Warblers are selective in what they eat, yet little is known about the dietary cues used by warblers as they decide what to eat. Semisynthetic diets may be useful for investigating how specific dietary cues, such as appearance or nutrient composition of food, influences diet preference of warblers because these dietary cues can be easily and systematically modified with semisynthetic diets. We offered Yellow-rumped Warblers (Dendroica coronata) paired choices of live waxworms (Galleria mellonella) and a waxworm mash, or waxworm mash and a semisynthetic mash. Birds strongly preferred live waxworms over waxworm mash, suggesting that natural appearance of food strongly influences diet preference of warblers when the nutrient composition of diets is similar. When birds initially were offered the two mash diets, they consistently preferred waxworm mash over semisynthetic mash within the first 15 min with food, suggesting that they were using dietary cues that provided rapid feedback as would be provided by a cue such as taste. This initial preference for waxworm mash was maintained for the first two days, but then the warblers ate similar amounts of waxworm mash and semisynthetic mash during the last two days of the experiment. The decrease in preference for waxworm mash over time probably occurred because at least some of the cues used by the birds in determining their diet preference(s) required days for reliable feedback. Thus, diet preferences of warblers apparently were influenced by dietary cues that provided immediate and delayed, postingestional feedback. These results support the use of semisynthetic diets in studies of avian diet preferences and highlight the importance of adequate acclimation time on test diets.