Snowy owl

The snowy owl (Bubo scandiacus) measures up to 70 centimetres (28 inches) in length with a wingspan of up to 150 centimetres (28 inches). This makes it one of the largest owl species. It is also one of the heaviest, weighing in at up to 6 kilos (13 pounds).

As the name suggests, the plumage is largely white – almost entirely so in adult males but with black specks and spots in females and juveniles. The beak is black and the eyes yellow. The feet have large talons covered in feathers.

Snowy owls entered the popular imagination when J K Rowling introduced one as “Hedwig” in the Harry Potter books. The owl is a messenger in these stories – taking letters and packets between her owner and other members of the magical world – and always has an air of mystery about her. That is not inappropriate, given that snowy owls have long presented questions to naturalists that have only recently been answered.

The problem with the snowy owl is that it is a native of the far north – in Eurasia and North America – but those regions are almost inaccessible during the long winter months when there is extensive snow cover and the sun never rises. It has therefore only been possible to study snowy owls during the summer, so what do they do during the winter? What is their food source? How do they survive?

It is known that snowy owls sometimes migrate southwards in what are known as “irruptions”, when large numbers can appear in winter at latitudes far to the south of their usual range. This coincides with the unavailability of their main food source, namely the lemming, a small mouse-like rodent that regularly goes through huge population fluctuations. When lemmings are plentiful, snowy owls can raise up seven or eight young in a breeding season; however, a poor summer for lemmings can mean that snowy owls do not breed at all.

Snowy owls therefore tend to follow the lemmings, especially in autumn when snow cover in the north makes hunting progressively more difficult. Snowy owls do not exclusively eat lemmings, so other prey is sought when there are no lemmings to be had. This is why some irruptions take snowy owls much further south than lemming territory.

In the winter of 2013/14 there was a particularly notable irruption in North America when large numbers of snowy owls ventured south, some even being seen in Georgia, Florida and Bermuda. This provided a rare opportunity to study snowy owl behaviour by using technology, in the former of tracking devices and transmitters fitted to captured and released birds, to monitor their every move.

One assumption that was soon overturned was that snowy owls, unlike other owl species, were diurnal – i.e. that they hunted during the day rather than the night. Given the choice, snowy owls prefer to hunt under cover of darkness. The reason why they had always been seen hunting in daylight in the high Arctic was that summer in that region is always day time – the sun does not set so there is no night to hunt in.

Another misconception was that irruptions are dictated by hunger and that snowy owls swarm south because of starvation. Observations in 2013 showed that the birds were well fed and also that they were mainly younger specimens. What happens in a good breeding season, with plentiful food to hand, is that many offspring survive and they are not encouraged to hang around by their parents. As winter approaches, the older birds protect their hunting grounds and the younger ones must move elsewhere. A large irruption may therefore be the result of too much food being around, not too little!

A major surprise was the discovery that snowy owls will hunt other birds and not just small mammals. Some were found to prey on ducks and other waterbirds, especially in the Great Lakes region. Those large talons are used to tackle prey that is even larger that the snowy owls themselves, such as snow geese.

The study prompted by the 2013/14 irruption therefore showed the snowy owl to be a highly adaptable bird that is well equipped to survive in changing circumstances. J K Rowling’s choice of the snowy owl as her model for Hedwig – a wise bird who knows what’s what – would seem to have been an inspired one!

© John Welford