Geographic Variation in Wing-loading of the House Sparrow

Intraspecific variation in size of widely distributed birds often agrees with modern interpretation of Bergmann’ s Rule; that is, size is correlated with a combination of climatic variables that includes temperature and/or moisture (James 1968, 1970, Power 1969). N or th ern races of birds are frequently larger (Mayr 1963:320), but the adaptive significance of increased size has been widely debated (Scholander 1955, Mayr 1956, Salt 1963, Kendeigh 1969, McNab 1971). In general, the word size in ornithological literature refers to wing length, which is often an accurate indicator of body size (James 1970). Wing length varies independently of, or inversely with, body weight in some species (Snow 1958, Hamilton 1961, Rand 1961). Selection for larger body mass without a concomitant increase in wing length could result in conflict between opposing selective forces, particularly in species that depend on flight for foraging or escape from predators. Large birds with relatively small wings will obviously have greater wing-loads and may approach an upper limit at which flying becomes energetically expensive or restricts the ability of the bird to escape predation. In this paper I describe geographic trends in wing-loading in North American populations of the House Sparrow, Passer domesticus, and assess the importance of upper limits of wing-loading using data on the flying ability of experimentally loaded birds.