Ospreys have a unique appearance and behavior, which makes them relatively easy to identify. Their dark-brown upperparts and white underparts, and a stereotyped hunting technique separate Ospreys from similar birds.
Male and female Ospreys have similar plumage and body measurements but perform different functions during the breeding season. This article focuses on the appearance and identification of Western Ospreys or Ospreys of the Americas.
How to identify a perched Osprey
A perched Osprey from a distance looks like a relatively large and long bird with black and white plumage. The dark upperparts are actually dark-brown and white underparts have some streaks and specks on the breast. Ospreys have the same plumage throughout the annual cycle.
Ospreys are almost always associated with bodies of water that have abundant fish. They are confident and can often be seen on high perches, utility poles, or their bulky nests.
- The dark-brown upperparts include the back, folded wings and tail.
- The white underparts include the throat, chest, and belly.
- An Osprey’s tail is relatively long and gray-brown with whitish bars.
- The legs and feet are pale in color with a bluish hue that can vary among individuals and is difficult to see in the field.
How to identify an Osprey in flight
A flying Osprey shows a slender figure and disproportionately long wings. Ospreys have a characteristic silhouette or profile in flight. The outer parts of their wings are bent and seemingly dropping giving the appearance of an “M”.
- Viewed from below, the throat, chest, and belly appear all white.
- The underside of the wing shows white and gray-brown color pattern. The white part is the underwing coverts while the dark part are the flight or wing feathers.
- Ospreys have a typical dark carpal area that corresponds to the wrist of the wings.
- Ospreys in flight have a shallow wing flapping. They also soar with their fully extended wings.
Head appearance
Its head distinguishes Ospreys from other birds with a similar appearance. The head is mostly white with broad stripes across the eyes.
- Most of the head is white, with wide dark-brown stripes that run from the base of the bill through the eye.
- Gray streaks and spots on the forehead and part of the crown are visible at close range or with the help of binoculars.
- Ospreys do not have a supraorbital bone or supraciliary bulge (also known as the supraorbital rim) above their eyes. Raptors have a characteristic fierce appearance due to the supraciliary protrusion. It can be said that the Osprey is a bird of prey without the typical fierce appearance of its close relatives.
- The feathers in the nape area are elongated, with sharp tips that give the appearance of a nuchal crest.
- An Osprey’s beak is black with a hooked upper jaw, which is typical of birds of prey. The base of the bill and the cere are bluish.
- Adult birds have yellow eyes. Young birds have orange eyes that turn yellow as they mature. In juveniles and adults, the coloration of the eyes may vary.
- An Osprey’s nostrils are elongated in shape, and can be closed voluntarily by a valve system. When an Osprey plunges into the water at high speed to catch a fish, the nasal valves close the nostrils to prevent water from entering violently, which could be harmful to the diving bird.
How can I tell the difference between a male and female osprey?
The male and female Osprey look almost identical at first glance. Both sexes show some differences in plumage, but these are inconsistent showing overlap between sexes. The most consistent difference is the body weight; females are heavier than males.
Other body measurements such as wing and tail are also longer on average in females than in males. The size difference can be noticed when a pair of birds are perched next to one another.
Plumage differences between males and female Ospreys
The difference in plumage between male and female Ospreys include:
- Dark feathers in the chest: Typically, most females have more spots and streaks on the chest. Some females show a dark chest band. Most males have fewer chest spots and streaks and in some individulas the chest is nearly pure white.
Some males and females have intermediate spotting and straking making it diffucult to relay on this plumage character alone to visually determine the sex of an Osprey with certainty.
In a North American study including hundreds of adult Ospreys, only 50 to 70% of the individuals had their sex accurately identified using the density of dark spots and streaks in the chest. The rest of the individuals had intermediate chest color patterns, so their sex could not be determined with certainty. - Underwing plumage: As noted above, the underwing plumage is bicolor, white, and dark gray. The wing underside of females has more dark spots and bars on average. Males generally have almost entirely white underwing feathers. However, this character also shows overlap between males and females.
- Carpal Area: The carpal areas is dark in both sexes, but on average it has a lighter color in males. As with other plumage characters, the carpal areas in both sexes shows overlap between sexes.
Size Differences between male and female Ospreys
- Weight: Studies have shown that females Osprey weight up to 20% more than males. Wheight is the most consistent difference between females and males but requires the two sexes perched side by side to accurately determine the sex of Osprey.
Quantitative studies that included the the weight of female and male Osprey found that males vary in mass from 1200 to 1600 g while females vary from 1600 to 2000 g.
To judge from the range of weight there is also some overlap between male and female Ospreys.
- Wingspan: The wingspan of females (the distance between the tips of their outstretched wings) is about 5-10% longer than the wingspan of males. Because the wingspan of Ospreys also overlaps between sexes, it is not reliable for determining an Osprey’s gender.
Male and female behavior
During the breeding season, male and female Ospreys have specific functions that can help determine the sex of the individual.
Nearly always the male collects the nesting material while the female sits in the nest the male brings.
The female does virtually all the incubation while the male hunts for fish to feed her. Hence a bird sitting on eggs is more likely to be a female. Also, a bird seen diving and plunging for fish during the incubation period of the breeding season is likely to be a male.
The appearance of a juvenile Osprey
There are small plumage differences between juveniles and adults birds. These differences disappear once juveniles attain the adult plumage about 18 months after leaving the nest.
Juvenile Ospreys have beige to whitish edges or margins on the dark feathers of the upper parts giving the appearance of a scaly plumage. The pale edges fade and shrink in size as the plumage wears down over time.
Juvenile birds also have buffy tones on the base of the neck, which also disappear when the bird becomes an adult.
The juvenile Osprey’s eye color
Juvenile Ospreys have orange to reddish-orange eyes. Although eye color is variable among juvenile individuals it turns progressively yellow as the bird reaches the adult age.
References:
- Poole, A. 1989. Ospreys: A Natural and Unnatural History. New York: Cambridge University Press.
- Poole, A. 1994. Family Pandionidae (Osprey). Pp. 42-50 in J Del Hoyo, A Elliott, J Sargatal, eds. Handbook of the Birds of the World, Vol. 2. Barcelona: Lynx Edicions.