A chap I work with has several House Martin nests on his house - part of a colony in the neighbourhood. He said that last night he was out on his patio when an egg fell from one of the nest. He was fairly sure the martins had pushed it out and asked me why they would do this. I suggested that maybe the birds had ousted it because it was infertile, but I wasn't sure so I thought I'd ask your collected wisdom. So: can House Martins tell if their eggs are infertile and would they chuck them out? Or is there another explanation?