Science Corner - Osprey in Berryessa Snow Mountain National Monument
If you’ve spent time near the water while visiting Berryessa Snow Mountain National Monument, chances are you may have observed one of our native raptor species nearby. Sometimes called fish hawk, sea hawk or river hawk, Osprey are found in temperate and tropical regions of every continent except Antarctica. They are the second most wide-spread species of raptor and one of only six land birds with global distribution. There are four subspecies: Eurasian, American, Caribbean and Australian, each looking remarkably similar with only minor physical differences.
The subspecies of Osprey found within Berryessa Snow Mountain National Monument (and the rest of the United States and Canada) are the American Osprey (Pandion haliaetus carolinensis). Originally classified as a species of falcon, today Osprey are the sole member of the genus Pandion and the only member of the family Panionidae. Nearly all subspecies of Osprey are migratory and travel long distances to winter in warmer climates, however, most of the individuals that call Berryessa Snow Mountain National Monument (Monument) home are year-long residents. In fact, some populations from farther north may travel to our region and farther south to winter there.
Osprey are smaller than a bald eagle but larger than a red-tailed hawk. They range from about 21 to 23 inches in length with a wingspan of from 50 to over 70 inches. They weigh between two and a half up to four pounds with the females being slightly larger than the males. Males and females look very similar with dark brown underparts and a white breast both which serve as field marks to help identify them in flight. Females usually have brown streaking on the upper portion of their breast. Their heads are brilliant white with a striking dark mask across the eyes and down the sides of the neck. This mask reduces glare to allow them to see fish in the water better, similar to an athlete wearing “eye black”. They have golden brown irises with black beaks, blue ceres and black feet and talons and a short, rounded tail. In flight they hold their wings form an arch, similar to a gull.
Osprey are considered piscivorous meaning that their diet consists almost exclusively of fish and they have several unique adaptations to allow them to be efficient and extremely successful predators. Osprey and owls are the only two raptors (bird of prey) with a reversible outer toe which means it can face forward or backward helping them capture and control slippery fish with two toes in front and two toes in back if needed. They also have sharp spicules and barbed scales on the underside of their feet that also help grasp wet, wriggling fish and their nostrils seal tightly to keep water out when they plunge feet first after prey. Osprey also have specialized vision that helps them see prey species underwater from up to 100 feet above the surface. You may have seen them hover for a bit and dive at rates of up to 50 miles per hour, but what you may not have noticed is how they are able to adjust their flight angle to account for the distortion caused by the refracted image. Often becoming fully submerged, their dense plumage is oily to help shed water. After a successful dive, Osprey will carry their prey to a nearby perch or back to the nest.
Osprey breed near lakes and rivers but can also be found nesting near coastal brackish waters. Flirting begins with the male calling while performing aerial stunts and the offering of a stick in his beak. If the female is impressed, she will accept the stick and the two begin a lifelong chick-rearing relationship. Nests or eyries are constructed in the tops of trees, on rocky outcrops, the tops of utility poles, duck blinds and platforms intended for them to nest on. Males will carry in material which may consist of sticks, driftwood, reeds, lichen, seaweed or even urban trash such as plastic bags and nylon netting while the female arranges it. Nests are remodeled and updated year after year, some have been used by different pairs for as many as 70 years. Over time they become so big - up to six feet in diameter and weigh nearly 300 pounds - and so messy that other species such as tree swallows and grackles build their own nests inside them! Two to four eggs are laid and the female stays with the chicks while the male brings food back to the nest. Chicks begin to fledge after about two months.
Many Native American tribes consider the Osprey a species of eagles and give them the same reverence as Bald or Golden Eagles. Coastal tribal legends often consider Osprey a guardian and others consider them medicine birds and seeing one in a vision meant that the person had been granted spiritual power as a healer.
Osprey were almost extirpated during the late 18th and 19th century due to egg collecting and hunting and then again during the 1960’s and 70’s because of the use of the insecticide dichlorodiphenytrichloroethane (DDT), which interfered with the calcium absorption in many species of bird causing fatal egg shell thinning. Fortunately, Osprey adapt well to life near humans and will readily use man-made nesting structures which has helped the population rebound following the banning of DDT use.
Fun Osprey Facts:
- When flying with a fish, Osprey turn them so they are face first to reduce wind resistance
- Migrants travel individually rather than in flocks and follow the edges of bodies of water such as coastlines, lake shores, rivers and mountain ridges.
- The oldest known Osprey was over 25 years old and was banded in Virginia in 1973
- Bald Eagles sometimes chase Ospreys to steal their catch
A not so fun fact is that “Osprey plumes” were used to decorate hats in the 19th century. Despite their name, the feathers were actually from egrets which caused a sharp decline in the number of egrets.
Be sure to keep an eye out for Osprey next time you visit our favorite monument!
-Kristie Ehrhardt (kehrhardt@tuleyome.org)
Tuleyome Land Conservation Program Manager
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