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The group of birds called crossbills are a most interesting group of finches. The top and bottom halves of the beak do, in fact, cross. I did an earlier page on crossbills but the taxonomy is so interesting I thought I’d add to the confusion.
There are many populations oof crossbills some called subspecies and several groups that appear to be separate and real species. The definition of species is very fluid; if they can be separated by appearance that is a species according to many people. If they interbreed they are not separate species is another. If the populations do not overlap is another of the way to define species. Whatever a species is, the crossbills mess with your thinking.
The boreal forest is very widely spread. Coniferous trees and vegetation is even more widely spread. The crossed bill of these birds is used to open unopened cones to get at the seeds inside each cone; under the cone scales.
As there are forests and trees of this sort all around the world in the northern hemisphere there are birds that have adapted to survive in such a place; the most successful group are the finches and the most successful of those seem to be the crossbills. The confusion regarding speciation probably centers around group adaptations to differing tree populations and the necessary changes needed/evolved/developed to live within that specific crop. In North America, Canada and the US, there are about nine populations that seem to exist without interbreeding (at least for the most part). One group has already be determined to be a separate species, the Cassia Crossbill found in Idaho mostly. Most of the descriptions of these North American populations is based on the calls the birds make. Genetic studies have shown little to no genetic variation in NA birds – but something going on for sure.
Because of the nutrition available in the cone seeds crossbills can nest anytime, as long as there are unopened cones available. Most birds have to wait for insects to hatch or larva to emerge or flies to fly in order to secure enough protein rich food for their young. But crossbills can nest in the coldest weather utilizing the seeds of the evergreens. Evergreens will have bountiful years and less bountiful years – this impacts the breeding and breeding success of the various crossbill groups.