Failed cone crops cause pine siskins to irrupt

By Jeff Mitton

I wandered deep into a spruce fir forest, far from a road, not far from a small lake, and sat down in the shadows to see what would happen. In just a few minutes, a party broke out -- a dinner party.

A cloud of pine siskins settled in. Not a cloud, really, just a flock of about 20 siskins. But they have a large presence; they twitter continuously as they move among the trees, keeping in contact with one another, perhaps exclaiming the discovery of spruce branches drooping with heavy cones.

They worked industriously to harvest seeds from a cone, then fluttered or hopped to the next cone. They frequently hung upside down, dangling either from a branch or the tip of a cone to reach all of the seeds in dangling cones. They fed gregariously, with two to four birds jostling one another at the tops of spruces only 25 feet tall.

Pine siskins, Spinus pinus, are small birds, about 5.5 inches long, with forked tails, slender beaks and pointed wings. Their colors are grey, brown and black, conspicuously streaked on the breast and abdomen. When they flutter from branch to branch, yellow feathers flash from the wings and tail. They occupy conifer forests from Alaska to Newfoundland and from the boreal forest to the mountains of Mexico.

From the perspective of the siskins, the problem with this dinner party was all the uninvited, ravenous guests. White-crowned sparrows and dark-eyed juncos were the most numerous and they worked at the cones, but they spent more time on the ground, probably gleaning seeds that spilled from open cones. A pair of chestnut-breasted nuthatches chiseled at cones but favored the stiffer lodgepole pine cones over the papery spruce cones.

The uninvited guest with the largest presence was a pine squirrel, cutting cones from the tops of trees and dropping them to the ground. When a cone hit the ground, juncos and sparrows looked for spilled cones. An opportunistic least chipmunk, which was thrilled by the cones falling from the sky, fibrillated with joy and delight. The chipmunk would pounce on a cone, frantically extract seeds, pack them in its cheeks and dash away.

When the pine squirrel descended to the forest floor and found the chipmunk stealing his harvest, he dashed at the thief, vanquishing it from the site. But as soon as the pine squirrel climbed to cut more cones, the chipmunk returned.

Clearly, an abundant Engelmann spruce cone crop attracts many hungry mouths. But in addition, some insects develop inside spruce cones and eat the entire seed crop before the cones mature. In other years, the vicissitudes of climate stress the trees so that cones are rare or absent.

Pine siskins do not migrate to tropical forests in fall but are resident in the pine and spruce fir forests of the Rocky Mountains, where winters can be very challenging. These little birds are very hardy and can survive temperatures far below zero, for they have the remarkable ability to ramp up their metabolic rate to five times their basal or resting metabolic rate to survive the coldest nights. But their high metabolic rate demands regular, if not constant, meals.

Pine siskins rely on seed crops, which must be shared in the best of times, and they must endure cone crop failures, which produce the worst of times. During the worst of times, when cone failures are synchronous across species and regional in scope, pine siskins must move to find food elsewhere.

Birders have noticed that in addition to wandering, pine siskin populations vary dramatically in size from year to year. Abundant cone crops allow populations to expand quickly, but cone crop failures can trigger population crashes. Irruptive migrations and population crashes clearly illustrate that during winter, pine siskins are on the edge.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.