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
Gleaning (birds) aforementioned piculet are specialized for gleaning the bark of trees, as are nuthatches, woodcreepers, and treecreepers. Most bark-gleaners work their way up tree trunks or along branches, though nuthatches are well known as the birds that can go the opposite direction, facing down and working their way down the trunk, as well. This requires strong legs and feet on the part of the nuthatch and piculet, while birds that face upwards tend to have stiff tail feathers to prop them up. Birds often specialize in a particular niche, such as a particular stratum of forest or type of vegetation. In