From all reports the numbers of House Martins coming back, from their migration to tropical Africa, to the UK to nest are both later and fewer than in previous years. The British Population in 2006 was estimated at between 250000 and 500000 pairs, this is just about 2% of the European population.
Of course, although the majority nest on buildings, they evolved before man made structures existed. The first historical record is from the eighth century but fossil remains date back up to 120,000 years . At this time they would have built their nests on cliffs with over-hanging ledges. The "White Cliffs" are perfect for this and there is still a small population that can be seen on the cliffs at Kingsdown. I don't know how big this population has been over the years, the "Birds of Kent" mentions a "few" pairs. The nest above was isolated and quite high up the cliff.
This group of three nest was in a crevice and was being visited fairly frequently. I don't know how far the birds are along their breeding cycle. They normally have 4-5 eggs and two broods, but if they arrived late they may be a bit behind schedule.
I though that this nest, quite low down, was interesting, it's almost got a "bottle-necked" entrance like a Red-rumped Swallow, from southern Europe. Altogether I found six nests but from the number of birds in the air I think that I missed a few.
Getting pictures of the House Martins along the cliff is not easy, even the Fulmars were difficult until they "hovered" in front of their nests. They wouldn't have shared the cliff with the House Martins in prehistoric times as they have only colonised Southern England in the second half of the 20th century.
The predecessors of this Feral Pigeon, the Rock Dove would have been common here, and would probably have been the main food item for the Peregrines that would have nested on the cliffs then.
The sun disappears behind the cliff quite early, so I had to push the ISO speed up to 800. The House Martin above is at least showing its characteristic white rump.
Compared to its relative, the Swallow, the House Martin is a very compact bird, with a short tail and much shorter wings.