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Sunday, April 15, 2012

Coursing across the landscape

A beautiful female Northern Harrier (Circus cyaneus)

Harriers (formerly known as Marsh Hawks) have yellow eyes when fully mature, so the individual above with dark brown eyes is probably a younger bird (in its first or second year).

Harriers are medium-sized raptors with long wings, a long tail, and long legs.  Here's a silhouette of a bird viewed overhead.

Harriers are sexually dimorphic: females are predominantly brown, adult males are mostly gray.  (Juveniles show significant rufous coloration below.)  Females are also larger than males — about 50% heavier and about 12% larger in size.

I still don't have a great picture of a male, but here's one (below) so that you can compare the difference in coloration.  Note the pale gray feathers above and the black wing tips.  (You can also see a hint of the white rump patch at the base of the tail, a distinctive field mark for harriers.)

Northern Harriers are unusual among raptors in that they are polygynous — males mate with up to five females.  Males (and sometimes females) are known for sky-dancing to advertise breeding territories and court females.  Sky-dancing consists of a rollercoaster-like flight pattern: steep arcs repeated over and over (up to 74 times!), sometimes with an upside down flip at the top of the arc.  Harriers nest on the ground, often in wet meadows or densely vegetated grasslands (they have nested on Bodega Head).

Their hunting flight pattern is recognizable from a distance — coursing low over the vegetation, tilting back and forth, wings held in a strong dihedral (V-shape), with slow, deliberate wingbeats.  They can also hover briefly (and often do so just before a pounce).

If you get a close view of a harrier, look for the owl-like facial disc.  Like owls, they use auditory cues to locate their prey primarily small to medium-sized mammals (especially voles) and birds.  (In New England, I once saw a harrier chasing Snow Buntings!)

In many ways Northern Harriers epitomize the essence of the Bodega Head landscape (at least the terrestrial portions).  The familiar view of them flying in the distance is an indicator of the dominant local habitat types — open grasslands, dunes, and marshes.

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