Imagine this.

You buy a house and plan for the big moment when the new baby arrives, decorating a nursery and anticipating a lifetime of familial bliss.But consider this.

After the baby arrives, you step into the nursery to check on the little bundle of joy but instead of one baby there are three. They all look alike and are in the same crib all wearing the same colored bunting from head to toe.

Which one is which? How did all of those extra babies get there?

Such is the plight of birds whose nests are borrowed by cuckoos.

Female cucukoos are well known for this habit, sneaking into the nests of others and starting their families there.

But scientists say cuckoos not only prefer other nests but apparently are genetically able to produce eggs in the shape and color of those of the host.

A host warbler lays eggs only in pale blue? No problem for cuckoos who can lay them in blue, speckled, white or in whatever color the occasion demands.

Oval eggs? The cuckoo can do it. Round ones? A specialty of the house.

Scientists now are studying this phenomenon, compiling a new catalogue of data on cuckoo egg-laying habits and speculations on why these birds, whose typical call notes are a low throaty chuckling sound, attempt to exploit the parental instincts of others.

Ornithologists Paul Harvey and Linda Partridge of Oxford University in England wrote in a recent issue of the journal Nature that female cuckoos possess a special Z chromosome where genes for egg mimicry are located and passed from mother to daughter.

The Z chromosome, which determines female sex in birds, is equivalent to the Y chromosome, which confers masculinity in mammals.

Some host birds, such as the dunnock, can't tell a cuckoo's egg from one of its own, thus making dunnock nests frequent sites for cucukoo invasions.

But experts from the University of Cambridge in England say cuckoos are especially discriminating about where they lay their eggs.

New studies there show that preferences for the nests of certain birds are so refined that various subspecies of cuckoos seek out the nests of particular hosts.

The practice of laying eggs in host nests is known scientifically as "brood parisitism" and apparently occurs in several other species.

The cuckoos of Europe are best known for this habit but cowbirds and finches in the United States are likely to do the same.