Both red squirrels and crossbills are closely tied ecologically to conifers. Previous essays in this space have dealt with both red squirrels and crossbills, but there is still more interesting natural history involving these seed predators.
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Red squirrels are the major predator of conifer seeds in much of western North America. Cones are harvested before they open and stored in damp places to prevent later opening, which would expose the seeds to other seed-eaters, such as mice or chipmunks, that can't open cones for themselves very efficiently.
Red squirrels are selective foragers, preferring to harvest cones from certain trees. When foraging on lodgepole pine cones in the Rocky Mountains, they forage most heavily in trees that have cones with a narrow base (making them easy to detach from the branch), relatively thin basal cone scales (which don't hold many seeds but are easy to take off so the squirrel can get at the seeds toward the tip of the cone) and lots of seeds (providing lots of calories per harvested cone).
As a result, lodgepole pines, over much of their geographic range, have evolved wide-based cones, hefty basal cone scales and few seeds. The squirrels exert natural selection against trees that produce easily accessible seeds, so trees with few and better-protected seeds become more common in these populations.
On both sides of the main chain of the Rockies are small mountain ranges that are separated from the chain by a gap of nonforested habitat. In some of these isolated ranges, red squirrels are absent, and they have probably been absent for several thousand years. Here, without the selection pressure from the squirrels, the lodgepole cones are different: They have narrow bases, thin basal scales and many seeds.
There are red crossbills in these isolated ranges, and they tend to be much more abundant and less nomadic than elsewhere. Lodgepole cone crops don't vary much from year to year, in contrast to many other conifers, and the absence of squirrels leaves the cone crop on the trees, so the crossbills have a ready food supply.
Here, the crossbills are the main predator of lodgepole pine seeds, and the pines have evolved better defenses against crossbills. The scales near the tip of the cone, with most of the seeds, are thicker and harder to wedge apart by the crossed-bill tips. The cones are shorter with greater overlap of scales, which also makes them harder to open.
The crossbills are different, too. Their bills are deeper and more down-curved than those of crossbills that feed on lodgepoles elsewhere. This gives them a stronger bite, so they can open the thicker, overlapping cone scales.
In short, the crossbill-lodgepole system is quite fine-tuned. Not only is there a lodgepole crossbill, the lodgepole crossbills in some mountain ranges (minus squirrels) are adapted to the local lodgepole pines, and vice versa.
Nevertheless, this evolutionary fine-tuning can be disrupted. For example, in one isolated mountain range without red squirrels, the cones are smaller than expected if crossbills were the only major influence on cone characteristics. Here there are very high and chronic infestations of a seed-boring moths, which preferentially attack long, narrow, many-seeded cones that have thin scales with little overlap. So the pines in this area have evolved few-seeded, thick-scaled cones that are smaller, shorter and wider than expected, if crossbills were the only significant agent of natural selection on cones.
In another example, the crossbills of one mountain range have smaller bills than the most efficient size for harvesting lodgepole seeds. This crossbill population has suffered an infestation of a mite that causes lesions on the legs and feet. Infected birds may lose toes or feet, and the infection can even be lethal.
The mites attack large-billed, large-bodied males (with high levels of testosterone) more than other birds, and large males with mites have lower survival than others. As a result, the average bill size of crossbills in this population is smaller than the most efficient for seed extraction, and there is less difference in body size of males and females than in other populations. A perceptible change in bill and body size of males has occurred since 1997.
The complex scenario has some take-home lessons for naturalists. The salient lesson is that interactions between species are not the same everywhere - in fact, there is a geographic mosaic of interactions with different outcomes in different areas.
Another lesson is that evolutionary changes can happen rather quickly, in just a few generations.
Such considerations raise some ecologically interesting questions for Southeast Alaska:
1. What are the effects of red squirrels on the cones of our spruces and hemlocks?
2. Are the cones different on an island historically without red squirrels?
3. Has the introduction of squirrels to some islands affected cone characteristics?
4. Has the decreased abundance of spruce-adapted crossbills changed the evolutionary pressures on spruce cones on islands lacking squirrels?
To my knowledge, no one has studied these kinds of interactions in Southeast Alaska, but the questions feed the curiosity bumps of trail-walking naturalists.
Mary F. Willson is a retired professor of ecology and a Trail Mix board member.