Birds can be directly counted on breeding colonies, and at roosts, flocks, or Leks.
Large diurnal migrants, like many raptors, can be counted as they pass through migration bottlenecks.
It is a powerful method for studying bird migration, estimating population sizes and survival rates, and recognizing changes in productivity.
Birds can then be identified, measured, weighed, and marked with a small aluminum band bearing a unique number.
Birds may also be marked with a locally unique combination of colored plastic leg bands, leg flags, patagial tags, or dyes which allow the bird to be recognized in the field without requiring recapture.