Geographic-Variation in Egg Size, Clutch Size and Date of Laying of Australian Raptors (Falconiformes and Strigiformes)

Abstract

Approximately 5000 clutches of eggs, and associated data, in oological collections and the field, were used to describe date of laying, egg size and clutch size of all 24 diurnal raptors, and eight owls occurring in Australia. Within families, smaller species tended to have larger clutches and to lay later than larger species. Several patterns in geographic variation were detected. For several species, date of laying occurred about 2-3 days later for every degree of latitude southwards. Weak clinal variation in clutch size was found for ten species: clutch size of some Australasian falcons varied with latitude; that of the true eagles and three milvine kites varied with longitude. Weak clinal variation in egg size was found for ten species, seven conforming to Bergmann's latitudinal rule for body size. Some evidence was found for: the existence of a contested subspecies, the small northern subspecies of Brown Goshawk Accipiter fasciatus didimus, north of 23°; the subspecific separation of the Osprey into the northern race Pandion haliaetus melvillensis, and southern race P. h. cristatus with larger, broader eggs; the subspecific separation of the large Tasmanian Wedge-tailed Eagle Aquila audax fleayii from the mainland population; and the recognition of a small, western race of the Brown Falcon Falco berigora occidentalis.