The other day, I found a plastic grocery bag tied to my mailbox, with a softball-sized wad of … something inside. I admit my first thought was that someone had left me a bag of dog crap. I looked inside with some trepidation, but I discovered the sack contained something more surprising and interesting: five Red Crossbills. A friend, knowing I had a permit and would have the them made into study skins, had picked up these road-killed birds coming home from Michigan’s Upper Peninsula.
There were two females and three males. All were young birds, hatched
within the last calendar year, based on plumage characteristics, although one
female may have been older. Ageing crossbills is complicated by the fact that they can breed nearly any time of the year. In most bird species, feather replacement follows a seasonal pattern that is quite predictable. In Red Crossbills, the timing as well as the number of times a young bird replaces feathers during the first year depends upon when it was hatched.
Red Crossbills are intriguing. They are highly variable vocally and morphologically. There are eight different flight call types which roughly correspond with some of the recognized subspecies, which are in turn based on bill and body size. Although the types do not differ much genetically, there is a strong tendency for birds of the same call type to flock and mate with their own types. Bill and wing measurements verified that these birds fell neatly into "Type 3," Loxia curvirostra minor, the dainty subspecies and call type which would be expected in the UP.
The variation in bill sizes in Red Crossbills are adaptations that allow them to feed on different kinds of conifer cones. You might recall from my previous post on crossbill bills, the top mandible may cross the lower to the right or to the left. In my little sample, I had two lefts and three rights.
Crossbills, unfortunately, are frequently struck by cars. They are attracted to roads, which might be the only place in a snowy landscape where the grit they need to ingest is exposed. The birds may also be attracted to salt or other chloride-containing road de-icers. The salt itself may be fatal, but in the case of these crossbills, they were purposely struck by a car and a snowmobile. My friend said he say this behavior more than once, where cars did not slow down when approaching a large flock of birds that were easily visible. In one instance, a vehicle swerved and aimed at a group of these sociable finches that were gathered at the side of the road. Repugnant.
While of course I’d rather look at live crossbills in the trees than dead ones in my hand, when these birds are made into study skins, they will serve as teaching tools. Few people in southern Michigan see crossbills — or even know they exist. The curious bills of these birds present lessons in adaptation, the story of the various types illustrate selection and evolution. And the provenance of the little bodies provides an example of human ignorance and cruelty. Perhaps a newfound appreciation of these birds in death will result in an intolerance for such callousness. If so, it could be the most valuable lesson they have to offer.