The effect of rice cultivation on waterbird populations has rarely been assessed. A study in northwestern Italy estimated that breeding herons and egrets obtained 80% of their food from agricultural habitats. The estimate was checked by comparing the occupation of three contiguous sectors of northwestern Italy, one with mainly rice fields, a second with planitial rivers and a third with small upland rivers, from 1982 to 2002, a period in which breeding populations increased. During the increase, the breeders expanded into the second and third sectors only when their population exceeded a certain level, in accordance with predictions of the “ideal free” model of habitat selection. The initial estimate of the importance of rice cultivation is confirmed by this long-term comparison, with the “rice fields” sector hosting 76% of the population. The long-term analysis cautions against generalizations from instantaneous surveys, which can estimate the differential profitability of habitats but not their carrying capacity, unless habitat availability limits population size. Additional information is needed to predict waterbird population responses to environmental change; topics include: habitat use, particularly in Asia; landscape effects; prey availability, foraging intake and behavior; demographic parameters such as breeding success, survival, settlement patterns, site fidelity, and temporal variance; and population changes in relation to management practices.