Of Pines and Crossbills

I would like to tell you a tale that involves three major players, the pine squirrel, crossbills, and lodgepole pines. The tale I am about to tell is an interesting one about speciation due to coevolution. Few examples in nature seem to play out so wonderfully apparent as this one.

Throughout most of the Rockies where lodgepole pines grow, the main seed predator is the pine squirrel. Crossbills are out-competed for this resource wherever the squirrels are present. Because of this, lodgepole pine cones have evolved methods to deter squirrel seed predation as much as possible. In Idaho, there are a few isolated mountain tops that have lodgepole pines but no pine squirrels. On these mountains, crossbills are the main seed predator and the pines have responded accordingly.

Crossbills are named because their bills are crisscrossed so that they can pry open the cone scales to get at the seeds. The trees in these isolated mountains have developed stouter cones with thicker scales that are harder to pry apart. The birds have responded by evolving shorter, deeper bills.

The most interesting thing about this response is the effect the new bill morphology has had on mating dynamics for the crossbills on these isolated mountains. Crossbills utilize calls to select mates. Also, crossbills prefer to flock with other crossbills that have similar calls. This is because beak morphology directly affects calls so by flocking with birds with the same call, crossbills can guarantee that the flock will find food sources that they can all utilize.

As beak morphology changes, so does the sound of the calls. It has been shown that the crossbills on these isolated mountains are showing preference for mating only with crossbills with the same bill morphology. Also, because they are the only crossbills that can utilize these lodgepole pines, they are one of the few crossbill populations that can stay in an area instead of migrating large distances to find other food sources.

These pre-mating isolation rituals are so strong on these mountains that the resident crossbills rarely mate with crossbills migrating through in search of food. In essence, the coevolution between these isolated lodgepole pine populations and the resident crossbills is driving these birds towards speciation. Whats more, this coevultion is estimated to have arisen only about 5,000 to 7,000 years ago.

On a side note, this may be a world record for how many times the word "crossbill" can find its way into a story.

Photo Credit: Perry van Munster and
nebirdsplus (http://bit.ly/1N42Ytf)

Further Reading:

http://www.jstor.org/stable/10.1086/511961

http://onlinelibrary.wiley.com/doi/10.1111/j.0014-3820.2001.tb01293.x/abstract