The flightless Kiwi is nocturnal and feeds on food hidden in the soil. Its vision is poor and it is the only bird with nostrils at the end of the beak and sniffs while foraging. In general, olfactory senses of birds are poor, but there are exceptions and we might just think they are poor because we don't have evidence. Taste buds, concentrated on the posterior part of the tongue and pharynx floor, are similar to mammalian ones. The total number of taste buds is less, though.
Hummingbirds can distinguish different kinds of sugars and their concentrations.
Seed eaters show no preference for sweet or sour. For most birds there is a variety of salt discrimination levels. Sour - Birds have a wide range of tolerance for sour. Bitter - Again, a variety of responses. The classic study was done by Brower in 1969.