I was especially struck by a chapter called Snow Flurries about Snowy Owl (Bubo scandiacus) behaviour and its relevance to the area in which I live, where this magnificent denizen of the arctic tundra is a regular winter visitor, sometimes in substantial numbers.
It confounds some of my friends that we are able to see this species with such ease, and I suspect it causes not a little unseemly drooling!
Conventional wisdom has often advanced the notion that movement into the south (south for the bird that is) has been triggered by a shortage of food in the arctic, especially in irruption years, when the presence of large numbers of owls has been attributed to crashes in the Brown Lemming (Lemmus sibiricus) population. See the following references:
North American Owls, Paul A. Johnsgard (1998): ....incursions......generally occur at intervals of about 3 - 5 years, coincidentally with cyclic or periodic declines in lemming populations.
Owls of the World, Edited by John A. Burton (1973): It is a decline in the availability of prey, ......that is a cause of Snowy Owl irruptions.
The Owls of North America, Allan W. Eckert (1987): Migration is extremely cyclic and predicated on the availability or nonavailability of prey.
Owls, Tony Angell (1974): The migrations are stimulated by the shortage of their principal foods, lemmings and Snowshoe Hares.
The diet of Snowy Owls, however, is varied and includes other rodents, ptarmigans, songbirds and waterfowl. In the winter in southerly locations a Snowy Owl is very opportunistic and will take whatever prey is readily available. This female is eating a Mallard (Anas platyrynchos) in Bronte Harbour, Oakville, Ontario. For birds that regularly winter at this location, waterfowl forms the principal items on the Snowy Owl menu.
Snowy Owls are nothing if not peripatetic and have a highly nomadic lifestyle. Strycker cites the example of seven chicks banded on Victoria Island in northern Canada, three of which were subsequently relocated at Attawapiskat, Ontario (2,050 km), Clyde Forks, Ontario (3,250 km) and Sakhalin, Russia (5,300 km). These movements are seemingly unrelated to seasonal food shortages and reflect the inherent behaviour of an avian nomad. In fact Snowy Owls seem to manifest little or no inclination to site fidelity at all.
To return to the question of whether precipitous declines in lemming populations are the reason for mass Snowy Owl movements to the south, it appears that exactly the opposite is true, borne out as Strycker points out, by rigorous statistical analysis of both the biomass of lemming prey and the movement of owls.
All the evidence points to Snowy Owls having a highly successful breeding season in years of lemming abundance, rearing seven or eight chicks in an average nest, a prodigious feat indeed. When these young birds fledge, there are not enough territories to go around and this is what triggers a mass migration south, and results in the spectacular invasion years we sometimes see in southern Ontario, where in an exceptional year it is possible to count twenty-five Snowy Owls in less than a forty-five minute drive from my front door.
So, it is a bonanza year for lemmings that stimulates mass migration from the tundra, not the opposite.
There is support for this assertion in the literature:
Owls of the World, Rob Hume (1997): When the population reaches a peak after a good breeding season, more owls will move south, especially young birds working hard to survive their first winter.
Owls of North America and the Caribbean, Scott Weidensaul (2015): Although most people assume that hunger drives irrupting owls south, in major Snowy Owl invasions the cause is usually extraordinary plenty, not privation.
I would like to end this short piece with a plea to my fellow birders, and especially those who in recent years are far more likely to carry a camera with a huge lens. Please, do not trespass on farmers' fields to get closer to the owl. This causes problems for others, especially those of us who have working relationships with farmers who permit us to study birds on their property, causing them to become resentful of the birding community and withdrawing our privileges. Furthermore the owls are resting between bouts of hunting, and disturbing them so that you can get the perfect flight shot causes them to expend valuable energy unnecessarily.
The same admonitions apply to birds found in harbours. Do not try to gain access to private docks where you have no business trespassing. Think of the whole picture and not just your desire for a better image.
Enjoy the bird at a respectful distance and consider yourself one of the lucky few able to spend time with this iconic bird of prey.
