This time of year, snowy owls in the High Arctic are starting to ease into the final phase of parenting as their chicks gain mobility and learn the ropes of apex-predatorhood up on the tundra.

A few years ago, a viral photograph taken by Christine Blais-Soucy in the northern reaches of Quebec's Ungava Peninsula showed just how good living can be for families of these great white owls during one of the periodic population booms of lemmings, generally the birds' most important food in the breeding season. Blais-Soucy's snapshot showed a snowy-owl nest wreathed with a whopping 70-odd lemming carcasses! 

Denver Holt of the Owl Research Institute, who has conducted long-term research on snowy owls around Barrow, Alaska since the early 1990s, has seen such a sight quite a few times (though with brown lemmings substituting for the Ungava's collared lemmings). "During a good lemming year we see several nests with 20 to 25-plus lemmings on every visit," he says. "The most I have recorded at one nest on one visit was 72 lemmings."

Lemming time and the living is easy. Image: Denver Holt

Such a prodigious larder, Holt says, reflects "a good male, a good hunter". Snowy-owl pairs raise their offspring with a fairly clear-cut division of labour: the smaller male goes out hunting while the larger female incubates eggs and broods chicks.

Male owls advertise their hunting prowess from the get-go: they often tote dead lemmings during the ritualised flight displays they perform to attract mates.

A baby owl ensconced in lemmings. Image: Dan Cox

Males are also the primary nest-defenders, a role that sometimes sees them actually physically striking potential threats such as Arctic foxes (or owl researchers, for that matter: Holt has been hit "lots of times").

Some scientists speculate the sexual dimorphism in snowy owls derives from the sexes' different roles during the nesting season. The female's greater size might boost her ability to insulate eggs and shelter nestlings, while a smaller body makes the male owl more manoeuvrable – a payoff in both hunting and harassing potential nest-predators. When dive-bombing such a formidable creature as an Arctic wolf, after all, you want manoeuvrability:

Owls breed asynchronously, which means nests see eggs hatching every few days, and nestlings cover a range of ages. Holt says a given chick is usually nest-bound for about three weeks, then leaves to spend several more scurrying about the vicinity, hiding in "nooks and crannies on the tundra" and begging for food. The male continues to drop off lemmings (or other prey) for these mobile but still completely terrestrial fledglings, as well as for the female and younger chicks still tied to the nest.

Once all the chicks have left the nest, the female owl may hang around, but her parenting role diminishes, Holt notes. The male, on the other hand, will keep feeding his offspring even after they've begun flying – the youngsters may be working out the whole airborne thing, but predatory skills take time to develop.

Bountiful lemming seasons can translate to many snowy owls showing up the following winter in southern Canada and the Lower 48 States: so-called "irruptions" of these Arctic birds, which may extend as far south as Florida. (This was indeed the case in 2013-2014, after the previous summer's lemming bonanza.)

Some suggest these southern-ranging owls, usually on the younger side of things, have been kicked out of higher-latitude winter territories by other snowies, but Holt isn't sold on that theory. And the persistent widespread notion that irrupting snowy owls are forced south by lack of food definitely doesn't hold water: most snowy owls in southern territory appear perfectly well fed, pursuing a varied wintertime diet that may include everything from waterfowl to rabbits.

Snowy owls are "irruptive migrants" that exhibit somewhat nomadic movements from year to year: they seem to seek out nesting grounds where lemmings are flush. That means southerly winter irruptions may simply represent a healthy crop of young owls – representing an above-average breeding season – that have similarly travelled until winter digs with lots of available prey are found. (The northern states of the conterminous US usually see a few overwintering snowy owls every year, though it's the occasional large-scale "invasions" that draw the most attention.)

During an irruption year, it's typical to see multiple young snowy owls hanging around together in the same winter quarters (often open landscapes such as lake shores, sea coasts and prairies that mimic Arctic barrens). "That probably has to do with migration and topography: moving along the same waterfronts and same topographic features, and instinctively moving south until they find food," Holt explains.

Many snowy owls don't move south during the winter at all. Some, in fact, go north and spend the cold, dark months on the sea ice, hunting eiders and other Arctic birds at the patches of open water where they overwinter. ("Wintering at high latitudes may be advantageous to owls by allowing them to start prospecting very early in spring for areas with high lemming densities, a prerequisite for a breeding attempt," the authors of a study documenting this on-the-ice sojourning wrote.)

Image: Dan Cox

Holt says that the next big push in the Owl Research Institute's study on Barrow-area snowy owls – the longest-running of any on the species – is investigating local declines in both owls and lemmings. "Owls are going down faster than the lemmings, but they're paralleling in a significant downward trend," he notes. Part of this research will involve assessing whether climate change has anything to do with these falling numbers.

The Barrow area hasn't seen a major, honest-to-goodness lemming boom since 2008, says Holt. Such lemming explosions don't just see owls, Arctic foxes, weasels and other rodent-eaters sitting pretty; ground-nesting birds, for instance, might benefit as well – if not for the rodent smorgasbord on hand, they might otherwise be more heavily targeted by those predators. 

Holt notes that "the lemming may be the best indicator of the health of the environment" where these rodents are found, but snowy owls – prospering directly when lemmings prosper – can serve as the flashier proxy of ecosystem health.

In that sense, Holt compares the snowy owl to that other grand Arctic hunter, the polar bear, as a similarly charismatic symbol. And now that we know snowy owls forage on the frozen Arctic Ocean in winter – weaving them into the marine food web, not just the tundra one – the bird's and the bear's vulnerabilities to climate change may directly overlap: both appear threatened by diminishing sea ice in the face of warming temperatures.

Meanwhile, to get a glimpse into the day-to-day fieldwork Holt and his team carry out up on the snowy owl's nesting grounds, check out this article from the Cornell Lab of Ornithology. And you can learn more about the owls' wide-ranging movements at the Project SNOWstorm website.



Top header image: Pixabay