Hearing the insistent peeps of a nest full of hungry chicks, a reed warbler dutifully flies off in search of more food. But researchers report that what may sound like a brood of starved warbler nestlings may actually be a single huge cuckoo chick imitating the calls of the baby birds it long ago threw out of the nest.

Researchers have puzzled for decades over how a cuckoo chick, which hatches from its egg in the nest of another species, manages to trick its foster parents not just into feeding it but feeding it more than they would feed a chick of their own. According to a research paper published earlier this year in the Proceedings of the Royal Society, a cuckoo chick can impersonate an entire brood of chicks so effectively that it can spur the host birds into feeding it as if it were a family of four.

''As soon as the cuckoo chick started calling, we thought, 'That sounds really weird,' '' said Dr. Nick Davies, a behavioral ecologist who carried out the research with two colleagues, Dr. Rebecca Kilner and Dr. David Noble, at Cambridge University.

Rather than the usual intermittent series of chirps -- peep . . . peep -- that a single chick makes, the cuckoo's call is an insistent stream -- peep, peep, peep, peep. And although young children seem to have an innate understanding of the power of a relentless begging call, researchers say they were surprised to discover the cuckoo's strategy.