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Osprey

Pandion haliaetus

Order:
Accipitriformes
Family:
Pandionidae
Sections

Appearance

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See Prevost 1983a, Clark and Wheeler 1987, and Dunne et al. 1988 for details. Large (approximately 1,400- to 2,000-g body mass), long-winged (150- to 180-cm wingspan) raptor with dark chocolate-brown back and upperwing coverts, mostly white breast (some speckling) and belly, white crown and forehead, and dark line through eye; Caribbean form (P. h. ridgwayi) appears more white on the head and breast. Iris yellow. Cere and legs pale blue-gray. Speckled brown necklace on breast of female (and some males). Juvenile similar to adult, but upperparts appear scaly because of light-buff feather edges on back and upperwing coverts; these generally wear off by first winter; iris orange-red through first year. Individuals often heard before seen; whistled guard calls (see Sounds: Vocalizations) are a prominent feature on breeding grounds. Upright posture when at rest; may open wings slightly.

Sexes difficult to distinguish in the field. Adult female averages 15–20% larger than male in body mass, wing (6%), and bill, but overlap in all metrics except mass (Poole 1989a). Females generally have fuller, darker breast-bands (speckling) than do males, but here again some overlap, especially in Caribbean and Baja California populations. See Prevost 1983a and Poole 1989a for details.

In flight, crook in wing (bends back at carpal joint) and dark carpal-patches on underside of wing are diagnostic (Dunne et al. 1988). Seen high overhead or from the side, head appears small. Seen head on, “. . . holds its wings in an exaggerated, uplifted bow completely above the horizontal axis” (Dunne et al. 1988: 165). Steady, languid wing-beat that is somewhat shallow; appears centered at elbow joint.

Similar Species

Not easily confused with other birds of prey, except maybe Bald Eagle (Haliaeetus leucocephalus), which is larger, usually dark-bodied, with all-white head and tail (adults), and soars with wings nearly flat. In flight, possibly confused with large, dark-backed gulls at a distance, especially adult Great Black-backed Gull (Larus marinus), but latter has all-white head and tail, white trailing edge to blackish upperwing, and no prominent crook to wings in flight.

Detailed Description

Ospreys have 10 full-length primaries, 18–19 secondaries (including 3 tertials), and 12 rectrices. They are diastataxic (see Bostwick and Brady 2002) indicating that a secondary has been lost evolutionarily between what we now term s4 and s5. Among primaries, p8 (usually) longest; outer web of p10 narrow; outer webs emarginated on p7–p9, and less so on p6; inner webs deeply notched on p8–p10 and less so on p7. Geographic variation in appearance is slight to moderate. The following molt and plumage descriptions pertain to the North American breeding subspecies P. h. carolinensis; see Systematics: Geographic Variation for appearance variation in three other recognized subspecies in the Caribbean and Eurasia. No geographic or sex-specific variation in molt strategies reported within North America, although variation in average timing and extent of molt occurs with latitude of overwintering location, due to variable environmental and migratory constraints and day-length regimes (see below).

Plumages

Following based primarily on detailed plumage descriptions in Bent 1937b, Witherby et al. 1939, Friedmann 1950a, Dement'Ev and Gladkov 1951, Oberholser 1974c, Cramp and Simmons 1980b, Henny 1988b, Forsman 1999, Wheeler 2003a, Wheeler 2003b, and specimens and photographs (Project VIREO, Academy of Natural Sciences of Drexel University); see Macnamara 1977, Baker 1993, Blanco and Rodriguez-Estrella 1999, and Pyle 2008 for criteria to determine age and sex. Sexes show average differences in Juvenile and Formative Plumages and can be used to separate most individuals in Basic Plumages. Definitive Plumage assumed at Second or Third Basic Plumage. Plumage descriptions below utilize popular color names familiar to most readers. Color numbers (given in parentheses) represent closest color match from (Smithe 1975).

