Final answer:
Assortive mating (nonrandom mating) would produce the smallest evolutionary change in a population of birds over a given period since it affects the distribution of genotypes without introducing new genetic variation.
Step-by-step explanation:
The student is asking which of the following would produce the smallest evolutionary change in a given period of time in a population of birds: 1) Migration 2) Mutation 3) Assortive mating 4) Natural selection 5) Gene flow.
Mutation is the source of new genetic variation necessary for evolution but typically has little immediate effect on allele frequencies in large populations. Assortive mating, also known as nonrandom mating, usually does not introduce new alleles into a population but can increase the proportion of homozygous individuals to heterozygotes, leading to a different kind of genetic structure without significantly affecting overall genetic variability.
In contrast, migration (gene flow) and natural selection can have a much larger impact on evolutionary change. Gene flow introduces new alleles to a population, and natural selection alters allele frequencies by favoring certain alleles that confer a survival or reproductive advantage. Considering the options provided, assortive mating would likely produce the smallest evolutionary change in a population of birds over a given period.