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 possibility of genomic expansion in mammals and reptiles. Also, there is no such genomic loss in any vertebrate group as great as that of birds. Its possible that the early chromosomic fragmentation event that lead to the appearance of microchromosomes in birds significantly contributed to this gene loss. These fragmentation events must have taken place in a common ancestor of most birds, since approximately every two out of three species studied have at least 30 pairs of microchromosomes, 2n = 80 being the size of the average karyotype of birds (with the only exception the family Falconidae, which are 2n