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This winter (2013-2014) was one I will never forget - the snowiest winter on record. That’s right, more snowy owls were seen in Massachusetts (my home state) than ever before.
Snowy owls breed in the arctic tundra where they feed primarily on lemmings. When the lemming population is high, the Owls will stock their nest, and lay many eggs. One Owl nest in Barrow, Alaska, contained 53 lemmings. The female laid 9 eggs that year. Other years when the lemmings are scarce, the owls may not lay any eggs. So, while snowy owls will take a great variety of prey, lemming numbers are clearly important in their breeding success.
During the winter, the Owls migrate south, inhabiting much of Canada and the very northern parts of the US, including northern Maine. However, every year, a few make it down into Massachusetts, showing up in places like Logan Airport and Duxbury Beach - two areas with wide expanses of open space.
Every four years or so, there is an “eruption” of Owls, with large numbers being seen throughout New England, and in fact in much of the country. This winter was just such an eruption.
Norman Smith, the Director of Trailside Museum in Milton, MA, has been banding ￼snowy owls for over 30 years. Usually he averages 8-12 Owls a year, with a high of 53 birds in 2011. This year, he banded an incredible 176 Owls! This year, the Owls were seen not only in New England, but throughout much of the country, with birds being spotted as far south as Bermuda, Florida and Louisiana.
In the early 1900‘s, noted ornithologist Alfred O. Gross studied these eruptions, and wrote a number of papers detailing his findings. What he noticed was that the Owl’s eruptions coincided with summers of high lemming numbers followed by a rapid population decline, the famous “lemming crash”. Typically, lemming numbers increase over a period of three to five years, after which their numbers crash, only to begin the process again.
According to Dr. Gross,
The Snowy Owl fits into the picture somewhat as follows. The owl lays from six to eight eggs, and some nests have been found to contain as many as thirteen. When the eggs hatch, it requires a tremendous amount of food to feed such a large family. Ordinarily the mortality is great, with-only two or three of the owlets reaching maturity. If the lemmings are scarce, the owls may not even attempt to nest. In a good lemming year, practically all of the large families survive, and thus the Snowy Owl population is built up to a peak along with that of the lemming. The same might be said of the other predators. Then with abruptness the 'crash' comes. *
Dr. Gross goes on to say,
If the peak of abundance of lemmings is followed by a sharp decline, then the Owls are forced to wander long distances to the southward to secure food for their maintenance.**
And so the mystery of why the snowy owl eruptions occur every four years or so was solved. Unable to find enough food, they must travel south to exploit new feeding grounds or starve. This reasoning makes so much sense that no one thought to question it. No one, until a 10 year old girl became interested in the Owls.
Danielle Smith was 10 years old when she started to accompany her farther Norman on his Owl banding excursions. She knew that snowy owl eruptions occurred after the lemming crash. But rather than just go along with the accepted belief that they migrated south because they were starving and must find a new food supply - a belief held by everyone because it is so obvious - she asked her father a question. She wanted to know how it was possible that starving birds can travel the 1,500 to 3,000 miles from their breeding grounds in the tundra to Norman’s traps in Massachusetts? Shouldn’t they die in the arctic or along the way?
What Danielle had done is what every good scientist should do - question. Question ￼everything. When a scientist makes a discovery, the most important thing that can happen is that the discovery is published in a journal. The reason for this is not just to inform other scientists of the discovery, but to allow them to question the findings. Only by questioning and looking at the problem in new and different ways with a fresh set of eyes and minds can we be sure that the discovery is as accurate as possible. A good scientist never blindly accepts previous studies, but always questions.
Danielle’s questioning caused Norman to take a new look at the data and take a fresh view. Here is what the data said:
So the question now becomes does this data suggest another reason for the cyclic southern migration of snowy owls following the crash of the lemming population in their breeding grounds? Take a few minutes to think this through. Using this new data, would you still come to the conclusion made by Dr. Gross that a sharp decline in lemming numbers forces the owls to wander long distances to the southward to secure their food? Would you modify this statement in any way? Or would you come to a different conclusion altogether?
When you have given this some thought, read on to see what this data suggested to Norman.
Like many others, Norman accepted the rational that snowy owls move south after the lemming crash because they were starving. Danielle’s question caused him to stop and take a fresh look at the problem, applying common sense rather than blind faith on a long-held belief.
If the owls were starving, why did they arrive in Massachusetts in good health, apparently well nourished? It is possible that they had fed along the way and gained back their weight on the trip south.
But why was it mostly the hatch-year birds that were showing up? If food was a problem, then these young, less experienced hunters should have been the first to succumb to starvation, resulting in more adult birds in the eruption.
Banding records across North America showed that the smaller Owls migrated further south. In territorial raptors like the snowy owl, the larger birds - the adult females - will chase the smaller birds away from prime territory - the smaller the bird, the farther it is displaced.
During the winter, the Owls leave the tundra and migrate south. If this is so, why should a crash of lemmings in the tundra affect the Owls in more southern parts of Canada and northern US?
All of this data put together - owls leaving the breeding grounds after mating season, birds migrating south showing no sign of malnutrition, most of the birds traveling south are immature, and the smaller birds traveling the furthest south - suggests that the snowy owl eruptions after a lemming crash is not because of the lack of food. Rather it suggests that owl eruptions occur after a really good lemming summer and not becasuse of a lemming crash. Many lemmings indicate many snowy owls being born. Typically in birds of prey, young birds, after they have left their parents care, are forced out of the territory by the adults and must seek new hunting grounds. With so many young owls surviving because of the abundance of lemmings, they get forced south into the US. The biggest birds, the adult females, force the smaller adult males south. These, in turn, force the still smaller young females and young males even further south, into New England and much of the US.
So it appears that the snowy owl eruptions may be caused by breeding success. With so many young Owls being born, they are forced further south by the larger, more dominant adult birds.
One final word from Dr. Gross:
The overall picture is still hazy and there remains much more work to be done in the Arctic before it becomes crystal clear.*
Although Dr. Gross wrote these words in reference to the reason for the lemming crash, they are equally true of the snowy owl eruptions we experience periodically.
*Cyclic Invasions of the Snowy Owl and the Migration of 1945-1946
The Auk, 1947, p. 584-601
** Snowy Owl Migration 1930 - 1931
The Auk, 1931, p. 501-511