The Cenozoic Forest is a forest in Early Pliocene - Early Pleistocene. The fact that Terror Birds live there would suggest that the forest is located somewhere in North America.
An Anomaly opened up in the forest and three Terror Birds (one juvenile and two adults) went through to Vancouver in the present day. The two adult Terror Birds (one of which had died) were returned through the Anomaly, while the juvenile was secretly kept in the present. (Angry Birds (NW 1x4))