I have more to add to the winter colours theme of the last couple of posts, but feel like a change of pace today. I haven’t done any posts yet about birds, which is a little surprising given that birds are really my primary interest in nature (first birds, everything else second). So here’s a post on birds.
This winter, southern Ontario, and indeed most of northeastern North America, is enjoying a phenomenon called “irruption”. An irruption is similar to migration in birds, but takes place irregularly, usually every two to four years (depending on the species), rather than every year. Most irruptions are the result of food shortages in the areas where the birds usually spend their winters. Because the birds can’t find sufficient food there, they start to move south in large numbers. In many of these species, small numbers may be seen every winter, but an irruption is marked by a great abundance of the species south of its usual range. This winter seed crops, especially of deciduous trees, did very poorly in much of the north, resulting in low food availability for most seed-eating species.
The above photo is of a Common Redpoll, named for the red cap on its head, a regular irruptive species that usually comes south into southern Ontario and the northeastern states every couple of years. On their wintering grounds, redpolls feed primarily on the catkins of birch and alder trees. In a year of poor catkin production, redpolls will begin to move out of their regular range in search of an area with good food availability. In the south, this is often in the form of bird feeders. Redpolls love nyger seed (thistle seed), and will swarm nyger feeders in large numbers. They’re rarely seen in small numbers or individually, and flocks can reach 40 or 50, to upwards of 100 birds. This year is a bigger year for redpolls.
Another frequently seen irruptive species is the Pine Siskin. This year they seem to have carried on through southern Ontario to places further south, but in some years they can be just as, or often more, numerous at the feeders than the redpolls. Siskins depend on evergreen cone seeds, but are also enthusiastic visitors to nyger seed feeders. Although they’re not very flashy, they can be distinguished from some other brown, streaky finches by their sharp, narrow beak (not well seen in this photo), and the yellow tints to their wing feathers.
Pine Grosbeaks and Bohemian Waxwings have also been reported in large numbers this winter. I haven’t had a chance to go out to look for either, yet, unfortunately. The last time I saw a Pine Grosbeak was some four or five years ago, and the only ones I’ve seen in Ontario were at the University of Guelph, back when I was a student there. They’ve been reported there again this year. That gives you an idea of the frequency of their irruptions this far south. I’ve never had the luck to see a Bohemian Waxwing, although I’ve gone looking for them.
These aren’t the only species that come south in years of low food availability. Red-breasted Nuthatches are seen periodically in larger numbers, and this year they moved out early in the fall, to destinations further south. My parents have one coming to their feeder this winter, however, and they’re usually gone by mid-fall. Black-capped Chickadees are usually year-round residents on their territories, but in years of good breeding success (that is, lots of babies!) coupled with poor winter food supply, large numbers of primarily young birds will move south looking for food. Chickadees moving through in the fall was slightly elevated this year, but 2005 was the biggest movement over the last few years. Blue Jays will also irrupt in larger numbers some years than others. We had a moderate movement this year, but the best year since I’ve been keeping track was probably 2003.
Seed-eaters aren’t the only group of birds that undergo periodic irruptions. The seed shortages that cause birds to move also affect rodent populations in those areas, which depend heavily on seeds as their food source. In years of poor seed crops, rodent populations suffer sharp declines (often called “population crashes”). This year rodent populations had an especially severe crash, as last year’s seed crop had been good, encouraging a good breeding season this summer. That breeding success was followed by this fall’s seed shortage, causing a precipitous decline in numbers.
Birds that prey on rodents, such as owls, tend to follow their population cycles fairly closely. Because rodents were so abundant, owl populations, particularly the Northern Saw-whet Owl (pictured above), had a very successful breeding season. When rodent populations crashed this fall saw-whets began moving south in huge numbers. Saw-whets usually follow a four year cycle, where every fourth year their rodent prey, Red-backed Vole, peaks in number and so does their population. Saw-whets are naturally migratory and will move south every year, but the numbers encountered in the south vary according to the size of the movement. The combination of high saw-whet numbers due to this year’s breeding success and the low prey availability because of poor seed crops resulted in a larger-than-normal movement of saw-whets this fall.
Great Gray Owls follow a similar pattern, although they usually only move as far as they need to to find food, which means they don’t often make it as far south as most human communities. A bird of northern Ontario, they often just move to another part of the north when prey shortages occur, since such shortages are often regional in nature, although small numbers are usually seen as far south as cottage country every winter. A few years ago, in the winter of 2004-5, a huge movement of these beautiful northern owls occurred in southern Ontario, and I had the opportunity to get out and see several. They’re the only ones I’ve seen.
Another species of owl that comes south every year, but can move in larger numbers some years, is Snowy Owl. There’s usually one bird that winters at Tommy Thompson Park (home of the research station, and as close to a backyard as I have here in the city) every year, although I haven’t seen reports of it this year. However, in years of larger movements, such as 2005-6, many Snowy Owls can be seen in a relatively small area (of suitable habitat, of course). The photo below was taken on Amherst Island, near Kingston, where we had up to 13 individuals during one day.