It is fascinating to watch House Martins build their nests. Recently I was staying in a house in Scotland near Aberdeen. The house was near a river and I was delighted to find that Martins were building their nests under the eves. I was to be able to observe a pair nest-building from a few feet away. One bird would arrive with a beak full of mud and then carefully and diligently work a new pellet into the nest structure. After a minute or two the other Martin would arrive with another mouthful of mud and its mate would fly back to the river for more building material.