The idea of an extended phenotype refers to the expression of genes in the behavior of an organism rather than its physical composition. The way a beaver makes a dam is influenced by the genetic makeup of the beaver, even though none of its genes code for dam making.
If a new allele arose that somehow improved dam making in beavers or nest building in birds, that allele would grow in prevalence among the beaver and bird populations, respectively. Baby birds with the allele would build better nests than those without it and be more likely to survive to pass it on to their offspring. Richard Dawkins, the man who proposed the idea of the extended phenotype, argues that once the new allele was introduced, the population of birds would evolve because of the allele.