House martins - swarming.

Hello - we have had the pleasure of House Martins nesting under the roof of our house - Up to 9 separate nests in all. However, this morning I have noticed that two such nests have been subjected to a number of House Martins "swarming" around them. I have not noticed this before. Up to a dozen or so birds at each nest - flying in and out, glinging onto the wall and generally making a quite a racket! Can anyone explain this behaviour? 

Thanks in advance.