I studied nutrient reserves, digestive organs, molt intensity, diets , and seasonal changes in food resources available to postbreeding adult male Northern Pintails (Anas acuta) and Cinnamon Teal (Anas cyanoptera) in marshes near Great Salt Lake, Utah. Total lipid and protein reserves of Northern Pintails were lower during the flightless period than before or after. For Cinnamon Teal, total protein was lowest during the flightless period, and total lipid showed a similar trend. Total mineral did not differ among plumage classes in either species. Use of nutrient reserves in these species may be an endogenous rhythm in response to increased predation risk or unpredictable food resources while flightless. Digestive organ metrics of Northern Pintails generally were lower while flightless than before or after, apparently due to decreased dietary consumption. Cinnamon Teal digestive organ metrics changed little from preflightless to flightless stages, but generally increased while postflightless. Changes in Cinnamon Teal digestive organs may be related to increased dietary intake or increased dietary fiber consumption. Total molt intensity in these species was generally high before and during wing molt, and decreased during the postflightless period. Mathematically weighting total molt scores produced results similar to those obtained without weighting, but selection of body/feather regions is critical to obtaining unbiased estimates. Changes in relative availability of plant and animal foods during July and August were marked . Animal foods made up 96% of total foods measured in the study area during early July, but plant foods comprised 95% of available foods by late August. Changes in abundances of food resources are probably an important determinant of diet selection in postbreeding adult ducks and ducklings in marshes near Great Salt Lake. Diets of postbreeding adult male Northern Pintails and Cinnamon Teal did not differ between species, but did among flight classes. Changes in use of food items and in use of animal and plant foods were consistent with changes in relative abundances of food resources. Postbreeding adult males of these species apparently foraged opportunistically.