Natal Down

Present primarily May–July in North America, in the nest. First down very short and covers entire head (including lores) and body, but down sparse on crown, nape, and sides of body behind wings. Head and underparts pale buff with dark-brown patch on auriculars (except directly below eye) and in front of eye, extending on to lateral sides of forecrown above eye and to center of forehead. Brown markings on head patterned as in adults. Upperparts including wings grayish brown, paler than brown markings on head, and with paler mottling and conspicuous wide, pale buff stripe down center of upperparts. A second down develops around 11 d; is woolly and slightly darker than first down but with pale stripe still present on upperparts.

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Juveniles have pale-fringed upperparts and orange eyes. Eye color can change rapidly, turning yellowish in some cases by early fall.

© Michael Brown, Indiana, United States, 28 August 2011
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Juveniles have brown upperparts with extensive pale edging, and usually a buffy wash to the underparts, especially notable in the underwing coverts.

© Nick Rosen, Delaware, United States, 4 August 2015
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Juveniles have dark brown upperparts with extensive pale edging in wing coverts, and orange eyes. Extent of pale edging variable, but usually notable in the field.

© Jeff Bryant, Illinois, United States, 19 September 2015
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Juvenile on migration. When seen from below, can be somewhat difficult to age, but note the very fresh, same-age flight feathers in the wings and tail, and the buffy wash below, especially prominent on the underwing coverts. Also note the M-shaped wings, a classic field mark for identifying Osprey by shape in flight.

© Brian Sullivan, Virginia, United States, 15 September 2005

Juvenile (First Basic) Plumage

Present primarily August–November in North American populations. Similar to Definitive Basic Plumage except dark feathers of upperparts, including all upperwing coverts and remiges, conspicuously tipped (and margined on distal portion of smaller feathers) with buff (121D; Figure 291 in Pyle 2008), that fades to white when worn. Elongated feathers of crown and nape slightly shorter than in adults. Tail with both inner and outer webs of all but central rectrices (r1) dull whitish and crossed by greater number of narrower and more distinct, dark-brown (20) bars and with broader and more-distinct pale tips (Figure 292 in Pyle 2008). Lower nape and sides of neck mostly whitish instead of mostly dark brown (20), and these areas, along with lower throat and breast, strongly washed buff (121D) or rufous (40) in unfledged juveniles (color quickly fades); remaining underparts plain white or washed rufous with variably light to heavy dark streaking across breast (formed by breast feathers with dark shaft streaks that widen into dark subterminal marks distally) forming breast band. On underwing surface, secondaries pale, with most primaries barred across both webs, and both a greater number of bars and more-distinct dark bars than in adult; smaller underwing coverts white but often tinged buff (121D) to rufous (40); proximal coverts whiter than in adult, and coverts on outer half of wing (not including greater primary coverts) have broad white tips, making carpal-patch more mottled with white and less conspicuous than in adults. On average, underparts of females tinged more strongly with buff (121D) or rufous (40) and breast with more extensive dark markings (see Figure 290 in Pyle 2008), but these differences show overlap between the sexes and generally cannot be used for reliable sexing by plumage alone.

Formative Plumage

"First Basic" or "Basic I" plumage of Humphrey and Parkes 1959 and later authors; see Preformative Molt and revisions by Howell et al. 2003 and Pyle 2005a. Present primarily October to December–March in North American populations, depending on when Second Prebasic Molt commences. Similar to Juvenile Plumage but with scattered replaced formative feathers on upperparts and underparts, the new feathers on upperparts darker, with reduced or no pale fringes.

