Here’s a little bit of bird lore recently arrived at. A new idea so new that I infer from the tone of the report that it is still suspect. But that’s so often the way with a new idea: immediate rejection (even ridicule), eventual acceptance (reluctant and after much proof), attribution (to someone else).

This particular idea is that the bills of many bird species, especially those of insect-eating songbirds, are dark. And darker on their upper mandibles than on their lower mandibles.

The reason, according to Sean. M. Williams and Edward H. Burtt Jr., writing in Birding magazine in September: to reduce glare caused by sunshine, glare that could interfere with a bird’s vital, precise vision.

A quarter of a century ago, Burtt postulated that among North American wood warblers, those that forage primarily in sunlight have darker upper mandibles. Warblers that forage in shade — the water thrushes and the ovenbird — have the lightest upper mandibles. Burtt concluded that dark bills reduce glare and help a bird finds its prey more efficiently. Or perhaps the melanin that produces a dark bill acts as an ultraviolet radiation shield.

And so Burtt took some willow flycatchers and, well, experimented on them. Their bills are dark brown, lighter below. He painted the bills of some of these flycatchers with white nail polish, to increase the reflectivity of them. Another group of willow flycatchers suffered their bills to be painted with a clear, satin-finish nail polish, thus retaining the bills’ natural reflectivity. A central group of flycatchers was unaltered.

In tests, the unaltered flycatchers foraged from shaded perches to catch flying insects — their normal behavior — only 2 percent of the time. The birds with satin polish on their bills used shaded perches from which to sally forth some 5 percent of the time. The white-billed birds, however, used shaded perches 28 percent of the time. Burtt and Williams focused on the glare-reduction hypothesis.

So far, they have observed the foraging behavior and light environment of more than 400 North and Central American birds. Their data shows that birds that spend more than half their time in sunlight have darker bills than birds that spend most of their time in shade.

Bird bills come with huge variation, not only in coloration but also in shape and adaptations. Some are pale or uniformly colored, or — . It reminds me of one of Raynes’ Laws, which states that the only unifying characteristics of birds are they have two legs, lay hard-shelled eggs so that embryonic development takes place outside the female’s body, and are feathered. Beyond those, seemingly infinite variation.

Williams and Burtt find that birds that require more than 50 percent of their annual diet to be insects are “more likely” to have dark bills than birds that feed on seeds or fruit, and that is independent of hunting technique (i.e., aerial sweepers like swallows, perch-and-sally hunters like flycatchers, or foliage gleaners like many wood-warblers.)

In their paper, Williams and Burtt point out they have studied songbirds, but roughly half of the world’s birds are not songbirds, and what about them?

“By and large, upper mandibles tend to be dark. Lower mandibles tend to be dark, but they show more variation than upper mandibles.” The authors solicit more data from bird watchers. Another fascination about birds.•

Field Notes: Jackson Hole gave vivid demonstration of its geography last week as persistent fog settled in, while, in every compass direction, science and generally clear skies prevailed.

Residents might want to consider the air pollution implications.

Large four-legged critters are moving about the valley, often at dusk or night. Exercise caution on foot or in a vehicle.

Some observations: A white-throated sparrow, Spring Gulch Road, Nov. 1, Bru Wicks.

Around Colter Bay in Grand Teton National Park, Tony White and Bobby Hughes did well bird-watching on Nov. 3 — with a shrike, brown creeper, various seedeaters and several ducks. They also spotted a red admiral butterfly, perhaps a late date. Sam Kravetsky welcomed a Clark’s nutcracker, a first at her feeder in Porcupine Creek.

On the National Elk Refuge, Eric Cole reported large flocks of both rosy finches (already) and cedar waxwings, seen sporadically.

Mike Casey satisfied a 35-year ambition to see otters at the Oxbow in Grand Teton National Park. Mike, Bronwyn Minton and son Oden enjoyed the sight on Saturday. Satisfaction.

Keith and Diane Benefiel are fascinated at interactions between a white-breasted nuthatch and a chipmunk. The bird opens its wings and sways back and forth on its legs to intimidate. Ultimately the mammal prevails. Wonderful nature note.

The Jackson Hole Bird Club will meet at 7:30 p.m. Sunday in Jackson Town Hall. Observations, initial arrangement for December’s Christmas Bird Count, socializing, refreshments and a trip to the Amazon by Lori Kahn and Ann Harvey. Everyone welcome and free.

© Bert Raynes 2010

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Bert Raynes writes weekly on whatever suits his fancy with a dash of news on nature and its many ways.

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