In general, breeding success in a colony or population is calculated by registering a sample of nest contents in the colony at least twice during the breeding season. These data will often yield the number of chicks produced per pair. The minimum solution relies on two visits to the colony; one visit at the start of the breeding season to register the number of active nests, and one at the end of the season to note how many chicks have been produced in those nests. More detailed data demand more frequent visits. Standardized sampling methods have been developed for each species, but methods may vary slightly between locations due to local conditions (topography etc.).