pairs in the 1970s (Van Daele and Van Daele 1982).

Nesting platforms (described in Conservation and Management: Management) have been particularly important in south-coastal New England and New York State, and in Wisconsin, where 70–75% of nests were on built on such structures in late 1970s, > 90% in 1990s (Poole 1989a; AFP). In 1980–1993 in southern Baja California, 18–46% of population shifted to artificial structures (Castellanos and Ortega-Rubio 1995). Use of power/lighting poles and especially cell towers are causing conflicts with humans (see Conservation and Management: Management).

Height of nest depends on location. On islands and over-water locations, where predators are absent or their access limited, nests are generally low or on the ground; e.g., in cypress swamps, North Carolina, 1–9 m above water in trees (Hagan 1986); in southern New England salt-marsh islands, 2–5 m high on nesting platforms built for the species (AFP). In boreal forests of Canada, spruce (Picea sp.) 15–18 m tall provide nest sites in Labrador and eastern Quebec (Wetmore and Gillespie 1976); shorter spruce (6–8 m tall) and low (2–3 m) rocks in rivers and bays in west-central Quebec (Bider and Bird 1983).

Nests generally close to water; e.g., in Ontario during 1990s, 93% of 179 tree nests were within 500 m of water; median distance to water for tree nests 10 m (vs. 4 m for nests on artificial platforms; (Ewins 1997). In Oregon, 83% of 78 nests were within 1 km and all were within 2 km of water (Henny and Anderson 1979). When appropriate near-water sites are lacking, however, pairs may nest farther inland: e.g., ≥ 14 km from main foraging areas in Nova Scotia and North Carolina (Greene et al. 1983, Hagan and Walters 1990); 1–5 km (median 1 km; 151 nests) on hydro (power) poles in New Brunswick (R. Stocek in Ewins 1997). Likewise, predator-proof nest sites over hyper-saline and fishless Mono Lake in California are so attractive that Ospreys will nest on them and commute to fish freshwater streams and lakes in the surrounding landscape (Fields and Pagel 2016).

Thus Ospreys will search out and use good nest sites up to 10–20 km from water, probably the energetically feasible commuting distance from nest to food. Historically pairs may have done so regularly because favored canopy trees were more available before extensive lumbering of recent decades and because Bald Eagles may have limited access to favored near-water nesting sites.

Throughout the U.S., pairs have produced significantly more young (about twice as many) at artificial sites as at natural sites (Table 8.3 in Poole 1989a). This in part because trees are generally less stable than artificial sites; more likely to blow down during the breeding season and between seasons (over winter; Table 8.4 in Poole 1989a), AFP; but see Ewins 1996 for Canadian Great Lakes, where natural and artificial sites proved equally stable). Thus Osprey populations with access to artificial nesting sites tend to grow more quickly than those confined to natural sites (Table 8.5 in Poole 1989a).

Nest

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Adults continue to bring sticks to nest even though incubation may have begun.

© Elizabeth MacSwan, Virginia, United States, 16 April 2016
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Adult bringing a large stick to build its nest.

© Bradley Anderson, Michigan, United States, 21 March 2016
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Although Ospreys are most often seen nesting on artificial platforms, a variety of natural nesting locations are used where available. Here an adult and young at nest on a rocky outcrop.

© Nick Varvel, Wyoming, United States, 12 July 2015
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Osprey nest at J. N. Ding Darling National Wildlife Refuge on Sanibel Island.

© Melissa Versiga, Florida, United States, 19 April 2016
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Pair at nest site.

© Ron Sempier, Florida, United States, 3 May 2011

Construction

Well described in Bent 1937b, Cramp and Simmons 1980b, and Palmer 1988e . “Prodigious builders” Palmer 1988e: 87. Generally male brings bulk of material (Levenson 1979, Jamieson et al. 1982) to the nest, female arranges it once there. May break dead sticks off nearby trees in flight or (more often) snatch from ground. Often a surge of nest building right after hatch, generally by female; regular nest building continues throughout the nestling period and even after nest failure. Older young will rearrange nest material. Nest shape changes during the breeding cycle. During incubation the nest is distinctly bowl-shaped. After hatching the nest flattens out, but a rim of sticks in maintained, sometimes by the young themselves, while the young are beginning to move clumsily about the nest. In the last weeks of the nestling phase, the nest often becomes completely flat.

