An online resource based on the award-winning nature guide

Birds

An Owl’s Digestion Process

11-26-18 -barred owl coughing up pellet2 _U1A1839Most owls do not bother to tear small prey such as mice and voles apart but instead swallow them whole.  After eight to sixteen hours, all the nutrients available in the eaten prey have been absorbed by the bird.  Owls cannot digest the fur, feathers, bones, teeth and nails of their prey, so these parts remain in the bird’s gizzard (specialized organ that grinds up food in most birds but serves as a filter for holding indigestible parts in birds of prey).  This accumulation of indigestible parts takes on its pellet form (which is the shape of the gizzard) about eight hours after ingestion, but is sometimes retained by the owl for another six hours or so before being coughed up. As a rule, bones are on the inside of the pellet, and the fur and feathers form a soft coating on the outside.

The stored pellet partially blocks the entrance to the digestive system so it must be ejected before the owl can eat again.  This process takes anywhere from a few seconds to several minutes.  The owl appears to “yawn” several times before regurgitating the pellet.  Note that the pictured Barred Owl has prey (a Deer or White-footed Mouse) in its talons, but out of necessity is getting rid of a pellet before devouring it.

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Lingering Great Blue Herons

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Most fish-eating birds that breed where most bodies of water freeze over in the winter migrate further south in the fall, including Great Blue Herons.  Movement of this large wading bird takes place largely from September to mid-October. According to Christmas Bird Count data, the Great Blue Heron has the widest wintering distribution of any heron species in North America.

While the number of Great Blue Herons in the Northeast is greatly diminished in November and December, it’s not uncommon to spot lingering birds at this time of year.  Come January, when most bodies of fresh water are inaccessible to herons, sightings become rare until they begin returning in March.

Where open water remains in the Northeast, those Great Blue Herons braving the cold continue to consume fish, insects, amphibians and crustaceans.  Small mammals, especially voles, and birds remain a warm-month delicacy, when mammal hair is cast in pellets and bones are digested.

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Ospreys Migrating

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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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American Goldfinches Dining On Thistle Seeds

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American Goldfinches are almost exclusively granivorous (consumers of seeds/grains).  Very few insects are consumed by these birds, even when feeding nestlings.  This is highly unusual in that spiders and insects are an essential part of 96% of N.A. terrestrial bird species. At the very least, most seed-eating birds feed their nestlings insects. (Brown-headed Cowbirds lay their eggs in other birds’ nests, leaving the raising of their young up to the host bird. It is rare that a cowbird chick will survive to leave an American Goldfinch nest, probably because it cannot thrive on a diet of virtually all seeds.)

The seeds of plants in the Composite family (sunflowers, thistles, dandelions, etc.) are the preferred food of goldfinches. Thistle seeds, being high in fat and protein, are high on the list. There appears to be a correlation between the late nesting period of goldfinches (late June or early July) and the flowering of thistles.  By the time American Goldfinch eggs have hatched, there is an ample supply of thistle seed for the nestlings.

Now is the time to keep an eye on the seedheads of thistles, dandelions and other composites for the acrobatic seed-plucking antics of American Goldfinches.

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Barred Owls Fledging (But Not Flying Yet)

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If you heard Barred Owls calling this winter, and have occasionally spotted one in the same vicinity this spring, now is the time to start looking up at the canopy to see if they produced any young owls. Having spent four or five weeks in a tree cavity being fed and cared for by their parents, Barred Owl nestlings get the urge to spread their talons (and eventually their wings) and leave their nest about this time of year. It will be roughly another month before they begin short flights; until then the fledglings are referred to as “branchers,” as that is where you will find them, perched and begging for food from their parents, who will continue to feed them until late summer or fall. (If you know a youngster who is captivated by owls, they might enjoy reading Otis the Owl by yours truly!)

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Common Ravens Fledging

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Amazingly, there are birds that are fledging from their nests this early in the spring, among them Common Ravens. The intelligence of this species is well known, but perhaps less familiar are their antics, especially those of young birds.

Common Ravens have been observed “sliding down inclines on their belly, lying on their side grappling sticks, dropping and catching objects while in flight, hanging upside down by one or two feet, snow “bathing,” giving vocal monologues, caching inedible items, playing “tug-of-war” or “king-of-the-hill” with other ravens, and pecking predators on the tail.” (Birds of North America Online) If you hear their guttural call from above, be sure to look up and enjoy the show. (Photo: Common Raven fledgling. Thanks to Erin Donahue and Charlie Berger for photo op.)

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Changing of the Guard

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In approximately 85% of bird species, both the male and female of a mating pair contribute to the feeding and guarding of their offspring. Red-bellied Woodpeckers are of this ilk. Both members of a pair help excavate a nest cavity, incubate the eggs, brood the young and feed the nestlings for up to 10 weeks after they fledge. As seen in this photograph, when tending to their nestlings, one member of a pair wastes no time in departing as soon as its mate appears.

Thanks to the extension of the Red-bellied Woodpecker’s range northward, even northern New Englanders now have the opportunity to observe the nesting behavior of these medium-sized woodpeckers. (Photo: female Red-bellied Woodpecker leaves nest as food-bearing male arrives. Note continuous red crown on male, which is broken in female.) (Thanks to Sadie Brown for photo op.)

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