Osprey in Yellowstone National Park
The Osprey population of Yellowstone National Park continues to show signs of a serious downward trend over the last six years. Cutthroat trout, a major food source for Ospreys, have severely declined on Yellowstone Lake due to the predominance of a non-native lake trout. At a spot on the lake where more than 70,000 cutthroats were counted in the 1970s, a mere 470 were tallied in recent years, the lowest number since record keeping began in 1945.
Other factors in the decline of the Osprey include a 2003 wildfire on Frank Island, in the middle of Yellowstone Lake. The fire burned 570 of the island's 600 acres, wiping out nearly all the old-growth trees that were a popular spot for nesting ospreys.
The Osprey population of Yellowstone Lake are not dying out but they have likely flown to areas outside of the park to feed in a more abundantly stocked body of water and to nest in the tall trees that they prefer. Listen for their call, it is a series of sharp whistles, described as cheep, cheep or yewk, yewk. Near their nest, the call is a frenzied cheereek!
Osprey (Pandion Haliaetus)
The Osprey is a large bird of prey or raptor weighing about three to four pounds. It is about 24 inches in length with a wingspan of up to six feet. The upper parts of the bird are a deep, glossy brown, while the breast is white and sometimes streaked with brown, and the under parts are pure white. The head is white with a dark mask across the eyes, reaching to the sides of the neck. The osprey’s eyes are golden to brown and the bill is mostly black.
The Osprey's diet consists almost exclusively of fish. It has evolved specialized physical characteristics and exhibits some unique behaviors to assist in hunting and catching prey. The osprey’s keen vision can sight fish from 32-130 feet above the water. After is prey is sighted the bird hovers momentarily then plunges feet first into the water. It is able to dive to a depth of 3.3 feet the bird has nostrils that close to keep out water during dives.
The Osprey’s feet have black talons which have backward facing scales that act as barbs to help hold its catch. It has a short tail and long, narrow wings with four long, finger-like feathers and a shorter fifth. The sexes appear fairly similar, but the adult male can be distinguished from the female by its slimmer body and narrower wings. The breast band of the male is not a noticeable as that of the under wing areas of the male are more uniformly pale. It is easy to determine the sex in a breeding pair, but harder with individual birds.
Osprey typically build their nests of sticks on the forks of trees, rocky outcrops, utility poles, artificial platforms or offshore islets. They prefer to be near fresh water lakes.
Generally, Ospreys reach sexual maturity and begin breeding around the age of three to four years. If there are no nesting sites available, young Ospreys may be forced to delay breeding. To encourage nesting, posts may be erected to provide more sites suitable for nest building.
Ospreys usually mate for life. In spring the pair begins a five-month period of partnership to raise their young. The female lays two to four eggs within a month, and relies on the size of the nest to conserve heat. The eggs are whitish with bold splotches of reddish-brown and are about 2.4 x 1.8 inches, and weigh about 2.4 ounces. The eggs are incubated for about five weeks to hatching.
The newly hatched chicks weigh only two ounces, but fledge in eight to ten weeks. When food is scarce, the first chicks to hatch are most likely to survive. The juvenile Osprey may be identified by buff fringes to the plumage of the upper parts, a buff tone to the under parts, and streaked feathers on the head. During spring, barring on the wings and flight feathers is a better indicator of a young bird, due to wear on the upper parts.
The typical lifespan of an Osprey is 20–25 years. In North America owls and bald are the only major predators of both nests and young adults.
Osprey in the Past and Future
In the late 19th and early 20th centuries, the Osprey population was threatened by egg collectors and the hunting of the adult birds. Osprey populations declined drastically in many areas in the 1950s and 1960s which was due, in part, to the toxic effects of insecticides such as DDT on their reproductive cycle. The pesticide interfered with the bird's calcium metabolism which resulted in thin-shelled, easily broken or infertile eggs.
Possibly because of the banning of DDT in the early 1970s, together with reduced persecution, Osprey have made significant recoveries. Artificial nesting platforms are common in areas where preservationists are working to re-establish the birds.
Sadly, the Osprey habitat in Yellowstone Park is declining. Tree nest site instability coupled with weather continues to play a major role in influencing Osprey productivity in the park. Frank Island is a major Osprey production area on Yellowstone Lake, but after a the fire of in 2003 only one breeding has pair nested here in the last three years and fledged only one young per year.
In 2006, a mere 23 young fledged from 41 nests, compared to only 15 young fledging from 48 nests in 2005, and 19 young fledging from 54 nests in 2004. This represents the fourth lowest production experienced in the last 20 years of collecting detailed osprey population data.
Eagles and ospreys frequent similar habitats and sometimes battle for food. Eagles often force osprey to drop fish that they have caught and steal them in midair. The incidence of Bald Eagles taking over Osprey nest sites has been observed on numerous occasions. Monitoring the population dynamics of Ospreys and other fish-reliant bird species is especially important as we chart lake trout numbers over time.
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