Structure and Composition Matter

Well described in Bent 1937b and Palmer 1988e. Large sticks (about 2–4 cm diameter; up to 1+ m length) at base, smaller sticks from there. As nest nears completion, clumps of grass, dried seaweed, Spanish moss (Tillandsia usneoides; in central Florida), mats of algae, bark, etc. are added to provide a cushion for the eggs and to insulate them from below. Later in the nesting season all nature of objects are delivered to the nest (e.g., paper and plastic bags, rope, nylon mesh bait bags, dried cow manure, beach toys).

Dimensions

Surprisingly few data. Nests on artificial platforms are often small and flat, especially in the first season; e.g., about 0.7 m diameter and 8–15 cm deep (AFP). Nests in solid trees or on ground can become huge, 3–4 m deep (Bent 1937b), 1–2 m diameter, probably the result of generations of building by Ospreys. Human can easily sit in such nests.

Microclimate/Exposure

No data. Nests generally open and exposed; young especially vulnerable to heavy rain (even if brooded, some nests become wet; often significant mortality of eggs and small young after prolonged rain; (Poole 1984). Young pant on hot summer days; small young are especially vulnerable; female shades young until they become too large for this to be possible.

Maintenance/Reuse of Nests

Maintained throughout the season, especially after spring arrival, pre-egg-laying, and after nest failure (AFP). Extensive reuse year to year; nest an investment in time and energy, so reuse the following season allows earlier laying, and in turn more surviving young (see Demography and Populations: Measures of Breeding Activity). Of 68 nests monitored in the Gulf of California in 1977, all but 9 reused in 1978; birds changing mates most likely to shift sites (Judge 1983). In southeastern Massachusetts, 1979–1984, an expanding population (artificial nest sites), about 95% reuse year to year; lower rates (70–80%) in Florida Bay, where nests are in natural sites and more prone to decay (Poole 1982b, Poole 1984).

Alternate/Nonbreeding Nests

A significant feature of breeding ecology of this species. Failed breeders especially likely to build alternate nests and use them in subsequent years (but few data on use; AFP); males with access to more than 1 nest or nest site in close proximity often defend both, promoting polygyny (see Behavior: sexual behavior, above). In a large (85 pairs), dense breeding colony in southern New England (artificial platforms), about 10% of males had alternate sites (AFP). Percentage apparently higher in habitat with more dispersed nest sites: only 49 of 98 nests surveyed in the boreal forest of the Great Whale/Little Whale basins, Quebec, were occupied, suggesting many pairs had alternate sites (Bider and Bird 1983).

One possible 19th century (T. Schulenberg, personal communication) and 2 recent (L. Lopez and M. Cohn-Haft, personal communications) confirmed records of complete nests constructed in South America. Similar reports of nests regularly constructed along southern Texas coast south and west of the known breeding range (O. Fitzsimmons, personal communication) are presumably young birds that have delayed their migration north but are responding to changing photoperiod.

Eggs

Shape

Elongate to oval-elongate (Palmer 1988e); small eggs often rounder (AFP).

