Avian clutch size of increasing clutch size with increasing latitude. The proximate and ultimate causes for this pattern have been a subject of intense debate involving the ideas of group and individual selection. David Lack observed a direct relationship between latitude and avian clutch size. Birds near the equator laid approximately half as many eggs as those that resided in northern temperate habitats. He observed an increasing clutch-size from the equator towards the poles (something he referred to as the “latitude trend”) for many passerine (perching) birds, near-passerine (tree-dwelling) birds and in various other groups: Strigiformes, Falconiformes, Ciconiiformes, Laridae, Ralliformes, Galliformes, Podicipedifomes, and
Origin of birds flight, in which developing wing structures served as a sort of parachute. Second, they achieved active flight by flapping the wings. He used "Archaeopteryx" as an example of this second stage. Finally, birds gained the ability to soar. Current thought is that feathers did not evolve from scales, as feathers are made of different proteins. More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false. Feathers are very common in coelurosaurian dinosaurs (including the early tyrannosauroid "Dilong"). Modern birds are classified as coelurosaurs by nearly all palaeontologists,