Hi Debbie,
from my experience, the same family of House Martins will return year in-year out. I destroy the nests now after the birds have migrated. Its not mean at all....in fact it helps them!!!....The nests suffer from humidity/water/frost during winter and become weak. the house martins however won't build new nests unless they have to and so will try and raise a brood in a structually dangerous mud cup!.....everything is fine until the chicks reach 2 or 3 weeks, and then, ussually during the night when the parents are roosting too, the nest falls......and I am left with the task of re-housing....
Also, House Martins carry a rather horrible parasite that overwinters in their nests.....these rarely, but occasionally cause problems by invading the donor house.
....
These birds are wonderful to live with.....they chat loads, and are very inquisitve. The oddist thing is that they all help each other out, even from nests miles away...
I would advise to get hold of a few man-made nests, this stops the problems of their nests breaking up.......some of the birds are awful at building their nests........Debbie, it would be very interesting if you put up 2 of these manmade nests, as HM's like living in families and would prefer to be close together......