Waterfowl, grebes, loons, and coots undergo a complete, simultaneous wing moult that leaves them flightless. The moult occurs during the brood-rearing period of geese, the post-breeding period of male ducks and the post-brood-rearing period of most female ducks. Feather regrowth requires three to five weeks depending on the species. Moulting periods are the ideal time to capture birds for mark-recapture studies because they are flightless.
Birds often move to areas away from their breeding grounds to moult. Although little research has been done, it is believed that these areas are rich in food and/or offer good cover for protection from predators. The use of cover by moulting birds makes them more difficult to detect in surveys. High foraging rates of moulting birds can also make it difficult to achieve an accurate count of diving birds. When feeding frequency is high, counts will be more accurate if birds are scanned slowly several times and the highest count used (Breault et al. 1988).
It can be difficult to identify many species during moult because of their irregular plumage (e.g., male Cinnamon Teal are difficult to distinguish from females after their mid-July moult).
Moulting sites are known to be consistent from year to year for some species of geese and sea ducks (e.g., Surf Scoters, Savard 1988). This provides an ideal situation for tracking long term trends in the number of birds using specific moulting areas.
Food supplies determine where and when large segments of waterfowl and allied species migrate. Migratory patterns of many species of geese and ducks have changed in response to increased availability of agricultural grains (Bellrose 1976; Fredrickson and Drobney 1979). It is important to consider these changes when interpreting yearly variation in populations at traditional migration stop-over sites.
Timing of migration is also related to weather, particularly the availability of open water in early spring and late fall. Surveys should be flexibly timed with respect to freezing and thawing in local areas.
Wintering birds are concentrated into much smaller areas than at other times in their life cycle therefore, winter is an efficient time to count a large number of birds over a small area. Unfortunately, the potential for error (at least in determining total population sizes) is great (Eng 1986). Flocks of birds that are concentrated in open areas are highly visible but difficult to count, particularly if flocks contain many species of different sizes. Howes (1987) devised a method for estimating numbers of birds in flocks by sight, and aerial photography has been used to increase the accuracy in counting conspicuous species, e.g., geese (Boyd, pers. comm.), swans, and even Canvasbacks (Haramis et al. 1985). It is possible to determine the accuracy and precision of observer's estimates of flock size by counting computer simulated flocks (e.g., Hodges 1993).
Flocks of migrants and wintering birds of the same species are often mixed for variable periods of time in the same location (especially along the south coast). Resighting marked birds repeatedly over the winter months is the best way to distinguish wintering birds from migrants.