One theory is it requires multiple phases. First an unintenional egg mixup. First there are eggs laid by one bird, then it's abandoned for some reason and another bird of a different species lays eggs on top. All the chicks are raised by the parents as if of the species of the latter bird, but one or more of the hatchlings are actually from the first batch, a different species. The chicks will absorb enough of the habits as to mate with their "social species" instead of their "biological species". (Am not a biologist and don't know the precise terms).
One theory is it requires multiple phases. First an unintenional egg mixup. First there are eggs laid by one bird, then it's abandoned for some reason and another bird of a different species lays eggs on top. All the chicks are raised by the parents as if of the species of the latter bird, but one or more of the hatchlings are actually from the first batch, a different species. The chicks will absorb enough of the habits as to mate with their "social species" instead of their "biological species". (Am not a biologist and don't know the precise terms).