"Tastes like chicken" is a common declaration used when trying to describe the flavor of a food a listener has never tasted. The expression has been used so often that it has become somewhat of a cliché. As a result, the phrase also sometimes gets used for incongruous humor, by being deployed for foods or situations to which it has no real relevance.[1]
As an explanation of why unusual meats would taste more like chicken than common alternatives such as beef or pork, different possibilities have been offered. One idea is that chicken has a bland taste because fat contributes more flavor than muscle (especially in the case of a lean cut such as a skinless chicken breast), making it a generic choice for comparison. Also, chicken reportedly has lower levels of glutamates that contribute to the "savory" aspect of taste known as umami; processing or tenderizing other meats would also lower glutamate levels and make them taste more like chicken.
Another suggestion, made by Joe Staton of the Museum of Comparative Zoology, is that meat flavors are fixed based on the evolutionary origin of the animal. Specifically, he noted that tetrapods largely taste like chicken, whereas other animals usually do not. Accordingly, birds (the most numerous form of meat by type) would (in most cases) naturally taste more like chicken than mammals. Furthermore, based on evidence for dinosaurs as the ancestors of birds, reptile meat might also taste somewhat like chicken and therefore dinosaurs would most likely also taste like chicken.
Seafood, however, would logically have a more distinctive flavor. Also, although mammals are tetrapods, very few mammals taste like chicken, which implies that there had been a mutation that changed their flavour on that branch of the evolutionary tree.[2] Where this theory falls apart is the fact that very gamey birds such as woodcock or grouse, or red-meat birds such as ostrich or squab do not "taste like chicken," while other (bland) sources of protein including insects, earthworms and numerous mammals such as guinea pigs and rabbits do "taste like chicken."
Another possibility is that since much of the meat of a chicken is taken from the chest, which contains the white 'fast fibers' that are necessary for the short, fast flight of a fleeing chicken, it tastes like these other animals due to similar concentrations of fast fibers in the parts that are used for meat.[3]
(From Staton[2] except as noted.)