Sunday, January 4, 2015

Woodpecker Niche


Just a few minutes up the trail after seeing the hairy woodpecker, we again heard a tapping noise through the fog and sleet. Like déjà vu, a black-and-white checkered woodpecker clung to a birch tree, using its chisel beak and kickstand tail to look for bugs along the knobby twigs. Only this time, there was no red on its head, the overall size was smaller, and her beak was tiny. Here was a downy woodpecker, the smaller cousin of the hairy woodpecker we had just seen.

Telling apart these two black-and-white woodpeckers is one of the rites of passage for beginning birders. My dad initiated me early, with this simple comparison: when you see a hairy woodpecker, you think “What a beak!” because it is so large (as long as the head). When you see a downy woodpecker, on the other hand, you might ask yourself “What beak?” because it is so tiny and inconspicuous.

To go with their smaller beaks, downy woodpeckers are only two-thirds the size of a hairy, and weigh only one third as much. This size difference has a big impact on the places that each woodpecker can forage, and allows them to exploit almost exactly the same range and habitats without too much direct competition for resources.

For example, the featherlight downy (who weighs under an ounce) gleans insects from bark crevices on smaller branches, and will often be seen dangling acrobatically on twigs like a chickadee. Meanwhile, the heavier hairy woodpeckers dig for wood-boring insects on trunks and large limbs that won’t swing under their weight.

Hairy woodpeckers also specialize by following pileated woodpeckers. After a pileated excavates a large hole and moves on, a hairy might clean up the crumbs of insects that the pileated missed. Hairy woodpeckers have also been seen drilling sap wells into sugar cane, and nabbing sap from wells made by yellow-bellied sapsuckers.

Downy woodpeckers have their own set of special feeding habits. Their small body and sharp beak make it easy for them to cling to the dried stalk of a goldenrod plant and drill precisely into a gall on the stem. Inside, they might find a sweet, juicy gall fly larvae for dinner. Downies compete with chickadees for this food source. If you find a bird-eaten gall, you can guess at the culprit by the neatness of the hole. Chickadees have much blunter beaks and excavate a messy, wide-angled hole.

When downy woodpeckers forage on trunks, as the females are more likely to do in the winter, they are better able to move horizontally and downward. This might give them a different angle on the bark crevices, and a better view of food that larger species missed.

Angles are important for their nest sites, too. Hairy and downy woodpeckers both choose a trunk or branch that is leaning to one side, and chisel the nest entrance into the underside. Scientists hypothesize that this keeps flying squirrels or sapsuckers from moving in. Both hairies and downies prefer excavating their nest holes in a living tree with a rotted core, or a soft dead tree, although the downy can use smaller trees and limbs.

The slight but important differences between hairy and downy woodpeckers that allow them to share a forest without pushing each other out are a function of their ecological niches. In shorthand, we sometimes define “niche” as “how an organism makes a living.” More technically, a niche can be defined as “the sum of the habitat requirements (including both the living and non-living elements of the environment) that allow a species to persist and produce offspring.” A niche also includes the ways that an organism responds to the distribution of resources, and to competition from other species.

All species have a niche. As we hiked across a snowmobile trail on the way back to our car, it struck me that human adventurers fulfill more than just one niche, too. With trails for skiing, skijoring, snowmobiling, hiking, and fat biking winding through our forests, everyone can get out and enjoy the woods in their own favorite way. What will you do?

If snowshoeing fits your niche, then join me on a snowshoe and tracking adventure this winter! Find the dates, descriptions, and registration instructions at www.cablemuseum.org.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! The current exhibit, “Nature’s Superheroes—Adventures with Adaptations,” opens in May 2014 and will remain open until March 2015.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com.

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