Size/Mass

Length × breadth (mm): 59.5 ± 2.46 SD × 45.0 ± 1.30 SD (n = 1 egg each from 20 clutches in New Jersey, Maryland, and Virginia; F. Preston in Palmer 1988e). Average 61.0 × 45.6, with extremes 68.3 × 50.4, 55.2 × 45.5, 60.0 × 41.7 (Bent 1937b). No regional variation noted (e.g., Florida vs. southern New England; Poole 1982a, Poole 1984). First egg in clutch significantly larger than others; in Westport, Massachusetts, mean reduction in volume of subsequent eggs: 2.1% for second laid; 5.6%, third; 8.2%, fourth—thus young from first and second eggs have potential size advantage over siblings, although hatching asynchrony appears a more critical advantage (Poole 1982a). No significant difference in mean size of eggs in 3-egg vs. 4-egg clutches (Poole 1984). Fresh egg mass, southeastern Massachusetts: mean 66.0 g (range 58–75, n = 31 eggs, 12 clutches; AFP); 3.6% of body mass of 1,850-g female.

Color/Surface Texture

“. . . the handsomest of all the hawk’s eggs . . . considerable variation . . . coloring very rich; a selected series of them is a great addition to an egg collector’s cabinet” (Bent 1937b). Ground color creamy white to pinkish cinnamon; usually heavily wreathed and spotted with reddish browns, especially larger end. Surface smooth but not glossy.

Eggshell Thickness

In uncontaminated eggs, about 0.42–0.51 mm, with a mean about 0.46 mm (see Fig. 9.7 in Poole 1989a; also Spitzer et al. 1978). For effects of contaminants on shell thickness, see Conservation and Management: Effects of Human Activity, and Poole 1989a.

Clutch Size

Ranges from 1–4 eggs, with 3 eggs being the mode in most populations; 1-egg clutches are rare and probably represent an incomplete clutch; most likely in renests. In Lower Peninsula of Michigan: mean of 2.92 eggs, with 76% 3-egg clutches (n = 537 clutches, 1967–1987; Postupalsky 1989b). Westport, Massachusetts: mean of 3.3 eggs (range 2–4), with 20–30% 4-egg clutches, depending on year (n = 94 clutches, 1979–1984;Poole 1989a).

In non-migratory populations, clutches generally smaller, although 3 still the mode, 4 almost nonexistent; e.g., Baja California: 2.63 ± 0.58 SD (n = 86), with 5.8% 1-egg, 24.4% 2-eggs, and 69.7% 3-eggs (Castellanos et al., unpublished data); southern Florida: 2.7 ± 0.6 SE (n = 22, 1978 and 1979; Poole 1982b); Sonora, Mexico: 72% 3-eggs, 25% 2-eggs, a few 1-egg and 4-egg nests (n = 105, 1992–1997; Cartron 2000).

See Demography and Populations: Measures of Breeding Activity.

Egg-Laying

Generally soon (1–3 d, sometimes longer) after nest takes shape and nest-lining added; continue to add nest material throughout laying and incubation, but at reduced rate (AFP). Eggs often laid in early morning (Birkhead and Lessels 1988); 1 egg/2–3 d (Poole 1982b). Male vigorously guards mate during this time; follows her whenever she leaves nest, roosts nearby, away only when foraging. Clutches generally replaced if lost early in incubation (1–3 wk); single eggs not replaced (apparently a determinate layer; AFP). Egg-dumping not known. But rival males contending for a vacant territory will kick eggs out of nests (I. MacLeod, personal communication).

Incubation

From AFP and others as indicated. Both sexes incubate, but female generally does most; e.g., females did 70% of incubation during daylight hours at 3 nests in northern California (Levenson 1979). 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; needs study. Incubation often begins with first egg but sometimes sporadic until second egg is laid. Incubation period (first egg to first hatch) 37 d (range 34–40 in United Kingdom; Green 1976); 38.5 d ± 1.1 SD (range 36–42 in British Columbia; Steeger et al. 1992). Incubation period is substantially longer than expected for a raptor of this size, perhaps because females develop only a small brood patch, potentially limiting their ability to maintain high egg temperatures; needs further study.

Hatching

From AFP. Egg “starred” (first cracks) 2–3 d before hatching; pip hole shows beak working 12+ h before. Chick makes faint peeping, especially if distressed. Shell-breaking and emergence: few data on time of day or duration of hatching/egg, nest cams have begun to provide important information here—of 10 observed hatches at a nest in Bremen, Maine, 7 were between 02:44 h and 09:42 h, 1 was mid-afternoon, and 2 were around 21:30 h (O. Bronhammer, personal communication); for intervals between hatchings, see below. Parental assistance and disposal of eggshells not known.

