Housemartins

We live on a newish estate (6 years old) and housemartis have found us in some numbers this year - lots of nests and young. Currently the birds are exhibiting behaviour I have not seen explained anywhere. They are flying round in quite large numbers and then they fly up to the eaves of a house and grab onto the rendering and 'sit' there for a while, making their normal calls and then suddenly all let go together and swoop around before returning. Sometimes they will perch on the roof instead (it's a very steep pitch). A couple of pictures of this are attached.

Can anyone tell me what they're doing?

        

Many thanks

Ian