crossbills: which way to turn?

December 20, 2005

in Birds,Natural history

The Auk is the journal of the American Ornithologists’€™ Union, and it publishes some pretty dense material.  The last few years, it has featured an increasing number of papers on phylogeny, taxonomy, and genetics.  This stuff tends to sail right over my head like a frigatebird on a stiff wind.

A genetics paper by Edelaar et al. in the recent issue caught my eye because it addressed a very interesting phenomena: the direction in which the bills of crossbills cross.

Red_cross_woodruff_1Crossbills (Loxia spp.) are finches of coniferous forests, of which two of the five species are found in North America: the Red Crossbill (L. curvirostra, shown here in this great photo by Michael Woodruff) and the White-winged Crossbill (L. leucoptera).  The tip of the lower mandible in crossbills crosses the upper; no other birds have naturally-occurring crossed mandibles.  This configuration is an adaptation that helps the birds pry open partially closed conifer cones to extract the seeds, their primary food.

The lower mandible may cross either to the left or right of the upper, and the two forms occur at an approximate 1:1 ratio in most species.  Below, we see Red Crossbills in which the top bird is a “lefty” and the bottom is a “righty.”  (The bill sizes are different, too, indicating that these individuals probably represent two of the eight or so different races of Red Crossbill, some of which may one day be considered distinct and separate species. Bill sizes are correlated with the type of conifer each race favors, e.g., larger bills being optimal for larger cones.)
Crossbills usually feed on cones that are still attached to trees. The tip of the lower mandible has to be pointed towards the axis of the cone in order for the scales to be effectively pried open. Unless the bird can move all around the cone (or remove it), it is limited to consuming seeds from one side of the cone.

The prevailing theory on how the 1:1 ratio is maintained was presented by Benkman (1996) and is summarized by Edelaar et al.:

“Birds that visit cones previously foraged on by birds with the same mandible crossing direction would…suffer from reduced food intake because of resource depletion,”€ they explain.  Ergo, lower survivorship for those birds, a situation favoring birds with the other (rarer) mandible crossing direction.  This give and take results in a frequency-dependent selection that is stable at a 1:1 ratio.

This whole scenario hangs on the assumption that bill crossing direction is heritable.  This has not really been tested.  Observing nests in the wild to record bill directions of parents and chicks is labor intensive.  And while most songbirds appear monogamous, many species actually engage in “extra-pair copulations”€ (they cheat with neighbors), so the parents of all the chicks in a nest may not be the birds that rear them.

The Edelaar et al. study examined the offspring of 31 pairs of captive crossbills of four species, kept away from their horny neighbors.  They were unable to find evidence of heritability in bill crossing direction.  Alas, however, they noted that their sample size was too small to be statistically powerful, and that nearly every pair was made up of one “lefty” and one “righty.” The authors noted that, on average, the expected bill crossing direction ratio of the offspring of these pairs would be 1:1, unless there was a strong sex linkage in heritability of bill crossing direction.

It’s difficult to imagine how the crossing direction is not at least partially genetically determined. While crossbills have straight bills for about two weeks after hatching, during this period they are still being fed by the parents and the chicks are not exposed to cones, so it is not an adaption to local resources. If the direction is random, deviations from the 1:1 ratio, seen in some populations of crossbills, would be hard to explain. What am I saying?  This is hard for me to explain! Geneticists and evo-devo types are welcome to contribute clarifying thoughts and elucidations in the comments.

Crossed bills in crossbills are an obvious physical trait.  As it turns out, there are also interesting behaviorial dichotomies in the avian world, including “footedness” in parrots and sparrows that favor one eye.  Topics for a future post!

Benkman, C. W. 1996.  Are the ratios of bill crossing morphs in crossbills the result of frequency-dependent selection?  Evolutionary Ecology 10:119-126.

Edelaar, P., E. Postma, P. Knops, and R. Phillips.  2005.  No support for genetic basis of mandible crossing
direction in crossbills (Loxia spp.). Auk 122:1123-1129.

Groth, J. G. 1992.  Further information on the genetics of bill crossing in crossbills. Auk 109:383-385.


Wisecrow December 21, 2005 at 5:04 pm

Well, I read 2 of the 3 papers, and all I have to show for it is a headache… Good questions. My only thought that I didn't see mentioned is that some species of Pine (and I don't know if this is relevant here) have clusters of cones, each oriented in different directions. I'm not sure what this would mean in the face of all the genetics, but it would definitely factor into the resource availability. Otherwise, combine all the discussions in the two papers and it looks like every possible explanation was speculated on, nothing was rejected, and anything is possible, including stuff that isn't know yet. Don't you just love science… I should have been a circus clown.

Aydin December 23, 2005 at 6:00 pm

Did they look at only the 1st generation offspring? In snails the direction of shell coiling in the offspring is dictated by the genotype of their mother, but the expression of the gene is delayed by a generation. So, if one looked at only the 1st generation offspring, it would look as if the direction of coiling wasn't genetically determined.

Wisecrow December 25, 2005 at 11:17 am

Reading Aydin's comments, I'm wondering why delaying the expression of a gene for a generation for something like this would be selected for? Are there examples of this in vertebrates?


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