In addition, if only a portion or even none of the population actually nests, the breeding population as we have defined it would be severely underestimated. The method would require that breeding population be defined as hens that have at least one nest, and a correction for renesting would be required. Dzubin recommended correcting population counts for unbalanced sex ratios, but estimates of sex ratio require detailed field studies that may not be available for operational surveys. Definition of breeding population for a local area requires that transient and breeding birds be distinguished. This behavior can cause biases because visibility varies among pond classes.
Individual hens change social status as the season progresses.
Breeding birds can be separated from migrants to some extent by timing of the census. Counts made too early or too late will be seriously biased because they may contain a high proportion of migrant birds, or birds that have already attempted to breed may have left the local area. In addition, the proper time for conducting pair surveys varies among species, years, and locations. Dzubin set two counts as a minimum but recommended an average of four or five counts. Single counts of breeding ducks are frequently used, however, to estimate the breeding population, a procedure that can lead to highly biased estimates.