No. 119: Sep-Oct 1998
In a recent issue of BioScience, R.B. Payne authored an excellent review of brood parasitism in birds. Brood parasites lay their eggs in the nests of other species, which then incubate the eggs and raise the alien chicks. The parent parasites are thus free to forage, hang out, and parasitize more nests. Brood parasitism is such a successful and easy way of life that 136 species of cuckoos, 5 species of cowbirds, 20 finches, and South America's Blackheaded Duck have adopted it.
Brood parasitism fascinates ornithologists because it involves war between the parasites and their hosts. Since host species may eject parasite eggs or fail to nurture parasite chicks, brood parasites have evolved mimicry as a powerful weapon in these battles.
Mimicked are host eggs, host nestlings, and host vocalizations. But the most insidious weapons of all involve the outright murder of host chicks. To this end, parasite chicks have evolved some special weapons and behaviors.
Some cuckoo chicks evict host eggs or chicks by squirming under them and positioning them in a specially configured hollow on their backs. Then, pushing upward and outward to the rim of the nest, they dump their cargo over the edge.
Other brood parasites are more direct and bloodthirsty.
"Nestling African honeyguides have bill hooks to stab and kill their nestmates and the brood parasitic American striped cuckoos have independently evolved hooks and pincers to kill."
(Payne, Robert B.; "Brood Parasitism in Birds: Strangers in the Nest," BioScience, 48:377, 1998.)
Comment. The hollow in the cuckoo's back and the deadly hook on the honeyguide bill disappear once their grisly work is done. Both strategies require bizarre, coordinated innovations in both weapons and behavior. These could, in principle at least, be the work of random mutation and natural selection -- but were they? Is there something we are missing in our theories?
|Euroasian cuckoo chicks maneuver under host eggs and chicks and dump them over the edge of the nest. Their backs have a neatly designed depression that just fits their potential competitor. (From: Biological Anomalies: Birds)|