House Martins gather nesting material - clay and moist earth that they fasten to the rock walls. (Pål Hermansen)
Today the House Martin builds its nests mostly under roofs of farm houses and barns, or on industrial buildings and petrol stations. But the House Martins built nests long before humans built houses. Sheer rock walls were actually the original nesting places for these birds, and some House Martins keep up the tradition.
This can be seen for example in Seglberget at Skjervet. This rock wall is especially well-suited, since it has many protrusions with plenty of room underneath. The nesting place must be situated so that water does not seep down the mountain and neither must rain get in. If so, the nests would easily dissolve and collapse. Probably many more swallows nested here in earlier times. The species has decreased dramatically in western Norway in recent years.