It was a cold morning with a hard frost covering all of Creamer's Field. We were excited to capture a variety of species today including an American Three-toed Woodpecker and a late Gray-cheeked Thrush. We were especially excited to capture a 35th first-of-season today - a Northern Shrike!!! This individual was easily identified as a hatch-year due to it's indistinct facial mask, dark baring on it's chest, gray rather than black bill, and brown rather than black wing bars.
The Northern Shrike is a migratory and typically breeds in taiga and tundra habitats. They are a predatory songbird that eats insects, amphibians, small mammals and even other birds. Unlike other songbirds, the "butcher bird" has a very unique eye adapted to it's specialized diet.
Unlike humans, birds eyes are fixed in place and they must move their entire head to change their field of view (our musculature allows us to remain stationary). The placement and stationary nature of most bird's eyes results in monocular vision (humans have binocular vision). Certain groups of birds have adapted a way to tackle this obstacle and attain a degree of binocular vision, something that is useful if you rely on your eyesight to capture quickly moving prey.
Humans and most songbirds have a single structure in the eye called a fovea. Fovea are small pits at the rear of the eye (located in the macula of the retina) composed of a very dense cluster of rods and cones; this structure is responsible for crisp and extremely detailed vision (i.e., visual acuity). A second fovea in some species provides a degree of binocular vision, increased visual acuity and depth perception. These are all useful if you are trying to catch quickly moving prey. Raptors (e.g., hawks, falcons, eagles, owls), hummingbirds, kingfishers, and shrikes are among the few birds that have more than one fovea (interestingly, a few bird species have no fovea - any guesses on who?). The only other songbird family that has two fovea are the swallows.
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