Female cuckoos rule the roost -- at least when it comes to choosing where to lay their eggs, Lisle Gibbs at McMaster University, Canada, and colleagues have found from a study of cuckoos as far apart as Britain and Japan.

Cuckoos (Cuculus canorus) are well known for laying their eggs in other birds' nests -- duping host birds into looking after cuckoo chicks as their own. But this is not an indiscriminate buck-passing exercise: cuckoos are very fussy about which species of bird they choose to dump their eggs upon.

In fact, there are different races or 'gentes' of cuckoos, each of which dupe a different host. And each gens (the singular of gentes) lays eggs of a colour that blends in with its chosen host's own eggs.

Researchers have wondered for some time how each gens maintains this behaviour and egg colour. Is this a learnt behaviour -- do young cuckoos observe and mimic their elders -- or is it inherited, in their genes?

One problem with the inheritance model is that each gens would have to be inbred to keep their distinctive genes. If cuckoos from different gentes never mated, then each gens would become more and more distinct, until they would actually become different species.

Gibbs' team took DNA samples from cuckoos, and used a technique similar to DNA fingerprinting, as they explain in Nature1. They found that different gentes all shared a similar clutch of 'marker' genes, suggesting that they weren't turning into different species.

So if the gentes are interbreeding, how do cuckoo peccadilloes persist?

Next Gibbs' team checked 'mitochondrial DNA' (mtDNA). Mitochondria are small particles in cells that are responsible for producing energy and contain a few genes of their own. Mitochondrial DNA is always inherited maternally -- from the mother only.

Gibbs and colleagues found that all females of a gens had the same mtDNA markers, which were different from other gentes' markers.

So female cuckoos inherit their gens-specific behaviour exclusively from their mothers. Meanwhile, male cuckoos are more promiscuous and have no gens preference as to which females they mate with. So, the cuckoos stay as one species overall.

Other researchers had hypothesized that this might be the case, for example, Richard Dawkins in his books The Extended Phenotype and Unweaving the Rainbow, but Gibbs' work is the first direct, genetic evidence.

"[This is] a beautiful example of how molecular genetic techniques can, with careful thought, be used to answer problems that have puzzled biologists for decades," comments Paul Harvey from the University of Oxford. "Cases such as this seamlessly integrate studies of evolution, genetics, ecology and behaviour".