Food resources within wetlands can be diverse and vary temporally and spatially. Birds are unique among vertebrates in their ability to use wetlands dispersed over hundreds or thousands of miles in their annual range. However, they are most restricted in movement during the nesting and rearing season or when they are flightless owing to age or annual wing molt. In addition to flight, they may have the flexibility of swimming, running, or walking as needed and efficient, and this reflects the end-products of long years of trial and error. This should not infer that all wetland birds are wandering long distances in search of food, but that they could and sometimes do. Typically, they are bound by a series of historical and evolutionary events that have resulted in the development of successful patterns of movement and breeding strategies for a given region, which is intimately tied to food resources.

Finding accurate information on foods used by birds seasonally and at various life-cycle stages and ages is difficult. An old but still valuable summary of food use by birds in North America is Martin, Zimm and Nelson (1951), but this and papers of this period contain some errors resulting from the examination of gizzards rather than esophagi alone for some bird groups. This underestimates soft animal matter and exaggerates hard seeds. More available but less quantitative summaries of general food uses are in Erlich, Dobkin and Wheye (1988) and Kaufman (1996) in the USA and in similar treaments of bird life histories in each region or country. In addition, there are numerous detailed studies of selected species; however review papers for various taxa, where they exist, are the best source.