This is an interesting and hopefully useful topic. All animals have built in instinct to feed. This is a primary instinct. A secondary instinct is what the individual animal identifies as food. The target of this feeding is formed by the environment. Piranha are a good example. The fish has a primary instinct to feed on moving items in the water column. If the piranha is living in the wild this will come in the form of smaller fish, aquatic crustaceans, and some vegetation. In captivity the piranha have been reared in an artificial environment. The target food item is a pellet or flake food (to avoid infection from living feeder organisms). The primary instinct to feed is shaped to identify these unnatural items as prey. Sometimes a predator fish will have a “pet” goldfish in their area for years without eating them. As long as the primary instinct to eat is satisfied they have no need to look for alternatives. If the primary instinct is unsatisfied, with a change in feeding regimen or other stressors, the piranha will look for alternatives. Usually this means that the “pet” becomes prey and alas a long friendship is gobbled up. This is a good argument for feeding pelleted foods only. If the racing pigeon only identifies pellets as food then they will not be distracted by other food sources in the environment. They will not identify wild seeds or garden birdfeeders as food and race home to satisfy the primary instinct to feed. Birds are not sensitive to smell or flavor but they are to color and shape.