THE BURROW‐EXCAVATION PHASE IN THE BREEDING CYCLE OF THE SAND MARTIN RIPARIA RIPARIA

Summary. Observations were carried out at a Sand Martin colony in a working sand‐pit, where fresh burrows have to be dug each year. Sand Martins arrive at the breeding pit from migration unpaired, and their social organization is then at its lowest. Burrow excavation occurs only as the climax to a form of group display. Digging is communal in character, and the number of holes dug is roughly equal to the number of individual birds present. The mounting excitement and mutual stimulation of the display period induces a climax at which the birds begin to build nests and lay. This period of display produces close synchronization which seems to be a necessary condition for the communal behaviour of the birds during the rest of the breeding cycle.