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
British Birds Rarities Committee existed in Scotland. In 2009, this species was removed, along with white-billed diver and cattle egret. Once removed, a species is classed as a "scarce migrant": "British Birds" publishes a separate annual report summarising the occurrence of these species, and the data is also presented on the Scarce Migrants website. In addition to parrot crossbill, four species, Cetti's warbler, little egret, long-tailed skua and Mediterranean gull, have become so common that they no longer qualify even as scarce migrants. The committee has ten voting members, a chairman (who may be one of the ten, or may be another, non-voting individual)