Second Basic Plumage

"First Basic Plumage" of some previous authors; see Howell and Corben 2000a and Howell et al. 2003. Present primarily March–February in North American populations but timing variable and transitional plumage present for protracted periods (see Appearance: Molts); can be present as late as May or June (at 24 mo of age) in some birds. Plumage variably intermediate between worn Juvenile and Definitive Basic Plumage, and indistinguishable from the latter after all flight feathers have been replaced. In molting individuals and some of those that have completed the Second Prebasic Molt but have retained feathers, juvenile body (especially rump) feathers and/or wing coverts, 1–4 juvenile outer primaries (among p7–p10) and corresponding primary coverts, 1–8 juvenile secondaries (among s3–s4 and s7–s14), and/or 1–4 juvenile rectrices (usually among r2 and r5) retained, contrastingly worn, and with rectrices showing juvenile pattern (see Figure 291 in Pyle 2008). A second wave of primary molt can commence (Third Prebasic Molt) before all juvenile outer primaries and middle secondaries have been replaced, but by completion of Third Prebasic Molt plumage not separable from that of Definitive Basic. Many individuals, especially females, can be sexed in Second Basic Plumage, as described under Definitive Basic Plumage (Figure 290 in Pyle 2008).

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At rest, this adult in Definitive Basic Plumage shows solid dark brown upperparts, pale head with dark eyeline, and large, sturdy legs and feet. Ospreys lacks the supraorbital ridge above the eye found in most raptors, making it appear somewhat dove-headed and less fierce in appearance than both eagle species.

© Brian Sullivan, New Jersey, United States, 5 May 2009
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Adult in Definitive Basic Plumage; note the dark brown upperparts, white underparts and head with dark eyeline; legs, feet, and talons are large and sturdy.

© Anita Regler, New York, United States, 17 April 2016
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In flight, note the white head and dark eyeline, brown upperparts, white body, and narrow, long wings often 'crooked' at the wrist, with dark carpal patch.

© Anita Regler, New York, United States, 13 April 2016

Definitive Basic Plumage

Present primarily June–May in North American populations.

Male. Head, neck, and underparts white except feathers of forehead, crown, and nape with blackish-brown (between 119 and 89) centers, becoming most heavily marked on center of crown and on nape and least heavily marked on sides of forehead, along sides of center of crown, and postocular area above eye; dark feathers of nape, sides of neck, and anterior crown elongated, forming short frontal and nuchal crests when erected; eye entirely bordered by narrow ring of blackish-brown (between 119 and 89) feathers; feathers of nape and sides of lower neck partly to entirely blackish brown (between 119 and 89) or dark brown (20) with white portions of some feathers washed pale rufous (40); wide blackish-brown (between 119 and 89) patch extends from just behind eye (except where eye margined on lower three-quarters by broad band of white feathers) back through auriculars and joining dark-brown (20) feathers along side of lower neck. Head markings vary substantially and can be used to identify individuals in the field (Bretagnolle et al. 1994). Back, scapulars, rump, and upper surface of wing dark brown (20) with many feathers on inner wing medium brown (119A), and most feathers edged slightly paler and very narrowly tipped dull white (Figure 291 in Pyle 2008). Upper surface of tail medium brown (119A); all but central pair of rectrices (r1) becoming dull whitish on inner webs and all crossed by about 5 (central feathers) to 8 (outermost feathers) dark-brown (20) bands with most distal band widest, and tail with narrow, dull-white tips (Figure 292 in Pyle 2008). Upper surface of outermost 5 primaries darker than rest of wing on distal, narrowed portion of feathers, appearing dull black (89), and basal portion of inner margins of all primaries dull whitish and incompletely barred or marked with grayish brown (21 and paler); primary coverts almost as dark as primaries.

Chin feathers often with dark brown (20) shaft streaks, and feathers along sides of throat and across breast similar but with variable amount of dark-brown (20) subterminal marks, some individuals with few or none (Figure 290 in Pyle 2008); front part of feathered portion of legs with medium-brownish (119A) wash or streaks, and rear flanks with small band of dark brown (20) feathers; lateral undertail coverts washed pale buff (121D). Underwing coverts white, becoming heavily barred or marked dark brown (20) on greater coverts; distal portion of median coverts, and all coverts on outer half of wing (from bend of wing outward), except primary coverts, with larger white spaces between dark brown (20) bars than other coverts with dark markings; dark coverts on outer half of underwing surface form conspicuous dark carpal patch; axillaries brownish white and often with darker terminal marks. Undersides of remiges dull whitish, barred with medium brown (119D; less distinct and paler bars than on Juvenile feathers), and becoming darker on distal narrowed portion of outermost 5 primaries.

