Many animals go to considerable lengths to make it obvious whether they are male or female, but in some species, such as the Australian long-tailed finch Poephila acuticauda, the sexes are completely indistinguishable - a male has to perform a courtship display to show a female that he is of the opposite sex. This would seem to be a waste of time when he could simply rely on permanent signals such as coloured plumage - so why be so secretive?
Naomi Langmore and Andrew Bennett of Cambridge and Bristol Universities respectively found that if a lone male finch is confronted with a group of unfamiliar birds, he is unlikely to advertise his sex by displaying. There is a ‘pecking order’ among males, and high-ranking individuals can be aggressive towards their inferiors. So, to avoid getting picked on, males keep their sex hidden.
Meeting a single unfamiliar bird is a different matter - a male will give the game away by courting the stranger. A subordinate male cannot afford to conceal his identity all the time, otherwise he will never find a mate. When confronted with one bird, the risk is worth taking - the stranger might turn out to be a female. But when a number of birds are present, there is a high chance of a male being subordinate to one or more of them, so it is better to stay anonymous.
But what if a male is approached by a stranger who starts to court him? Should he reveal that he is a male, too? Not if it means being attacked, say the researchers - they found that if a male is courted by a higher-ranking individual, he keeps his identity secret, rather than provoke a fight that he is likely to lose.
(Proceedings of the Royal Society of London B, vol.266, p543-50).