 Hummingbirds are specialized nectarivores and are tied to the ornithophilous flowers upon which they feed.Some species, especially those with unusual bill shapes, such as the sword-billed hummingbird and the sicklebills, are co-evolved with a small number of flower species.However, even in the most specialized hummingbird-plant mutualisms the number of food plant lineages of the individual hummingbird species increases with time.