Monday, 26 March 2018

Harriers and Kereru

Ex cyclone Hola recently passed east of the Peninsula only dumping 50mm of rain and packing moderate winds. It has been a wet year so far with more than 650mm of rain recorded at the house. It will have been a lot more higher up the divide. 
With continuous rain and wind I spent some time yesterday watching the forest for activity from the comfort of the living room. Flocks of silvereye feeding in the lower vegetation were blown from one spot to another and every so often a kereru would fly up into the gale to ride the buffeting wind before hurtling back into the canopy of an emergent rata. It is difficult to imagine any purpose to these sallies other than sheer enjoyment in the power of the elements and their mastery of them. 
Watching the kereru got me to thinking about one characteristic behavior we have frequently observed. Whenever a Swamp Harrier (also known as the Australasian Harrier or Harrier Hawk) drifts up the valley quartering the forest, kereru will fly up from the forest canopy and fly around apparently in some panic until it moves on. The harrier is quite a small raptor and no threat to an adult kereru. It feeds mostly on carrion and small animal prey (although no doubt given the opportunity, a harrier would take a squab from a nest). The adult kereru's response to these harrier visits seems excessive. However, the swamp harrier is a relative newcomer to New Zealand - having arrived only a few hundred years ago.

Swamp Harrier

What is fascinating is that 500 years ago or so these hillsides would have been the domain of a far larger predator that would have targeted kereru as a main source of prey. The Forbes' Harrier (Circus teauteensis ) known from fossil remains, was four times the size of our swamp harriers today. Of NZ's aerial predators only the Haast's Eagle was larger. Forbes' Harriers were formidable killers taking large prey even including small moa. They became extinct in the prehistoric period presumably as a result of human hunting although the introduction of pacific rats could have resulted in egg and chick loss.

Is it possible that todays kereru, which evolved for millennia alongside the Forbes Harrier have transferred their flight response to the smaller, but similar shaped, swamp harrier? Perhaps, deep in the kereru's DNA, the survival of the species over thousands or millions of years still drives the behavior of our birds today. 
What else that we witness in our forest is a vestige of an extraordinary lost past?
Forbes Harrier
Image from a painting by Paul Martinson in Extinct Birds of New Zealand by Alan Tennyson and Paul Martinson.

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