From the BTO Website:
"Over the last 25 years we have lost two-thirds of our breeding House
Martins, with the greatest losses in England and Wales. During the last
year, we've received several reports of long-established colonies being
abandoned, possibly due to the cool, wet 2012 summer but in order to
fully understand the declines, we need more information. You can help by
telling us where House Martins are building their nests or by supporting a new tagging project to discover where House Martins over winter."