Young Birds

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Osprey hatchlings.

Newly hatched Osprey chicks, Westport, Massachusetts. About 20–30% of the pairs in this population lay 4-egg clutches.

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Figure 4. Osprey chicks often hatch sequentially.

Osprey chicks usually hatch sequentially, often up to 4–5 d apart, giving older chicks an advantage when food is limited. Photo by D. Schmidt from Schmidt 2001, used with permission.

Condition At Hatching

Figure 4. Semiprecocial; down-covered; body mass 50.3 g ± 10.3 SE (n = 31; Steidl and Griffin 1991); culmen length about 10 mm (Poole 1984); weak in movements but can beg, briefly. Brooded almost continually and fed small bits of fish by female parent (Poole 1989a). In southern Florida and 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 1982b).

Growth And Development

Growth particularly well studied in southern New Jersey (Steidl and Griffin 1991) and in Nova Scotia (Schaadt and Bird 1993). In New Jersey, growth best described by logistic curve (k = 0.173); little or no difference in growth related to year, brood size, or broods with or without nestling loss; number of days required to grow from 10 to 90% of asymptotic mass = 26 (range 25–29 d); period of quickest growth (linear phase) about 10–30 d of age; gain in body mass peaked at 50–75 g/d between ages 20–35 d; sibling weight differences greater in 3-young vs. 2-young broods—up to 60% difference at 7–8 d old (vs. < 50% for 2-young broods), 50% at about 17 d old (vs. < 25% for 2-young broods). Significantly slower growth for Ospreys in Chesapeake Bay (Stinson 1977a), and Florida Bay and Gardiners Island (but not eastern Long Island, New York Poole 1982b); apparently related to less dependable food supply. Brood reduction appears to adjust brood size to levels at which minimum growth rates can be maintained (Steidl and Griffin 1991).

In Nova Scotia (Schaadt and Bird 1993), where sex of nestlings was determined, body mass and tarsus length the only measures to show well-defined asymptotes at fledging (talon length, cranium width, and culmen length within 10% of adult values); males differed significantly from females in having lower asymptotes of mass and tarsus length, but not in rate of growth; no difference in growth rates among individuals in broods of 1, 2, or 3 nestlings, or within broods as a result of hatching asynchrony; no male/female differences in age at time of feather emergence or in length of nestling period. Thus, no evidence for rapid growth in males to compete with larger females (see Appearance: Measurements).

Both culmen length (Poole 1982a) and wing length (Odsjo and Sondell 2001) used to age nestlings; latter seems best for older young.

See Schaadt and Bird 1993 for details on growth of body parts. See Appearance for details on molt into Juvenal plumage. No precise data on when young can thermoregulate, but brooded almost continually until about 2 wk of age (see Breeding: Parental Care).

Parental Care

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A pair of adults tending to young in the nest.

© Tresa Moulton, Florida, United States, 1 May 2016
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Adults tear small pieces of flesh off prey items to feed to younger nestlings.

© Tresa Moulton, Florida, United States, 1 May 2016

Brooding

Almost entirely by female. Small young (1–14 d old) brooded almost continually; brooding intermittent thereafter, up to about 4 wk of age, as necessary (e.g., rain, sun; Cramp and Simmons 1980b). In hot sun, young 3–4 wk old often just shaded (not brooded) by female (AFP).

Feeding

Onset and Duration. No precise data regarding onset; young generally fed within 1 day of hatching. For duration, see Fledgling stage, below.

