As the name suggests, we tend to think of House Martins as nesting under the eaves of houses. However, this is only a relatively recent adaptation for House Martins & I'm guessing it has only been something they have been commonly doing for a few hundred years, after the first stone buildings were constructed on a regular basis. Some of the early Norman castles & medieval cathedrals probably hosted the first House Martin colonies on buildings.
Before that date, House Martins in the UK would have most likely have nested on cliffs. These days there are probably few sites in the UK where House Martins still nest on cliffs. One location where a small colony still do so is Old Harry, on my Studland patch. I've been watching this colony for quite a few years now & over the last decade, I've been trying to get photos of them nest building. This is surprisingly hard to do, given they are nesting on the underhanging cliffs. Finally, this year I've had some success in locating a nest being built. The colony size appears to be between five & ten nests in size.
