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Osprey

Mystery Photo: Bald Eagle or Osprey Pellet

 

Congratulations to Jill Osgood (“osgoodjill”), the first reader to correctly identify the pellet of either a Bald Eagle or an Osprey.  Many people are familiar with bird pellets – lumps of material consisting of the indigestible parts of a bird’s diet which are regurgitated by the bird hours after they’ve eaten their prey.  Raptors often consume their prey whole, including parts that are not easily digestible such as fur, feathers, bones, teeth, nails, etc.  These parts get as far as the proventriculus, an organ located between the esophagus and the gizzard, where they are packed into a pellet.

We often associate pellets with owls, but many species of birds, in addition to owls and other birds of prey, form pellets.  They include grebes, herons, cormorants, gulls, terns, kingfishers, crows, jays, dippers, shrikes, swallows, and most shorebirds.  The size of the Mystery Photo pellet (3” long) indicates that the bird that regurgitated it was very large – in general, the larger the bird, the larger the pellet.  It was found near the shore of Lake Champlain, where Ospreys and Bald Eagles are not uncommon.

If I had to, I would guess the pellet was regurgitated by a Bald Eagle. Osprey are piscivores, eating primarily fish, and bald eagles are carnivores, eating a variety of fish, mammals and amphibians. A close look at the pellet reveals, in addition to fur and fish scales, the upper mandible of a very small rodent on the left hand end of the pellet.  An Osprey’s pellets consist of primarily scales and bones, whereas a Bald Eagle’s pellets are composed primarily of hair (its stomach acid breaks down the bones and scales).

Twelve to eighteen hours after consuming prey, a Bald Eagle casts a pellet. Relatively odorless and light-weight, these pelleted remains can reveal the varied diet of this raptor.

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Juvenile Ospreys Fledging

Their red eyes tell you that both of these Ospreys are this year’s young (adults have yellow eyes).  During the last week they are in the nest, the young often exercise their wings by hovering over the nest.  After their first flight, fledglings generally remain at the nest or nearby.  Eventually they begin hunting for themselves, but the parents continue to bring fish back to their young for ten to twenty days, supplementing the food that the young start to catch on their own.  Within a month or so of fledging the juvenile birds begin their migration south. (In the accompanying photograph, one fledgling is returning to the nest after quickly circling a nearby field while its parents were off fishing.  Dinner was delivered shortly thereafter.)

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Sibling Rivalry

The male Osprey typically provides fish for the chicks, often feeding first and then presenting the remainder to the female who tears the fish apart into small pieces and feeds it to the chicks.  When food is delivered, there can be significant aggression on the part of the older Osprey nestlings if the parent hasn’t fed them in a while.

Incubation begins with the first egg, so they hatch sequentially, producing a brood of chicks that are not  the same exact age.  Once dominance is established, the older chicks feed until satiated and then allow the younger ones to eat.  If food is scarce, it’s not unusual for the younger chicks to starve to death, but if food is plentiful, a peaceable kingdom reigns.

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Osprey Nesting Behavior

Naturally Curious is back!  Different ecosystem (western vs. eastern Vermont) but same curiosity!  This week’s posts are going to be devoted to the nesting behavior of the Osprey — the only raptor that plunge-dives feet first to catch live fish as its main prey source.

Ospreys nest within six to twelve miles of water (usually much closer).  The male collects most of the nesting material and brings it to the nest site where the female arranges it. Sticks as large as an inch-and-a-half in diameter and three feet long are collected from the ground, or (less commonly) snapped off a tree while the Osprey is in flight.  Nest-building continues throughout the incubation of the eggs as well as the brooding period — even if a nest fails, Ospreys will continue to add material to it.

Although nests built on platforms are relatively small, those built in trees or on the ground can be 10 -13 feet deep and 3 – 6 feet in diameter (the largest nests are most likely the result of several generations of nesting Ospreys).  The shape of an Osprey nest changes during the breeding cycle.  When the eggs are being incubated, the nest is bowl-shaped.  After hatching the nest flattens out, but a rim of sticks is maintained. By the time the nestlings fledge (around 50-55 days) the nest is often completely flat.

