For breeding, sand martins dig burrows in sandy cliffs. The distribution of burrows and broods is non-random in different respects. The study deals with proximate factors responsible for this feature and with the question, whether differences in breeding success are correlated with differences in the distribution of burrows and broods.
A series of field experiments shows that characteristics of the cliff (upper and/or lower limit, existing burrows, ledges) and of the conspecifics (social attraction, defence of burrows) influence the birds' selection of a burrow-site and that site-attachment is also involved.
Birds in the highest burrows suffer less predation by a beech marten; the breeding success is correlated with the length of the burrows and with the synchronisation of broods in the sub-colonies.