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
Hawking (birds) for certain kinds of aerial insect as a food source seems to correlate with gregarious or colonial behavior versus territoriality. For birds that take advantage of swarming insects, which are by nature found in local concentrations, colonial breeding can be a successful strategy. An example is the cliff swallow of western North America. Its relative the barn swallow hunts larger, non-swarming insects, and is more solitary. Certain neotropical tyrant flycatchers will join mixed-species foraging flocks, as will some Asian drongos. Such flocks stir up flying insects, which can then be picked off in quick sallies. Hawking (birds) Hawking is a