Thursday, January 5, 2012

SNOWY OWL FLIGHT YEAR


The winter of 2011-2012 will go down in history as a time when arctic-breeding Snowy Owls (Bubo scandiacus) came down in great numbers to southern Canada and the northern United States. This happens at irregular intervals, and this time it has been six years since the last flight year in the Pacific Northwest.

These flights are called irruptions, a word meaning 'into an area' just as eruptions means 'out of an area.' The latter is used for volcanoes, the former for unusually large flights of birds or movements of other animals, usually detected where they arrive. In this case, our baseline is no more than a few birds observed each winter, whereas the irruptions involve dozens to hundreds of birds.

Snowy Owls have been reported from many localities since November, some of them birds that were seen for only a day or two, others birds that arrived and stayed put. Numbers inland are generally small, not much higher than in non-flight years, but the majority of birds are congregated near the coast.

Up to a dozen birds have been present on Damon Point, a spit on the north shore of the entrance to Grays Harbor, and these are the birds featured here. Even more are present along Boundary Bay in southern British Columbia. At both of these localities, numerous birds can be seen from the same spot, sometimes several perched together on a log. They seem not at all aggressive to one another. In addition, a few birds have taken up residence at Port Susan Bay and the Nisqually Flats.

There is still some confusion about exactly why the owls come down in numbers only at intervals. Earlier thoughts were that they did so after a lemming population crash, the absence of their primary food sending them south. But this has never been entirely satisfactory, because lemming population cycles in different regions are independent of one another, yet the irrupting owls appear over a very wide area.

An alternative gaining more credence is that during years of lemming population highs, the owls do very well, and so many young are produced that densities in the fall are higher than normal, and this sends many birds southward or perhaps in all directions. Thus our numbers are because of lemming booms, not lemming busts.

Lemmings are absent from this area, but voles are locally abundant, and some of the birds that make these flights may subsist on these lemming relatives.

In lieu of these rodents, many owls switch to a water-bird diet, which is why coastal areas are favored. Water birds usually roost right out in the open, either on shore or on the water, and if the owls go out at dusk to hunt them, their prey is at a great disadvantage. A white owl approaching in low light levels may be just about impossible to see against a paler sky. We still don't know if they continue to hunt through the night, but they clearly don't during the day, which is why we rarely see them doing anything but turning their heads and occasional preening.

Studies based on the collection of regurgitated pellets find these coastal owls eating shorebirds (Dunlins, Sanderlings, Red Phalaropes) and diving birds (Horned Grebes, Buffleheads) as well as Black Rats that live among jetty rocks. After the breeding season, some female owls fly out to the pack ice and presumably have nothing to eat but water birds.

One last thought. These owls are magnificent additions to the local scene whenever they favor us by their presence. Although they seem tame and placid, it's always best to avoid getting too close. Each owl has its own flight distance, and observers and photographers should be sensitive to that.

Dennis Paulson

1 comment:

Krista Allen said...

It is absolutely amazing to me that it has been six years since the last flight year for Snowy Owls! They are such beautiful creatures and the mystery of when and why they will fly down to Canada and the U.S. just adds to their appeal. :)