Are Mallards Numbers Still Driven by Ponds?
Of all the environmental variables that can affect mallard numbers, the one given most attention is the number of May ponds. We developed a regression model relating annual changes in the estimated continental mallard population to pond counts during the 2 previous years. The model implies that the change in mallard numbers from, say, 1965 to 1966 is affected by pond numbers during the current year (1965) and the previous one (1964). The 1965 pond count reflects the extent and quality of habitat during the current breeding season. The 1964 count is an indirect measure of the age structure of the population in 1965; high counts indicate favorable conditions for breeding the year before, and thus a high proportion of yearlings in the present population. Yearlings are less effective reproductively (Krapu and Doty 1979, Cowardin et al. 1985), so a high pond count in 1 year might favor a population increase the next year, but a decrease 2 years later because of the high proportion of yearli
