The only two species of birds mentioned in the Torah which can be offered as sacrifices are the turtledove and the pigeon. Turtledoves are migratory birds, but pigeons dwell permanently in the land. A turtledove must be mature to be fit for a sacrifice, but a pigeon is acceptable only before it reaches maturity. We therefore have the following rules.
What is acceptable in the case of turtledoves is unacceptable in the case of pigeons, and vice versa. The beginning of yellowing in both types is unacceptable. That is when a turtledove is not fully mature yet, and a pigeon begins to become mature. The reason for this is that the Torah calls pigeons “bnei yonah,” “sons of pigeons”; conversely, we understand that turtledoves must be mature.
But logic should dictate otherwise: if turtledoves, which are not acceptable when young, are acceptable when mature, then pigeons, which are acceptable when young, should surely be acceptable when old! To dispel this, the Torah never called a pigeon a pigeon, but only a “son of a pigeon” to emphasize that only young ones are acceptable.
Art: James E. Bourhill - Pouter Pigeons