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
Genomic evolution of birds several modifications and deletions which occurred in the exons of the genes implicated in the formation of enamel and dentine. It is thought that the common ancestor of birds already lacked mineralized teeth, and that later genome changes pushed the situation to the current status. Also, birds have the best vision system known in vertebrates. They have a higher number of photoreceptors, and most birds are tetrachromats. The only exception are penguins, which have only three functional opsin genes (and hence are trichromats). This exception could be related to the aquatic lifestyle, since marine mammals have also lost either one