Female. Similar to Definitive Basic male in plumage except dark markings on chin, breast, and sides of neck average greater, the subterminal marks on breast becoming larger, often distinctly paler at center of breast, where some feathers mostly medium brown (119A or paler; see Figure 290 in Pyle 2008).

Definitive Basic Plumage is often characterized by mixed generations of basic body (especially rump) feathers, and 2–3 sets of basic feathers among primaries and secondaries, in Staffelmauser patterns; replacement sets in primaries are defined by a worn feather immediately distal to a fresh proximal feather, and those of secondaries showing mixed generations in various sequences (Pyle 2005b, Pyle 2006, Pyle 2008). Number of sets in primaries equates to minimum ages of 2–3 years; individuals overwintering in temperate latitudes may average more sets in older birds than those overwintering in tropical latitudes. Definitive primaries, secondaries, and rectrices are broader than juvenile flight feathers, showing less distinct barring as described above (see Figure 292 in Pyle 2008). Occasionally, individuals may undergo a complete Definitive Prebasic Molt and can be indistinguishable from birds in Second Basic Plumage.

Aberrant Plumages

A leucistic male from South Africa is in the collection of the Academy of Natural Sciences of Drexel University (catalog no. 2298). One or 2 cases of melanism (AFP).

Molts

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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.

General

Molt and plumage terminology follows Humphrey and Parkes 1959, as modified by Howell and Corben 2000a, Howell et al. 2003, Howell et al. 2004, and Pyle 2005a. Under this terminology a Formative Plumage is recognized during the first cycle and the first flight-feather molt, beginning at age 5–11 mo, is considered the Second Prebasic Molt (see below for alternative interpretations). Ospreys exhibit a Modified Basic Strategy (Howell et al. 2003, Howell 2010b), including incomplete-to-complete prebasic molts and a limited preformative molt (Pyle 2005a), but no prealternate molts (Bent 1937b, Oberholser 1974c, Cramp and Simmons 1980b, Prevost 1983b, Edelstam 1984, Henny 1988b, Marchant and Higgins 1993, Forsman 1999, Pyle 2008; Fig. 3). Study needed on the precise extents and timings of molts during predefinitive cycles (see below).

During flight-feather molt, primaries are replaced distally (p1 to p10), secondaries are replaced proximally from s1 and s5 and distally from the tertials, and rectrices possibly replaced in sequence r1–r6–r3–r4–r2–r5 on each side of tail, as in other Acciptrid hawks (Prevost 1983b, Pyle 2008). Molt pattern among primaries and secondaries exhibits Staffelmauser (Stresemann and Stresemann 1966a, Prevost 1983b, Edelstam 1984, Pyle 2005b, Pyle 2006), whereby incomplete molts result in a series of termination points from which molt continues during the next prebasic molt along with new series commencing at p1, s1, s5, and/or the tertials. Primaries are replaced in successive overlapping waves, each beginning with p1 and progressing out to p10, with number of feathers separating successive waves fewer in Definitive than in Second and Third Basic Plumages. By the Definitive Prebasic Molts, replacement often proceeds in 2–3 waves through the wing, resulting in 2–3 "sets" of feathers following completion of molt (Pyle 2006, Pyle 2008). Staffelmauser appears to be a product of insufficient time to undergo complete wing-feather molt; resultant multiple small gaps in the wing, as opposed to a single large gap, is adaptive for maintaining optimal aerodynamic capabilities of the wing in this and many other large bird species that are heavily dependent on soaring (Tucker 1991, Shugart and Rohwer 1996, Pyle 2005a).

