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Posts tagged “Proventriculus

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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Avian Digestion

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What happens to food after a bird swallows it? It may be stored in their crop, a pouch which is actually an enlarged part of its esophagus that some species of birds (and bees, fish and earthworms) have, or it may go directly to their stomach.  Birds have a two-chambered stomach.  The first chamber, the proventriculus, secretes acids that help break down food, including bones. A shrike’s well-developed first stomach chamber can digest an entire mouse in only three hours!  From the proventriculus the food goes into the second chamber, which is referred to as the gizzard, before entering the intestines.  The gizzard is a muscular organ which grinds up tough food, sometimes with the help of grit that some birds ingest.  The gizzard grinds the gravel and stones against the nuts and seeds, a process which smashes the food.  Wild turkeys can actually pulverize walnuts in their gizzards!  In some species, the gizzard remains small and insignificant during the summer when their diet consists of soft food such as flesh, insects, or fruit, but it grows more powerful during the winter when seeds are their main food.