Ospreys will reuse their nest year after year, saving themselves time and energy which allows earlier laying and more surviving young. Birds whose nests fail are likely to build alternate nests and use them in subsequent years. (Birds of the World Online).

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Ospreys Migrating

10-15-18 osprey 014

Banding birds, and the retrieval of these bands, has provided valuable information on the movement of birds. Today we also have the benefit of satellite telemetry, in which a bird carries a tracking device and its location is calculated via satellites that orbit the Earth.  The following is just a sample of what this tracking technology has revealed about the migration of Ospreys.

There are significant differences in male and female timing of migration (females leave up to a month before males), distance traveled and overwintering locations.  There is strong fidelity to overwintering sites as well as to migration flyways.  Breeding pairs of Osprey do not migrate or overwinter together, and adults do not migrate with their offspring.  Ospreys rarely migrate at night over land but inevitably migrate at night when undertaking longer (more than 12 hours) water crossings.

Subtle insights into migratory behavior can be gained by the findings of satellite telemetry, as well.  Their first flight south by juvenile Ospreys is often largely over water. A majority of juveniles migrating over the Atlantic Ocean from Massachusetts to the Bahama Islands flew as many as 1,500 miles over a period of up to 58 hours. The fact that no adults or 2nd year birds took this route over water suggests that juvenile Ospreys learn the coastal migration route during their first trip north.

Overwintering habitat preferences have also been assessed.  Of 79 Ospreys tracked by satellite, 30.4% overwintered on coasts, 50.6% overwintered on rivers, and 19% overwintered on lakes or reservoirs, with differences based on both sex and region of origin.

These few facts don’t begin to exhaust the information gathered from banding and satellite telemetry on Ospreys, much less many other species. They just serve to illustrate how modern tracking technology compliments and increases the information formerly gathered by firsthand observation and banding.

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Ospreys Migrating

email-osprey 014Adult female Ospreys begin their fall migration in August, before their young are completely independent. After females leave, males continue to feed this year’s young and don’t reach the peak of their migration until the middle of September. Ospreys tend to migrate during the day, except when crossing over large bodies of water, which they do at night. Unfortunately, the nocturnal flights of northeastern Ospreys over the Caribbean (a 25-hour nonstop flight) on their way to their wintering grounds in South America often coincides with the hurricane season. As treacherous as this is, 80 percent of adult Ospreys survive migration, according to the National Wildlife Federation.

 


Ospreys Laying Eggs

5-5-15 osprey 266Ospreys are late-season breeders compared to other raptors of their size, and are just starting to lay and incubate their eggs in the Northeast. This is thought to be an adaptive delay to allow ice to break up and to allow fish to move into shallow waters. (In years of late ice-out, ospreys may not breed.)

Both male and female ospreys incubate their 1 – 4 eggs, but the female generally does a majority of it, and nearly always is the incubator at night. The male typically brings the incubating female food, which she takes to a nearby perch to eat while he sits on the eggs. Once the eggs hatch (in about 5 weeks) the young are brooded by the female. The male does the fishing, bringing his prey back to the nest, eating his fill and then giving it to his mate to tear into small bits and feed to their nestlings.

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Avian Toe Arrangment

3-17-14 mourning dove tracks 004As opposed to humans, who use the entire bottom of their feet for support, birds stand and walk only on the ball of their foot and with their toes. When you look at a bird’s leg, what appears to be its knee, bending backward instead of forward as it does in humans, is actually its heel.

Most birds have four toes, arranged differently according to the life style of the bird. Songbirds, as well as most other birds, have three toes pointing forward and one pointing back. Most woodpeckers, being active climbers, have two toes pointing in each direction, which provides added clinging support. The outer toe (of the three forward toes) of ospreys and owls is reversible, so that they can have two toes in back should they need to get a better grasp on slippery fish or other prey. Some birds that do a lot of running, such as sanderlings and most plovers, have only the three forward toes. (Photo: Mourning Dove tracks)

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