Tuesday, April 3, 2012

SNOWY OWLS ARE ORNITHOPHAGOUS!


The Snowy Owls (Bubo scandiacus) that came down to Washington this winter, which I have written about before, finally contributed some pellets to the cause of science.

Of course, you know what owl pellets are. Birds of prey, and actually quite a few other birds, eat a lot of stuff that doesn't make it through their digestive tract. Hair and feathers are difficult to digest, as are bones and mollusk shells. So even if they are broken into smaller pieces when eaten and crushed by heavily muscled gizzards, even the smaller pieces can't pass through the hindgut very well. Rather than sharp-pointed bones coming up one by one, they are coated in hair or feathers and barfed, urped, hurled, vomited and/or regurgitated back into the environment.

It's not easy to find these pellets unless you know right where the bird has been roosting. After they are produced, they get covered up by detritus, even blown around, and eventually decay into pieces. But they hold together for a while, and ornithologists have long used them to get a handle on the diet of birds such as hawks and, especially, owls. Snowy Owl pellets look like fuzzy three-inch cigars. It's been said there is nothing like a good cigar, but I personally prefer owl pellets.

Paul Bannick, well-known bird photographer and author of The Owl and the Woodpecker, recently sent me three pellets he picked up from one spot at Ocean Shores. At the museum, we soaked them in water and stirred them up until the feathers floated and the bones sank. We recovered a surprising amount of bones, arranged them by type, and identified them by comparing with our skeleton collection.

I had a pretty good idea what birds were out there, and it wasn't difficult to identify the majority of the bones as sandpiper bones. The only confusion would have been between Sanderling and Dunlin, both common birds in Grays Harbor. Sanderlings were common right where the owls were roosting, so I favored them. Sure enough, there were several lower mandibles present, and they clearly belonged to Sanderlings.

In total, at least five Sanderlings were present in these pellets, as indicated by counts of tibiae and tarsometatarsi, long, slender bones that were well represented because birds of prey tend to swallow legs of smaller birds whole. In addition to all the sandpiper bones, there were quite a number of larger bones. Many of them were broken up, but a few were intact, and two coracoids and a femur allowed identification as a Horned Grebe. Probably all the bones, including many vertebrae, were from the same bird.

I also examined single pellets from Sandy Point, near Bellingham, furnished by Isa Werny and Andrea Warner. They were mostly feathers, but one of them contained a few Horned Grebe bones, the other a few Bufflehead bones. The second pellet was found at the foot of a utility pole along with parts of a dead Bufflehead, making the identification easier. Both of these species are known Snowy Owl prey.

Snowy Owls are well known to subsist largely on water birds in the winter on our coast, and there wasn't a trace of a mammal in these five pellets. By now you may have figured out that ornithophagous = bird eating.

Dennis Paulson

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