I have seen a few of these, and heard of one very violent one (one’s on Youtube), where males gang “rape” one female. What would drive so many males to mate with an obviously already mated female? Is this usual in other birds or animals?
I saw a David Attenborough thing on whales once, in which he mentioned that several males would often mate with the same female. Whales have huge testes and deliver gallons of semen despite the fact that, just like us, they only need one cell to get in there and do the job. The theory they have to explain this seeming waste of resources is that if you’re the last guy to make it in, your gallons of sperm might wash out all that came before you (sorry), thus making it more likely that you’ll fertilize the egg. Interesting idea.