Geographic Variation in the Clutch Sizes of Seven Owl Species

GEOGRAPHIC variation in clutch size and its possible adaptive significance have been examined by many authors, most notably Moreau (1944), Lack (1947, 1948, 1954, 1966, 1968), Skutch (1949, 1950, 1967), WynneEdwards (1962), and Klomp (1970). Although these authors examined a broad spectrum of orders, the majority of data are for passerines. This paper reports an examination of clutch size in owls. In many species an increase in mean clutch size is noted as one moves toward the poles and from 'the coast inland. The north-south trend in the New World has been examined in several species, including Common Eiders (Somateria mollissima) (Paynter 1951), Song Sparrows (Melospiza melodia) (Johnston 1954), Black-capped Chickadees (Parus atricapillus), and Carolina Chickadees (P. cardinends) (Brewer 1963). The east-west trend in North America has been studied in Song Sparrows (Johnston 1954), House Finches (Carpodacus mexicanus) (Wagner 1957), and Longbilled Marsh Wrens (Telmatodytes palustris) (Verner 1965). Among the New World Strigidae, Snowy Owl (Nyctea scandiaca) clutch size and its relation to food abundance was reported by Pitelka et al. (1955), while the north-south and east-west trends were examined in detail only for the Great Horned Owl (Bubo virginianus) by Henny (1972). Other references to geographic trends in the clutch sizes of New and Old World owls were made by Moreau (1944), Lack (1947, 1948, 1954, 1966, 1968), von Haartman (1954, 1971), Klomp (1970), and Foster (1973). von Haartman (1971) presents a graph plotting average clutch sizes of several species against latitude ranging from 70 ø N to 40 ø S. His data were obtained "from various local faunas and handbooks." The species examined are not indicated. The most widely known theory to account for the increase in clutch size with an increase in latitude is that proposed by Lack (1947, 1948) and applies primarily to temperate, r-selected passetines on mainlands. This hypothesis is that clutch size is adapted to the largest brood size for which the parents can find food. Longer daylength in northern latitudes during the breeding season allows more time for food collection, and thereby the support of more young. This explanation is perhaps adequate for diurnal passerines, but would not explain similar trends among nocturnal species. However such trends have been reported (von Haartman 1971), suggesting that the problem is more complex. The