Roles and behavior of parents/Method of feeding. Male brings fish to nest, or nearby, often feeding first (usually consuming the head and foreparts of the fish), before presenting the remainder to the female. Female distributes food to nestlings, seemingly feeding whichever begs closest and most vigorously ( Poole 1979, Poole 1982b, Forbes 1991). Female dissects small pieces of fish, and carefully presents them in her bill; generally from anterior portion of the fish, working toward the tail; female discards or eats bones and hard parts herself and often eats the entire tail in one gulp. By about 40 days of age, young begin to feed on their own, taking prey from the male as he lands at the nest or from the female parent after she has fed. With large young, however, female generally feeds last (Poole 1984).

Food of young. Not different from that of parents; see Food Habits, above.

Rate of feeding. Varies significantly, among and within populations; difficult to measure accurately because fish differ greatly in body mass and caloric content, and size is tough to judge accurately at a distance (Prevost 1982, Poole 1984, Poole 1989a, Green and Ydenberg 1994). At 2 locations in eastern Long Island, at nests with unfledged young: 92 g/h ± 2.5 SE and 59 g/h ± 3.1 (1,426 and 915 g/d, respectively); in Florida Bay, 71 ± 3.6 g/h (= 816 g/d); brood reduction higher in nests with low rates of food delivery (Poole 1982b). See also Appendix 3 in Poole 1989a (southeastern Massachusetts breeding vs. western Africa overwintering), which shows: 6–8 versus 1–3 fish/d; 1,250 versus 300–350 g/d, respectively. At 8 nests in British Columbia, mean ± 95% CI intake (kJ/h) of males feeding prefledged young: 50 ± 29; females, 29 ± 10; chicks, 50 ± 8 (Green and Ydenberg 1994).

Sibling aggression/food allocation. See Poole 1979, Poole 1982a, Poole 1982b, Forbes 1991. When hungry, older nestlings often aggressive toward younger siblings (particularly at feedings), pecking them into submission and sometimes to death. Once such dominance established (usually by age 7–10 days), simple threat (head raised) is often enough to cause submission, allowing older young to feed until satiated. When food is limited, younger siblings often starve. With regular, abundant food, no apparent aggression or dominance, and young often feed side by side, female feeding siblings equally. Parents not known to intervene in aggression among siblings.

Nest sanitation/Invertebrate associates

Parents make no effort to clean nest. Dead chicks and partially eaten fish often remain in nest; may be covered by nest material, whether deliberately or by accident not clear. Larger nestlings defecate over rim. Mesostigmatic mites the most abundant microarthropods extracted from material in 2 of 5 nests in southeastern Massachusetts; oribatid mites numerically dominant in 3 other nests; about 1 mite/g dry weight, but 1 nest with 27/g (Philips 2000).

Cooperative Breeding

Extremely rare. One case of polygyny in Minnesota with 2 females laying a total of 6 eggs in 1 nest (Englund and Greene 2008). 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). Two males at a nest in South Carolina observed providing food to and defending the nest together (B. Price, personal communication). Once fledged, young sometimes land at nearby nests; in these cases, parents have been known to feed these fledglings that are not their own. Such visits are generally brief (Poole 1982a), but a New Hampshire fledgling moved into a nest 220 km south of its nest and was fed there for 12 d, apparently excluding the 2 young recently fledged in the nest (B. Lombardi, personal communication).

Fledgling Stage

Departure from nest

In southern New England, age at first flight generally 50–55 d (Poole 1989a). In United Kingdom, 52.8 d (range 49–59; Bustamante 1995). Longer period (62.5 d ± 4.9 SD) among Ospreys in Gulf of California (Judge 1983) and probably other subtropical populations; perhaps related to non-migratory status or lack of food (needs further study). During last week of nestling period, young often exercise wings hovering over the nest. Susceptible to premature fledging; e.g., if disturbed at nest, may jump before they can fly well, forcing a landing on ground or in water, and jeopardizing survival. Researchers banding young Ospreys must time nest visits carefully (young ≤ approximately 40 d old) to avoid such loss.

Growth

No information postfledging.