Prejuvenile (First Prebasic) Molt

Complete, primarily May–July in North America, in the nest. Juvenile feathering begins to appear at around 4 wk of age with emergence of primaries, followed by feathers of head and upperparts (Bent 1937b). A specimen in the collection of the Academy of Natural Sciences of Drexel University (Catalog no. 189580) has juvenile feathers emerging last on underparts below the neck, and a band down the center of the rump and lower back. Juvenile Plumage is fully developed at about age 60 d (Henny 1988b). Otherwise, little information is available on timing or sequence of pennaceous feather irruption and development.

Preformative Molt

Generally termed "First Prebasic" or "Prebasic I" molt previously in most birds (see Second Prebasic Molt). Preformative Molt is absent to limited, on migration and/or overwintering grounds. May include up to 35% of body feathers scattered over head, upperparts, and underparts, in September–March in North American populations (Pyle 2005a), but study is needed on body molts in the first December–March, when flight-feather molt can commence on birds overwintering at tropical latitudes. It is possible that the Preformative Molt occurs only in some birds in September–December, after which time body molt in December–March (concurrent with initiation of first flight-feather molt in some birds) should be considered part of the Second Prebasic Molt (see below).

Howell proposed an alternate view in which the Preformative Molt also includes some or all of the body and flight feathers during the first wave of Staffelmauser (Howell 2010b: 121-122). Here we maintain a more traditional view (e.g., Pyle 2005a, Pyle 2006, Pyle 2008) until further study on the details of variation in the first flight-feather molt can be performed, e.g., in Ospreys that overwinter in temperate vs. tropical latitudes. In either case it seems reasonable to conclude that "opportunistic" and "obligate" Staffelmauser strategies (Howell 2010b) originated with a common ancestor and thus should be considered part of the same evolved molt; here we maintain it to be part of the Second Prebasic Molt (rather than the Preformative Molt), as in most other birds that undergo Staffelmauser, but further consideration is warranted.

Second Prebasic Molt

Considered "First Prebasic" or "Prebasic I" molt of previous authors; see Howell and Corben 2000a and Howell et al. 2003 for initial revision to this terminology regarding Accipitrid raptors and other birds. See also Preformative Molt (above) for an alternate view on the designation of first-cycle and second-cycle molts.

Second Prebasic Molt in Ospreys is incomplete to complete, variable in timing, can commence (with the dropping of p1) as early as December or as late as April, and concludes as early as July or as late as November in North American populations (see Third Prebasic Molt). Occurs primarily or entirely on non-breeding grounds, especially in individuals that overwinter in tropical regions; such individuals may remain (over-summer) in overwintering areas during their first and second cycles. Occurs earlier in the year than Definitive Prebasic Molt owing to lack of constraints related to breeding.

Remiges exhibit Staffelmauser (stepwise) sequence and pattern of flight-feather replacement (see above). In some individuals that overwinter in tropical regions, the Second Prebasic Molt may complete in a single molt sequence and in others it may be continuous with the beginning of the Third Prebasic Molt; in individuals that overwinter in temperate regions, from 1–4 juvenile outer primaries (among p7–p10) and corresponding primary coverts and 1–8 juvenile secondaries (among s3–s4 and s7–s14) can be retained after completion of Second Prebasic Molt (Pyle 2005b, Pyle 2008). In tropical western Africa, primaries were found to be replaced at a rate of about 1/mo and rectrices grew on average 3.2 mm/d (Prevost 1983b); rates may be faster in birds that overwinter in temperate regions. Molt of rectrices also may be incomplete, with 1–4 juvenile feathers retained, usually among r2 and r5. Secondaries (typically among s4, and s9-s12) are the last feathers to complete replacement, at 17–19 mo after fledging (January–April) in birds overwintering in tropical western Africa (Prevost 1983b); in some birds this may be after the Third Prebasic Molt has commenced.

