Generally, birds do not have a developed sense of smell such as that found in mammals. However, this is not to say that birds may not use pheremones as a way to identify the opposite sex or for mating purposes. However, birds do use sight as a powerful sensory identifier such as increased or change in coloring during mating season. We know this about swans as there has been research in waterbirds and their use of the uropygial oil gland to maintain healthy and waterproof feathers. Some researchers believe that the oil may enhance ultraviolet light to increase sight and recognition/identification of other flock/family members.
As far as swans, we do know that hormone levels increase significantly during the breeding season as we worked with Univ. of Tennessee’s (Knoxville), Veterinary Medicine to conduct hormone research. How the swans determine this level in other swans such as their mate, we do not know at this time.
We can only theorize that there may be some type of pheremone or other sign that the birds recognize. You might do research in pheremones in other species of birds (if it has been conducted) and make the correlation that Class Aves are known to use pheremones or other sensory identifiers and attribute this to swans in general if needed. If no such research is available in birds, then this could be stated as a great research opportunity in the future.
Besides sight, hearing is also a very powerful sensory attribute of swans.
We hope this information is of benefit. The Regal Swan