Sunday, December 28, 2008

White-winged Crossbills

Yesterday was the Mohican Christmas Bird Count, and I went along for the ride. Our 15-mile diameter circle encompasses some of the most beautiful terrain in the central Ohio area, with the centerpiece being Mohican State Forest.

Many good birds were found, and at our lunchtime get-together, 76 species were reported with nearly all teams checking in. I'd think the count broke 80, but I haven't yet heard a final tally. Noteworthy and exciting were White-winged Crossbills, the biggest celebrity among this year's crop of winter finches. Twenty of them flew over our heads at lunch, and small roving flocks were seen elsewhere for a couple dozen birds in all.

Mohican State Forest, view from the top of the fire tower. I'm really glad that they've fixed this structure up and allow the intrepid to climb to the top. A commanding vista can be had, and in this photo we're looking in the direction of the Clear Fork Gorge - the heart of this 5,000-acre forest.

After hearing a report from Robert Hershberger's team that some cooperative White-winged Crossbills (henceforth WWXB) were seen at the gorge overlook, we headed there right after lunch. My team members Cheryl Harner and Marc Nolls pose next to the Eastern Hemlock, Tsuga canadensis, that was the site of great WWXB revelry only minutes before. A gang of two dozen descended on this tree and staged quite a show, twittering, seed-eating, clambering about like little parrots, and engaging in the odd brief tussle. All of the green trees in the backdrop are hemlock, which is abundant in the gorge. Tons of crossbill fodder here.

Two males, resplendent in coats of salmon-pink set off by bold white wing bars, search the hemlock for booty. Most of the time, the pack members feverishly destroyed cones, seeking the tiny seeds that fire their engines. Allegedly, a motivated crossbill at full tilt can consume 3,000 conifer seeds in a hard day's work. That means that this pack must have taken several hundred seeds from this hemlock in their short visit.

I had a hard time managing any real great shots due to the light, so I borrowed one of Dr. Bernie Master's wonderful efforts from earlier this month. His shot shows a female cracking into some ripe hemlock cones. Pay attention to the cone she is dissecting, and the one in the middle of the two behind her. Mmmm - very good! Note how the scales are separating and spreading outward, indicating ripe fruit and easy pickings. The other two cones still have the scales tightly appressed, indicating a higher moisture content and probably seeds that are not quite ripe. Plus, these greener cones are probably a heck of a lot harder to get into. I suspect crossbills instantly and accurately size up the ripeness of a cone crop as soon as they alight.

Eastern Hemlock cones are tiny and delicate - well-suited for the rather small bills of WWXB's. Note that the foliage of this species of hemlock has two conspicuous white lines running the length of the leaves on the underside, a good identification characteristic. These cones are not fully mature and would likely be shunned by foraging crossbills, I suspect.

Here I've largely simulated what a crossbill does much faster and more efficiently than I could hope to. These are the cone scales, which must be removed to access the seed, which is what they are after. You can see a seed in the scale on the far right, near the base. Feeding WWXB's are often remarkably quiet, only emitting soft twitters. Sometimes all one hears, if they listen closely, are the cracking sounds of cones being violated, accompanied by the sight of little brown scales fluttering softly earthward. Once the flock takes to the air, they don't shut up, though.

Eastern Hemlock seeds, separated from the chaff. It can take as many as 70,000 of them to make a pound. When the small size of the prey is known, one can understand why crossbills have become so efficient at withdrawing the seeds from the cones. If it is true that an industrious WWXB can eat 3,000 of these in one day, it might take the bird over three weeks to consume a pound. Of course, a pound of hemlock seed would make quite a pile.

An example of the disparity in cones among different conifers. The tiny cones up top are from the hemlock, and the whoppers are those of Colorado Blue Spruce, Picea pungens. There are several dozen native conifers in North America, and shapes and sizes of cones vary tremendously. It is not hard to see why Red Crossbills, which typically manage cone sizes larger than those preferred by WWXB, are actively evolving different bill morphology to better exploit different species of cones. Why aren't White-winged Crossbills in a similar evolutionary race? I don't know.

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6 comments:

dAwN said...

What a great sight..those are really beautiful birds...hopefully someday i will see one. thanks for the very informative commentary.

Jim McCormac said...

Thank you, Dawn - I really appreciate and enjoy your comments! I also got the idea for that little gadget that shows where visitors are coming from,from your blog, and just installed it. I am normally a techno-dumbhead, but figured it out. Thanks!

Jim

Julie Zickefoose said...

This is a wonderful post. Thanks for dissecting the hemlock cones, and for teaching me some things about crossbills I didn't know.

Jim McCormac said...

Thank you, Jules - that means a lot to me, especially coming from you!

Your husband was making fun of me on the Internet. Kindly sic that Pied Mountain-Boar on him if you would!

Jim

Scott said...

Jim, is eastern hemlock considered native to northeast Ohio? The USDA Plants database only shows it in Mahoning, Columbiana, Jefferson, and Hocking Counties. My wife and I participated in a bird count near Painesville, and one of the places we visited was an eastern hemlock-white pine forest that appeared to be very natural. I'm curious because I'm interested in finding out why eastern hemlock is so spotty in its distribution (it's known from the entire eastern US and Canada, but is virtually absent from the Chicago Region, where I live).

Scott.

Jim McCormac said...

Hi Scott,

Absolutely, Eastern Hemlock is native to northeast Ohio, and some of the finest hemlock gorges are found there. Lake County, in particular, has some outstanding examples. There are even a few hemlock-dominated swamp forests - one of the best, Padanarum Swamp, is in Ashtabula County.

Hemlock is a boreal remnant, a holdover of a time when our climate was much cooler and dominated by northern trees. Black Spruce was once common here! Today, hemlock only persists in exceptionally cool microclimates, such as steep gorges and a few cold boggy areas. However, head north of Ohio and you could find such habitats commonly.

Jim