You have noted that some of these sparrows are darker than others. In particular, you have noticed that darker colored females choose to breed with darker colored males within the population. There is differential reproductive success within populations. The environment creates favorable characteristics within populations . The fittest individuals leave the most offspring.
Populations tend to produce more individuals than the environment can support.
Allele frequency cannot be estimated from this information. They tend to increase the frequencies of deleterious alleles and decrease the frequencies of advantageous ones. They have no effect on allelic frequencies. They tend to increase the frequency of new alleles and decrease the frequency of old ones. They tend to selectively combine favorable alleles into the same zygote but do not change allelic frequencies.