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This could be a good year for snowy owls. We have had at least ten sightings in the Chicago area through Christmas. Three of the reports were from the lakefront in the city, four were from the Indiana dunes, and the rest came from open fields out beyond the 'burbs.

Snowies are birds of the open tundra, and when they come South, they look for someplace that reminds them of home. Meigs Field is about as windswept and forlorn a place as Chicago has to offer, and one of this year's sightings comes from there. Airports in general feel homey to wintering snowies. Eighteen were counted in a single day recently at the Boston airport.

We see some snowies every winter around Chicago, but the numbers vary wildly. Some years they are almost absent, and some years you can expect to find on e if you have the patience to look a few times. These fluctuations have been arranged by some into cycles of approximately five years, with heavy emphasis here on the word approximately."

Ornithologists have been tracking snowy owl irruptions since the winter of 1876. In the old days before those treehugging environmentalists got protective laws passed, the way to monitor owl invasions was to contact as many taxidermists as possible. Snowy owls were an ideal adornment above the bar in a saloon or in a barber shop, and people with an interest in nature might even want one for the front parlor.

During the flight of 1889-90, a taxidermist in Mandan, North Dakota, had 500 carcasses sent to him for preservation, and in Ontario, as many as 1,000 specimens were preserved during the flight of 1901-02.

The flight of 1926-27 was a record, with 2,363 sightings in the U.S. Owls were reported from the Atlantic to North Dakota and as far south as North Carolina.

These periodic invasions by snowy owls are directly tied to the population cycles of the arctic's most common rodents, the lemmings. Lemmings--there are several species --are plump, stubby-tailed animals about an inch longer than the average house mouse. Like the owls, lemmings are circumpolar animals. They can be found on the northern tundra in both the eastern and western hemispheres.

Lemmings can produce as many as three litters a year with as many as six offspring in each litter. Apparently-, predation and the other vicissitudes of life in the wild are not enough to offset this high reproductive potential, because lemming populations can build quite rapidly to a density of as many as 35 animals per acre.

At this point, things fall apart for the lemmings. Disease, food shortages, and other stresses kill most of them. Some of them try to move to less crowded land in migrations that may involve thousands of lemmings. Sometimes these would-be migrants try to swim across open water to reach new land, and many of them die in the attempt. This is the source of the legend about lemmings committing mass suicide by leaping into the sea.

Lemmings are a major food source for snowy owls, and for other northern predators --such as the arctic fox --as well. During a period when lemming numbers are increasing, life is good for the owls. They respond by producing more babies. In really lean years, the adult owls may not even try to nest. In ordinary years, they will nest but only lay two or three eggs. But when the lemmings are at their peak of abundance, the owls may lay ten or more eggs, and a substantial percentage of those will survive to adulthood.

So the snowy owl population is cyclic too, but its cycle lags behind the lemming cycle. Owl numbers reach their maximum just after the lemming population. has crashed. Suddenly, there are all these owls and nothing for them to eat. So they come south."

Linked population cycles involving predators and prey are common in the north. In the woods, the lynx and the snowshoe hare are similarly yoked. And doubtless the arctic fox rises and falls with the lemmings too. These mammals are not as mobile as the owls, so they mainly stay where they are and starve to death.

Owls have another method of dealing with food shortages. It is called Cainism, after the fratricidal son of Adam and Eve. Most birds don't start incubating their eggs until they have laid a full clutch. This ensures that all the young will hatch at about the same time. But owls, along with hawks, eagles, and some other birds, start incubating as soon as they lay their first egg. Subsequent eggs may be laid at intervals of a day or more, in good times when the female lays a large clutch, the oldest bird in the nest may hatch as much as two weeks before the youngest. If food becomes scarce, the older birds-- bigger, louder, and more able to attract their mother's attention --are likely to get most of it. And, if times are really hard, they will eat their younger siblings. The situation would make a perfect image for Gary Larson's Far Side cartoon. Imagine an exasperated mother owl telling her firstborn "No, you can't have any more lemmings. There aren't any more lemmings. Now shut up and eat your little brother."

Lemming cycles are not synchronized throughout the north. The lemmings of the Ungava Peninsula may be thriving just as the lemmings of Baffin Island are going down the tubes. Which means that the Baffin Island owls may come south in one year and the Ungava owls in another. The really big owl invasions may come in years when lemmings are buying the farm across a considerable expanse of tundra.

Cycles like these are a major reason why we should not drill for oil in the Arctic National Wildlife Refuge 'as the Reagan administration would like to do. The tundra is not a steady-state sort of place. It is a mosaic of overlapping, boom-and-bust cycles. The animals need an enormous amount of unbroken space so that they can move when times get hard and so that a bust in one place can be balanced by a boom, in another place.

The lakefront is the place to look if you want to see a snowy owl this winter. In addition to Meigs Field, birds have been seen at Montrose Harbor and on the far north side at Pratt Avenue and the lake. They are often active during the day, but they are quite wary, so you can't expect to get too close to them.

I saw my first snowy owl several years ago at Montrose on a gloomy day in early February. It was on the ground, well camouflaged in the old gray snow, and I didn't notice it until it reacted to my approach by flying off and perching on top of a light pole a few hundred yards away. I saw it three more times that winter, once perched on the light tower on the fishing pier at the east end of the beach, and twice sitting on a star float in the harbor. Rats are common around the harbor. They hide out in the old tires fastened to the harbor wall to serve as I bumpers for the boats, and they were probably the owl's main food source.

You do not need to be a skilled birder to identify a snowy owl. It is one of the largest and heaviest owls, with a wingspan of five feet and a length of two feet or more. The males weigh about three pounds, the larger females up to four pounds. Their coloring varies from almost pure white to white heavily barred and mottled with dark brown. The males are generally whiter than the females, but this difference is not absolute. There is a continuum from dark to light birds, and you can't tell the sex of birds that fall somewhere in the middle. However, pure white birds are almost certainly males; dark birds are likely to be female.

We have other big white birds along the lakefront in winter, but gulls look nothing like snowy owls. Owls have broad wings; gulls have narrow, pointed wings. Gulls are slim bodied; owls are heavy bodied and big headed. To confuse the two would be like mixing up Willie Gault and William Perry.

The snowies will be with us until late March. Over the years, ornithologists have debated the question of whether irruptive birds ever make it back to their homes in the north. Certainly many of them die during their long flights even when people aren't, stuffing them and setting them up on the mantelpiece. But we now have banding records that show not only that some get back but that they return to familiar wintering grounds in later years. Some of the snowies we are seeing this year may have been here before.

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