At these specific sites mist-netting is conducted at regular intervals and for measured periods of time. This allows useful information to be derived not only from the recovery of ringed birds but also from the actual numbers of each species trapped and their sex and age ratios. The seasonal and longer term patterns built up in this manner can be used to describe seasonal patterns of migration and dispersal - particularly when there is an extensive network of sites - and also to detect trends in population size. The higher rate of retraps with this technique also provides additional information on residency and longevity. In short, measured- effort sites greatly increase the rate at which useful information is accumulated for a given amount of ringing effort.