1.   They are largest in polyandrous birds, where several males fertilize one female, and it is not hard to see why.

2.   But for his part, the chance of fertilizing a female may mean the difference between offspring or no offspring at all.

3.   In this way, around one in eight females are fertilized by males they have never met.

4.   Two-thirds of the time, the second male to mate, rather than the first, wins the battle to fertilize the female.