Study is needed on the designation of molt terminology during the variable predefinitive molts in this species (see also Preformative and Third Prebasic molts, and Howell 2010b: 121-122).

Third Prebasic Molt

Incomplete to complete, variable and protracted, can commence on overwintering grounds as early as November of second calendar year or as late as April of third calendar year (Prevost 1983b, Pyle 2008), and typically completes in July–September on breeding grounds, at which point it may be continuous with Fourth Prebasic Molt entering the definitive cycle. Similar to Definitive Prebasic Molt but occurs earlier in the year owing to lack of constraints related to breeding; Fourth and Fifth Prebasic molts may also average earlier in pre-breeding individuals. Staffelmauser continues among flight feathers where Second Prebasic Molt is concluded, and begins a new replacement cycle at p1, s1, s5, and/or the tertials (see Definitive Prebasic Molt). Second Prebasic Molt can be continuous with Third Prebasic Molt in some Ospreys (see above), in which case the dropping of p1 for the second time could signify the transition point between these molts, sometime in November–July when the first sequence is often proceeding among p7–p10. Although evidence is lacking, some temperate-overwintering Ospreys may not complete juvenile primary and secondary replacement by the end of the Third Prebasic Molt, as in other temperate-overwintering raptors that undergo Staffelmauser.

Definitive Prebasic Molt

Incomplete (occasionally complete), primarily June–February in North American populations (Fig. 3), usually commencing on breeding grounds and concluding on overwintering grounds. Molt can begin during incubation, suspend for chick feeding, and resume after chicks have fledged; details need to be worked out in species that undergo Staffelmauser (Pyle 2005b). Suspension allows individuals to take advantage of a quiet period to molt, especially females, which are typically fed by males during incubation and brood rearing (see Breeding). Molt also can be suspended in August–September for fall migration and resume on overwintering grounds; e.g., resumes as early as 15 October in tropical western Africa and continues through February or early March (Prevost 1983b). Replacement patterns among primaries and secondaries exhibit Staffelmauser, as described above.

Bare Parts

Following descriptions for P. h. carolinensis based on Friedmann 1950a, Henny 1988b, and color photographs in VIREO collection at the Academy of Natural Sciences of Drexel University.

Bill and gape

Bill dull blackish in adults, with cere, base of lower mandible, and commissure (extending from gape distally to nostril area) bluish gray. Photos indicate that dorsal portion of cere (down to nostril) may be either bluish gray or blackish in adults. Cere in recently fledged young is black except around the nostrils where it is bluish gray. Lores unfeathered and dull blackish in adults.

Iris

Variable in juveniles and generally darker than in adults; described as “bright orange-red to dark brownish yellow” (Friedmann 1950a: 529). Yellow in adults but orange-yellow in some males.

Legs and feet

Feet and unfeathered portion of legs covered with small, rough, scales which become large and flat on distal portion of upper toes, and smaller with sharp points on soles of feet. Feet large; outer toe reversible; talons long, robust, and strongly curved. Unique foot structure facilitates retaining grasp of slippery prey (Henny 1988b). Legs and feet pale bluish gray or sometimes tinged greenish; talons black. In most photographs, legs and feet appear very pale and dull whitish, with bluish color difficult to discern.

Measurements

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Figure 5. Seasonal change in body mass.

Seasonal change in body mass of Ospreys breeding in southern New England. “E” shows data from adults (females above, males below) feeding experimentally enlarged broods. From Poole 1989a; data primarily from individuals weighed electronically at perches.

Detailed in Prevost 1983a and Poole 1989a. Females have longer wings, tails, claws, and bills than males, but overlap exists between sexes; see Appendix 2 . Adult females average 15-20% heavier than males (nearly 25% heavier in New England breeders), and do not overlap with males in mass (Poole 1989a). Mass of males and females declines during the breeding season; females significantly heavier at all stages of breeding cycle (see Figure 5). 

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