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FLIGHT LINES with AUDIO: Nomadic bird making a local showing

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Irruptions (bird invasions) of red crossbills can sometimes force the conifer seed specialist into visiting bird feeders for supplemental food. Photo by Keith Corliss. 2 / 3
While usually seen atop conifer trees, red crossbills will forage on the ground occasionally. Photo by Keith Corliss3 / 3

Jonathan Weiner's book, The Beak of the Finch, won the Pulitzer Prize in 1995 for general non-fiction.

It wasn't like I avoided it all these years; there were just too many other reading projects that always seemed to get in the way I guess. I finally picked it up and finished it.

While focused largely on one tireless couple—Peter and Rosemary Grant—continuing their decades-long data collecting on the finches of the Galapagos Islands, Weiner deftly weaves instances of other relevant research illustrating just how rapid and ever-shifting evolution is into a coherent and easily-read volume.

He touches on such disparate topics as pesticide resistance in mosquitoes, the overuse of antibiotics, and how the HIV-1 virus remains such a moving target, yet always returns to the Grants and their finches. Darwin's finches of the Galapagos naturally garner much attention in the evolution world since he collected and wrote of them. But included in Weiner's book is the frequent mention of a species of bird some of us are somewhat familiar with, the curious and highly variable red crossbill (Loxia curvirostra).

Making this relevant at the moment is the irruption taking place in our area. It started as a trickle in late June and early July with regional reports of crossbills. Hardly a day goes by any more without someone seeing or hearing small flocks of them.

This is a highly nomadic species that tends to wander great distances in search of its only food source: conifer seeds. The birds can and will nest at any time of the year once a suitable crop of cones is located.

Because tree cone production is so variable, the bird must necessarily relocate regularly to find adequate food. For those wishing for a clear cut description of a single species, there is bad news. As many as 10 different "types" of red crossbill have been recognized in North America, each with its own bill size and structure but, more importantly, its own slightly different voice. To help separate birds, call notes are viewed more or less like fingerprints: unique to species. And all 10 calls are uniquely different.

Similarly, the varying bill structure directly corresponds with the different types' preferred trees. One type is mostly Douglas fir dependant, one targets hemlocks. Some crossbills prefer pines, while others spruce and so on. Oh, and let's not forget Europe and Asia where even more types are found.

It's the peculiar twisted look of their bills that give them both their common name and Latin one:

recurvirostra ("curved bill"). It looks more like a deformity at first but once one observes how efficiently cones are pried open and seeds devoured, a healthy respect follows.

Genetic research suggests the red crossbill began to separate from a common ancestor about 5 million years ago. Imagine a single bird hatching with an ever so slightly twisted bill and the advantage it had.

Weiner writes, "The press of competition in the woods would have made the novelty of a crossed beak more and more desirable, because it would allow its possessor to eat food no one else could eat; the same competitive pressure would favor each new twist. New worlds kept

opening around the birds: pine cones, spruce cones, hemlock cones, fir cones."

From that ancestor, something called "adaptive radiation" has taken place giving us what some experts think are as many as 25 separate species worldwide, each with a unique ability to acquire food from a source no other bird can access: closed conifer cones. It is believed a crash in the Rocky Mountain cone crop is at least partially responsible for this most current irruption. The reports have been of Types 2, 3, and 4, all western specialists. With some practice they can be differentiated by ear (a useful link to hear the different call types is:

ebird.org/content/ebird/news/recrtype).

You need not travel to the Galapagos to see Darwin's finches nor witness evolution in the making (he actually viewed crossbills with great curiosity noting the large variations in their bill "length, curvature, and degree of elongation of the lower mandible"). Simply listen and look for red crossbills when outside, then try to determine just which type you have. Whichever one it is, it will likely be declared a new species one day soon.

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