Winter's nesting crossbills

Posted: Friday, March 11, 2011

Once again, this time in late February, the Auke Nu trail was in top shape with firm snow all the way to the cabin. As we sashayed along, I heard some soft and gentle warbling notes high in the canopy. I finally spotted some movement under a clump of moss that seemed to be growing on top of a witch’s broom (a parasite that distorts branch growth).

Photo by Monica O'Keefe
Photo by Monica O'Keefe

Underneath that clump of moss were three female red crossbills, busily selecting short twigs. They seemed to be the source of the songs I heard, although female crossbills don’t sing as much as males do. There is no indication in the literature that females sing a little song when gathering nest material. (But I’m tempted to suggest “Here we go gathering twigs and moss, twigs and moss, twigs and moss…”) I suppose it’s possible that some males were hanging around in the dense hemlock foliage nearby and providing the sound effects.

Crossbills often nest in loose clusters, several nests fairly close together. Females build the nests, although males may sometimes carry nest material. They can nest at virtually any time of year, even in the dead of winter. Nesting is initiated when females have enough food to develop eggs and can see that there’s lots of food still around for raising chicks. So when there is a big cone crop, as there is this year, they gear up for making babies.

Crossbills are nomadic, moving around from region to region, in search of good cone crops. If they find a really good one, they may stay and rear two or three, even four, broods in one place. If not, they move on and rear a series of broods in different areas. Breeding is so tightly linked to food supply that young females only six months old can sometimes breed in a good cone year.

Even when the cone crop is good, things can still go awry. Crossbills are specialized to pry open cones and extract the seeds. They are most efficient at foraging from conifer cones and less efficient at eating other kinds of seeds. But if the air is dry for a period of time, spruce and hemlock cones may open spontaneously and release the seeds to fly away on the wind. Dispersal of the seeds clearly reduces the availability of seed-laden cones for crossbills, but it allows the seeds a chance to escape seed predation by these birds and perhaps produce some seedlings.

As we walked along the trail, we noticed very localized patches where the ground was covered with tiny hemlock twigs and empty cones. We surmised that these patches were under trees where crossbills had been foraging. Crossbills often tear a cone off the branch, hold it in one foot, and pry out the seeds, letting the empty cone drop.

In addition to several groups of red crossbills, I heard what sounded like swarms of white-winged crossbills as we went up the hill (although one of Juneau’s ace birders says that one white-wing can sound like a whole bunch…). White-wings are probably nesting now too. Like the red crossbill, they are nomadic, can nest at any season if there’s a good cone crop and often nest in clusters. Their breeding range goes as far north as the treeline, but overlaps that of the red crossbill all across southern Canada and into the Rockies of the northern U.S.

Both species irrupt southward when cone crops fail in the north. Young crossbills and females are subordinate to older birds and males are the first to feel the pinch of hunger. They often move well ahead of the dominants, leaving an area earlier and sometimes going farther south, almost to the Gulf coast. They seldom breed in the southern regions, however, and move north again in search of good cone crops.

• Mary F. Willson is a retired professor of ecology.



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