Clark’s nutcrackers face lean, hard times

By Jeff Mitton

I am somewhat pessimistic about the welfare of Clark's nutcrackers in the next several decades.

The prospect of early fall colors lured me to the Bierstadt Trail in Rocky Mountain National Park. It was still too early for fall colors, but it was a glorious day in the mountains. I was descending the trail when the racket from a pair of Clark's nutcrackers grabbed my attention.

They were harvesting seeds from a limber pine beside the trail, 40 feet in front of me. Limber pine cones are not pendant, as they are in ponderosa pines, but are held horizontally to serve as landing platforms for nutcrackers. The nutcrackers would land on a cone, thrust their bill between pine scales and extract a seed.

This was fascinating to watch, for it seemed like they simply bent down, stabbed the cone and straightened up with a seed—it seemed to take only a second. Then a quick motion to maneuver the seed into their sublingual (beneath the tongue) pouch, and they were probing the cone again.

The birds are acrobatic and animated. While standing on a cone, they could reach down and extract a seed from the bottom of the cone.

It seemed that they would only take two, three or four seeds from a cone, then lift up into the air to settle on another cone. Several times they would lift off, fly off 200 feet, circle back and land on the same tree. Through all of this activity they called raucously. They seemed to be having a good time.

One flew off to the southeast, dropping lower into the valley, and the other followed a few seconds later. I tried to watch them, for they were surely off to cache their seeds, and I would have liked to have known how far they carried the seeds and where they were caching that day. But as they dropped lower, they were hidden by trees.

Field observations suggest that each nutcracker caches 30,000 limber pine seeds in late summer and fall and then recovers about 15,000 of them to get through the winter. Unrecovered seeds germinate and grow. The nutcracker effectively charges the tree one seed for the service of planting one seed.

Around the turn of the last century, foresters replanting clear cuts had need for an enormous number of seedlings. They found it economical and expedient to pay for seedlings to be raised in Europe, shipped to Halifax and Vancouver, delivered by train and then wagon to reforestation sites. Regrettably, the seedlings carried a fungus, white pine blister rust, that would decimate five needle pine forests a century later.

White pine blister rust spread south from Vancouver to infect the grand sugar pines in California and spread east to infect the whitebark and limber pines in Glacier National Park. Clark’s nutcrackers rely on those seeds to get through the winter, and the majority of both species are either dead or infected with blister rust. The nutcrackers are losing their winter food, so local populations will shrink, emigrate or go extinct.

White pine blister rust invaded southern Wyoming and northern Colorado in the time that I have been at the University of Colorado Boulder. More recently, the rust was found on limber and bristlecone pines on Mosca and Medano Passes in the Sangre de Cristo mountains.

In the last 10 years, the mountain pine beetle has hit limber pine particularly hard. Scott Ferrenberg and I have both noticed that, when they had the choice, beetles preferred limber pines over lodgepole pines.

Clark’s nutcrackers also harvest and cache cones from pinyon pines on the Colorado Plateau and singleleaf pinyon in the Great Basin. But the combination of a sustained drought and record hot spell produced the greatest bout of mortality ever seen in pinyon. It will not be the last.

The mutualisms between Clark’s nutcrackers and limber pines, whitebark pine, pinyons and singleleaf pinyons are fascinating examples of coevolution and the birds are a delight to watch. But because their hosts are dwindling, I fear that some populations of nutcrackers will be lost and others will shrink.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Sept. 12, 2014