At last, a bird that nails killer chicks. (Zoology).

For the first time, scientists have found a bird species--Australia's superb fairy-wren--that can spot a murderer.

The killers are chicks of a particularly aggressive cuckoo species. The crime begins when the cuckoo mothers sneak an egg into a non-cuckoo nest, duping foster parents into all the work of childcare. After the intruder chick hatches, it kills its young nestmates by shoving them overboard.

Any human being can spot that something's wrong in the invaded nest, since the cuckoo chick quickly grows ridiculously big, up to six times the size of its foster parent. Some victimized species catch the fraud in the egg stage and roll the cuckoo egg out of the nest. Yet to the bemusement of biologists, no bird had been known to catch the obvious cuckoo once the chicks hatched.

Now an international team of researchers reports that nearly half of superb fairy-wren mothers (Malurus cyaneus) actually do have a clue. If a chick of Horsfield's bronze-cuckoo (Crysococcyx basalis) hatches in a superb fairy wren nest and heaves out the rightful chicks, about 40 percent of mother fairy-wrens desert the nest. The cuckoo chick is left to starve while the mother starts another nest, report Naomi E. Langmore of Australian National University and her colleagues.

This ability might represent an escalation in an arms race between superb fairy-wrens and cuckoos, the researchers suggest in the March 13 Nature. The cuckoos lay eggs that usually fool the fairy-wrens, so an ability to detect phony chicks may have evolved as a countermeasure.--S.M.