Question

What, if anything, would happen to the relative phenotype frequency of the moths if there were no birds in this environment?

Answer 1

A population of moths, under the selective pressure of being eaten for their phenotype, could suffer changes in the prevalence of polymorphisms (e.g. colour type), if birds stop devouring them.

By removing the cause of the selection (birds) it would be expected that the population frequency would vary from year to year, until a balance is reached.