natural selection\nreproductive maturity\ngenetic variation\ngenetic drift

Brief Explanations:

Natural selection is the process by which organisms better adapted to their environment tend to survive and produce more offspring. Reproductive maturity refers to the stage when an organism can reproduce. Genetic variation is the presence of differences in sequences of genes between individual organisms of a species. Genetic drift is the random change in the frequency of an allele in a population. Without further context about what the question is asking regarding these chickens, if we assume it's about random changes in their genetic makeup in a small - scale context, genetic drift is a likely factor.

Answer:

genetic drift