Sunday, 20 April 2008
It's the time of the year for choughs. Trevor recently posted some photos showing their characteristic behaviour (and slightly crazy red eyes)
You rarely see a lone white-winged chough (Corcorax melanorhamphos). Where there's one, there's usually half a dozen or more. They move in mobs, searching through forest litter for insects, spiders and snails. Every now and then, they'll abandon the ground for a tree branch, where they'll preen and chat for a while before heading back to the leaf litter.
Given their behaviour, it's no surprise that they're closely related to apostlebirds. Both are mud-nesters that form tightly-knit, talkative groups and spend most of their time foraging on the ground. They also devise elaborate plans involving trapdoors and anvils with Acme engraved on the side, but that's another story. And I've already said too much.
In a chough mob, only one pair breeds at a time. The others — which are usually the older offspring of the breeding pair — help to care for the new young. They collect food and groom the chicks and probably encourage them to behave badly when the parents aren't around.
But some of those helpers aren't entirely selfless. When food supply is limited, they change their behaviour. They collect tidbits for the chicks. They bring them back to the nest. They'll even go as far as putting dinner in the gaping beaks of those chicks — but then they take it out again and eat it themselves when no one's looking. This 'false feeding' behaviour decreases when there's enough to go around. It also becomes much less common in older birds.
Despite the mealtime switcheroo, the younger helpers do spend time caring for the chicks. They undertake the sorts of activities that the older birds eschew. Young helpers preen their infant nest mates, whereas the older ones concentrate on gathering food. It works well. There are plenty of choughs around the place.
Boland, CRJ, Heinsohn, R. & Cockburn. A. (1997) Deception by helpers in cooperatively breeding white-winged choughs and its experimental manipulation. Behavioral Ecology and Sociobiology 41: 251 – 256.