Northern Harrier Hearing

As I mentioned in an earlier post, Northern harrier is the currently accepted common name for Circus cyanus (or cyaneus, as it is also written). But having originally learned marsh hawk as the common name for this bird, I continue to call it that. Indulge me!

Incidentally, “harrier” most likely comes from the Old English “herigan” which means to harass or plunder – an appropriate name for this bird which flies low over fields and pastures searching for small mammals. Britons call the marsh hawk a hen harrier.  To my way of thinking the scientific name makes a little less sense than harrier. “Circus” from the Greek “kirkos” means circle and supposedly refers to the flight pattern of the marsh hawk while hunting. I never think of marsh hawks as circling birds, but rather expect to see them sweeping low or hovering over the ground  in search of prey. The species name derives from “cyan” or “blue”, the color of the male. To me the male is grey, not blue.

The other day Leonard noticed a marsh hawk grab a small mammal, mole or vole probably, from the field next to the house and land on a fencepost to enjoy its meal. The wind was blowing toward me and the hawk was facing away from me so I was able to get quite close.

Marsh hawks are one of the few raptors where the male and female display different plumage. This male has grey upper parts, a spotted breast and white underparts. The dark bill is yellow at the base and the eyes are yellow, as are the legs. In contrast a female is mottled brown. Both sexes though have a distinctive white rump that aids identification in the field. Another interesting feature of the marsh hawk is a facial disc. In the female the facial disc is outlined in white making it even more visible. While most hawks use sight to catch their prey, marsh hawks also depend on sound.

Owls are known for their facial discs, one of the best known examples being the barn owl (Tyto alba). The facial disc of the marsh hawk is less pronounced, but serves the same purpose. The facial disc is formed of stiff concave feathers surrounding the eyes, which collect sound waves in the same manner as a circular parabola, and reflect the sound to a focal point – the ears. A facial ruff, stiff feathers around the neck of the marsh hawk, can be raised to enlarge the facial disc and further improve hearing. In the same manner that owls can locate prey in the dark, marsh hawks use sound to “see” their prey in tall grass or even under snow. Additionally, like owls, marsh hawks have soft feathers that permit quiet flight and increase their stealth. But like all birds that hunt during the day, marsh hawks  make shadows as they fly, warning their intended victims of their presence.

Taken next to our house (Lookout CA), one picture shows a male marsh hawk eating a small rodent. In the picture of the hawk moving to another fencepost, the distinctive white rump patch, tail barring, black wingtips and black line on the trailing edge of the wings are all easily observed.

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