Censusing Wading Bird Colonies: An Update on the "Flight-Line" Count Method

Censusing waterbirds has long been recognized as an arduous task (Cramp et al. 1974, Nettleship 1976, Erwin 1981). Not only is it difficult to accurately determine the numbers of breeding adults at a colony, but the census process itself can sometimes result in major disruption of the colony (Ellison and Cleary 1978, Tremblay and Ellison 1979). The flight-line method provides some promise as a technique that minimizes disturbance yet allows reasonable estimates of the nesting population (Erwin and Ogden 1980). The method requires simply that the number of birds flying to and from colonies (or discrete subcolonies) be monitored over a period of from one to several hours. Concommitantly, when testing this technique, a total nest count (or sampling method such as belt transect) is performed. Regression analyses are then performed to evaluate the relationship. Previous work (Erwin and Ogden 1980) showed that tide level, time of day, nesting phase, species, and colony location all influenced the numerical relationship between flight rates (i.e., number of flights per hour) and number of active nests in a colony, thus substantial variation in the results can occur.