Why do birds eat nectar from only certain flowers?

Explanation:

Birds with longer, thinner bills, such as hummingbirds, bananaquits, and sunbirds, insert their bills into the flower so they can lick at the nectar directly. ... Nectar-eating birds are called nectivorous if nectar does make up a large proportion of the majority of their diet, such as with hummingbirds.

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.