Nature: Crossbills an ever-evolving species

Red crossbills (female above, male below) have primarily stayed north of Ohio this year

Columbus Dispatch

February 18, 2018

NATURE
Jim McCormac

For birders, one of the most exciting avian winter events is an irruption of winter finches. Not eruption, as in a blowing volcano. Irruption refers to a mass migration of birds to a new region, usually because of food shortages.

Winter finch irruptives include northern species such as common redpoll, evening grosbeak, pine siskin and purple finch. Perhaps most exciting, though, are the crossbills.

There are two eastern species, the red crossbill and white-winged crossbill. This winter showed promise for the former, and there have been a number of Ohio sightings. However, the majority of red crossbills stayed north of Ohio.

Crossbills are boreal breeders, nesting in the great swath of coniferous forest that stretches across Canada and the northern U.S. Their range also extends along mountain ranges in the east and west, where pine or spruce forests occur.

These scissor-billed finches are uniquely adapted to harvest seeds from conifer cones. A crossbill’s mandible tips are elongated and curve strongly in opposite directions, as if the bird suffers a deformity. The bill’s odd shape is a perfect adaptation for popping open tight cone scales so that the bird can access the seed within.

A crossbill in a feeding frenzy can harvest seed at a remarkable rate. It will deftly grasp the cone with its feet, parrotlike, while forcing the cone scales apart. A long barbed tongue flicks the seed out in the blink of an eye. An ambitious bird might harvest 2,000 of the nutrient-rich seeds daily.

I’ve made two trips to Algonquin Provincial Park in Ontario, Canada, this winter. The abundant white spruce in that region are experiencing a boom year for cone production, and crossbills are everywhere. Even though it’s midwinter, the crossbills are showing signs of nesting. Breeding can occur at almost any time of year if food sources are plentiful. The tough finches are unaffected by brutal cold. Winter temperatures in Algonquin regularly plunge far below zero.

Crossbills are highly nomadic, a lifestyle forced by the irregular production of cone crops. Most conifers — spruce, pine, hemlock, etc. — produce bumper crops of cones only every other year, or at even longer intervals. Thus, what might be excellent habitat one year could be nearly lacking in food the next.

On a geological time scale, boreal forest habitats in their current distribution are modern. The last glacial advance retreated from Ohio only about 10,000 years ago. Up until a few thousand years ago, spruce, fir, pine and other conifers covered much of the state. Now one must travel a few hundred miles north to find this habitat commonly.

Red crossbills seem to be in a state of active evolution, still changing with the shifting coniferous forests following glaciation. Ten “types” have been delineated, and one of these, the western Cassia crossbill, was described as a distinct species last year.

The various crossbill types differ in bill shape and size, and are adapted for feeding on different cone sizes and shapes. Our ability to tell them apart is evolving, and identification rests largely on differences in call notes.

In a way, the red crossbills are the Darwin finches of the north, actively evolving to exploit shifting niches. Someday, ornithologists might split the red crossbill complex into 10 species.

Naturalist Jim McCormac writes a column for The Dispatch on the first, third and fifth Sundays of the month. He also writes about nature at www.jimmccormac.blogspot.com.