An encounter with a crossbill

I said I'd get distracted. Yesterday I again found myself in the New Forest in search of - can you guess - deer and birds. We saw deer alright, but mainly it was a finch-themed day ('we' = Southampton Natural History Society). As you can see from the adjacent photo, a lot of people were out, all hoping to see one particularly bizarre finch (that's me at the front, not a bizarre finch). But I'll come back to that later....

Our first sighting was of both male and female Common (= Red) crossbill Loxia curvirostra. They clambered around, feeding and calling, at the top of a pine, their attractive plumage well illuminated by the day's low-angle, bright wintry sunlight. Male are red, females green. These fantastic finches - found right across the Northern Hemisphere - are best known for associating with Scots pine, larch and spruce; they are acrobatic feeders that remove seeds with their distinctive bill. At their tips, the jaws cross, and by biting in between the scales of cones, crossbills are able to force the scales apart to expose the seeds concealed within (Benkman 2003). Because they rely on an ephemeral food source, crossbills occasionally 'irrupt', huge numbers suddenly invading regions where they were previously rare or absent.

The several crossbill species differ in the robustness of the bill, and correspondingly in feeding behaviour and ecology. How many species are there? This is actually one of the most discussed and argued about subjects within passerine systematics, and controversy remains over whether certain populations should be regarded as distinct species, or as subspecies of others. The best example of this sort of thing is the Scottish crossbill L. scotica, the only bird species endemic to Britain... if, that is, it is a distinct species. Including the Scottish crossbill, there are four species, but in one recent monograph it was proposed that the many L. curvirostra varieties present in North America actually represent no less than seven distinct species (Groth 1993). Indeed L. curvirostra is supposed to consist of twenty or so subspecies, several of which are notably robust-billed and about as distinct as L. scotica. Other subspecies are distinct for other reasons, such as the particularly small L. c. japonica of eastern Asia (and not just Japan as the name suggests) [adjacent image, showing male L. curvirostra, from here].

As a European person, I of course associate crossbills with cool, northern places. But subspecies of L. curvirostra occur as far south as Vietnam, the Philippines and Guatemala. Crossbills are cardueline finches closely related to redpolls and goldfinches, and in fact some studies find Loxia to be nested within Carduelis (Marten & Johnson 1986, Arnaiz-Villena et al. 2001). This is a problem if we want to keep Carduelis monophyletic and retain Loxia as a genus. Arnaiz-Villena et al. (2001) found crossbills to form a clade with redpolls, and implied that the generic name previously used for redpolls (Acanthis) could be resurrected for the redpoll-crossbill clade [adjacent pic of Common redpoll C. flammea from here].

Crossbills have a good Pleistocene fossil record (Tyrberg 1991) but working out what this record means for relationships and biogeography depends on how the taxonomy is interpreted. Workers have disagreed on how old crossbills might be, with some studies suggesting an origin about 10 million years ago (during the Miocene), and others promoting a Pliocene origin (about 5 million years ago). To date the oldest fossil crossbill is the fossil species L. patevi from the late Pliocene (Boev 1999).

So, drat, another post on birds. I was planning to write only 100 words or so on crossbills before moving on to the other finches we saw in the New Forest, but I'm now having to split the planned text over more than one post. More to come...

Refs - -

Arnaiz-Villena, A., Guillen, J., Ruiz-del-Valle, V., Lowy, E., Zamora, J., Varela, P., Stefani, D. & Allende, L. M. 2001. Phylogeography of crossbills, bullfinches, grosbeaks, and rosefinches. Cellular and Molecular Life Sciences 58, 1-8.

Benkman, C. W. 2003. Divergent selection drives the adaptive radiation of crossbills. Evolution 57, 1176-1181.

Boev, Z. 1999. Earliest finds of crossbills (genus Loxia) (Aves: Fringillidae) from Varshets (NW Bulgaria). Geologica Balcanica 29, 51-57.

Groth, J. G. 1993. Evolutionary differentiation in morphology, vocalizations, and allozymes among nomadic sibling species in the North American red crossbill (Loxia curvirostris) complex. University of California Publications in Zoology 127, 1-143.

Marten, J. A. & Johnson, N. K. 1986. Genetic relationships of North American cardueline finches. The Condor 88, 409-420.

Tyrberg, T. 1991. Crossbill (genus Loxia) evolution in the West Palearctic - a look at the fossil evidence. Ornis Svecica 1, 3-10.