A truly dreamboat mate for a blue tit girl would be an older man who, for his age, has a bright crown which blue tits can see in the ultra violet spectrum. He also sings long and loud to show he's still got it, and provides plenty of food during courtship and beyond, whether through effort or skill, which shows that he's more experienced at finding food. Even though, given how he's such a catch, he probably would be getting a fair bit of nooky on the side.

She might temper it a bit with the odd dalliance herself, just to make sure she's got a mix of genes (which makes good evolutionary sense), but not too much. She doesn't want to risk being caught, or daddy might be less inclined to help at home, and anyway, she's got a great bloke already.

But as there's never enough of these amazing males to go round, females have to trade off the different qualities, and do the best they can, maybe even balancing good traits in one's spouse against good traits in affairs. After all, it's much easier to persuade a man to no-strings-attached sex than it is to persuade him to commit.

Not surprisingly, with so many qualities to choose between, different studies come to different conclusions as to what is important to females. Overall age is perhaps most important, females seem to be more faithful with older males.