Friday, March 16, 2007

Make them an offer they cannot refuse

Cowbirds like cuckoos* are brood parasites. They lay their eggs in the nests of other birds and leave the hosts with the hard job of raising their young. But cuckoo chicks normally** kick the original nestlings out so they can monopolise the food supply. Cowbird chicks however tolerate their nestmates. Moreover, while eggs of cuckoos mimic those of hosts, the eggs of the cowbirds look completely different. What’s going on?

The victims of the racket are warblers. These birds don’t reject cowbird eggs even though they look different. The reason is that if the warblers tolerate the cowbird eggs they are allowed to breed with reasonable success but if they were to attempt to get rid of the cowbird eggs the adult cowbirds attack the nest destroying most of the warbler eggs. This was verified by ornithologists who deliberately removed the cowbird eggs from parasitised nests.

Providing food to the cowbird nestling amounts to paying ‘protection money’ which proves to be a good deal from the warbler's view. On average, they raise 3 of their own chicks when they support a cowbird chick. Yet they raise only 1 of their own if a cowbird egg has been rejected. This also explains why cowbirds don’t need to disguise their eggs to look like those of warblers.
The overall effect is that the cowbirds bully the warblers into ‘an evolutionary state of acceptance’.

The cowbirds' dirty tricks don’t stop here. Many warbler nests that never have cowbird eggs in them also get destroyed. This is something like ‘farming’. If warblers lose a clutch, they will often produce a second so if a female cowbird female fails to lay in a warbler nest in time for her egg to hatch with those of the host, she can reset the clock in her favour by killing the first clutch.
Thus warblers who lay too early for the cowbirds to cuckold them suffer retribution. The cowbirds spy on the warbler parents, find out where they were nesting anew, and sneak in to lay an egg at exactly the right time.
Even the Mafia never thought of that one.

* That humans suffer intra-specific cuckolding is the basis of the dads’s rights movement. This week the Bulletin features the Liam Magill incident. Liam raised 3 kids believing they were his own only to find up from a paternity test that two of them were fathered by another dads. His wife Meredith Magill, in her own words, ‘messed up big time’ but is angry with Liam for ‘taking it out on the kids’. The kids – even his biological son - also say they hate him. Liam’s initial award of $70,000 damages against Meredith has been overturned on appeal. He will get nothing.

**The Great spotted cuckoo runs a mafia-style protection racket against magpies living in Andalucia, Spain. If a magpie rejects a cuckoo egg laid in its nests, the cuckoo promptly returns to destroy the magpie's own eggs or kill its chicks.


Anonymous said...

Great article. Thoroughly enjoyed reading it.


Anonymous said...

You have uncanny insight into the way birds think--almost to the level of kinship or empathy: Great article!!