Association with parents or other young; Ability to get around, feed, and care for self

After first flight, fledgling generally remains at the nest or nearby; parents bring fish back to young, which can rip it apart to feed themselves, but some persist in being fed by adults well into the fledgling period. Young in northern, migratory populations are dependent on parental feeding for at least 10–20 d, supplementing food that young start to catch on their own (Stinson 1977a, Poole 1984); average 30.4 d (18–36 d) at Loch Garten, Scotland (Bustamante 1995); farther south (northern Florida), fledglings may stay near the nest and take food from their parents for 8–10 wk (Edwards 1989b). Parental feeding seems to provide a key transition to independence, perhaps one reason fewer young that fledge late (and thus get less parental feeding) survive (Poole 1989a; see Demography and Populations: measures of breeding activity, below). It appears that males provide most food for fledged young, especially older young; e.g., among mated pairs in U.S. East and Midwest, satellite-tracked females departed 7–39 d (mean 22.3 d) before their mates in all cases (n = 11; Martell et al. 2001a). Parental roles need study.

Development of foraging skills studied (2 yr) in north-central Florida (Edwards 1989b): (1) Young (n = 22) attempted to catch prey as early as 5 d after fledging, with first successful capture 11 d after; all had caught fish by 20 d after fledging. (2) Mean capture success increased with age; e.g., 22–23% success 30 d after fledging, 58–62% success 120 d after. (3) Siblings that fledged together remained together and foraged together, had higher initial capture success than single young (which foraged alone), but eventually achieved the same success rate as siblings foraging together and had similar foraging behaviors (e.g., foraging height). (4) Compared to adults, individual young showed little variation in diet, tending to catch the same species throughout the postfledging period.

Thus Osprey fledglings may learn some foraging skills from siblings, at least in populations with long postfledging periods like those in north-central Florida. At higher latitudes, by contrast, where breeding season is shorter, young siblings have much less time together (e.g., 20–40 d in southeastern Massachusetts) before migrating, which they do alone (AFP, ROB). Young that humans released (hacked) into the wild learned to fish on their own, without parents, so Osprey fishing behavior is innate (Schaadt and Rymon 1982).

Immature Stage

Banding and satellite data show that some juveniles (hatch-year young) lag behind adults in their first migration south, at least in northeastern U.S. populations, perhaps because they have less time to gain flight and foraging skills before migration begins, compared to young fledged farther south (Poole and Agler 1987, ROB).

Of 51 satellite-tagged juveniles in New England, 28 remained in their nesting area until migration began, 8 dispersed but returned to nest area prior to migrating, and 15 left the nest area and remained away until they started south (ROB).

Once on the overwintering grounds, considerable wandering may occur, often covering hundreds of kilometers, but all (n = 34) satellite-tagged U.S. juveniles that survived southbound migration had settled down within 3–4 mo of cessation of migration (ROB).

Almost all remain in the wintering range for approximately 18 mo, making their first migration north in their third calendar year (age 21–24 mo), based on wing-molt patterns of young trapped on overwintering grounds in western Africa (Prevost 1982), satellite data (ROB), and on ages of banded, apparent first-time breeders in northern populations (southern migratory and resident populations may differ, but no data; see Demography and Populations: measures of breeding activity).

The period of Osprey life history about which we know the least is the period between the first return trip and establishment of the first breeding territory. Satellite data is providing some insight here (see Nest Site: Selection, above).

On overwintering grounds in western Africa, foraging dives of immatures 6 mo old were significantly less successful than those of older Ospreys, although average time/capture did not differ significantly; i.e., young Ospreys were less successful at judging when to dive, but dove more often to compensate (Prevost 1982). Both young and older birds had little trouble meeting daily energy needs, but bad weather or other stresses might accentuate the differences; note that survival rates of immatures are significantly lower than those of older birds (see Demography and Populations: life span and survivorship).

Recommended Citation

Bierregaard, Richard O., Alan F. Poole, Mark S. Martell, Peter Pyle and Michael A. Patten.(2016).Osprey (Pandion haliaetus), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/osprey

DOI: 10.2173/bna.683