Hi John: Great question. Birds have a terrible sense of smell and must resort to eyesight. Birds can count so they know the number of eggs and young. If one fails to turn up then they consider it lost so they will not accept it back after any extended period of time.

Additionally, swans as well as most waterfowl possess a uropygial oil gland, commonly called the preen gland. Some researchers believe that the oil from the gland not only maintains the health of the feathers and keeps the bird water repellent, but may also increase the ultraviolet sensitivity of the eye. This possibly helps the bird pick up on subtle differences in color or other recognition attributes. The Regal Swan
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