Temperature Regulation and Climatic Adaptation in Black-Billed and Yellow-Billed Magpies

Abstract

-The Yellow-billed Magpie (Pica nuttalli) and Black-billed Magpie (P. pica) are closely related and morphologically similar species. Yet because of their allopatric distributions, they experience different climatic regimes. We sought evidence of climatic adaptation in the thermoregulatory responses of these species by measuring their body temperature, oxygen consumption, and evaporative water loss at stable air temperatures between 10 and 400C. At temperatures below the thermoneutral zone (TNZ), both species had similar metabolic rates, thermal conductances, and body temperatures. Within the TNZ, metabolism of Blackbilled Magpies averaged 7.52 mW g-', which is 22% lower than that of Yellowbilled Magpies and 18% lower than that predicted allometrically for a 159-g passerine. Above the TNZ, Yellow-billed Magpies lost significantly more heat by nonevaporative pathways, became more hyperthermic, exhibited a smaller increase in oxygen consumption with increasing temperature, and were more heat tolerant than their congeners. Our data suggest that climate acts directly to restrict Black-billed Magpies to the Cold Type Steppe Dry Climate, rather than ecologically through a secondary effect on food availability. Climate may influence a species' range directly (i.e., physiologically) through its impact on thermoregulatory processes, and/or indirectly (i.e., ecologically) through its influence on vegetation and food availability. Because birds are highly mobile endotherms, climate is generally believed to affect their distribution through its ecological consequences. Recent studies indicate, however, that climate's physiological consequences can be major, even dominant, determinants of distribution for some species (e.g., Weathers and van Riper 1982). Generalizations concerning the relationship between distribution and physiological capacity are lacking, however, and additional studies, especially of closely related species from different climates, are needed to resolve this issue. North America's two magpie species are ideal subjects for such a study. Although their affinities and evolutionary histories are uncertain, the yellow-billed species probably evolved from the black-billed form (a mutant of the European Magpie, Pica pica pica, occurs that has a yellow bill and yellow skin on the bottoms of its feet [Linsdale 1937]). These species occupy similar niches, but because of their allopatric distributions (Fig. 1) are exposed to quite different climates. Black-billed Magpies (P. p. hudsonia) occur in sparsely wooded areas of western North America from northwestern Alaska southward to northern Arizona, New Mexico, and western Texas. Their range so closely overlaps that of the Cold Type Steppe Dry Climate (Linsdale 1937) that their distribution and abundance are clearly limited by high temperature and humidity (Bock and Lepthien 1975). Whether these climatic factors exert their effect directly or indirectly through their impact on food availability is uncertain. The range of the Yellow-billed Magpie (P. nuttalli) is limited to the Sacramento and San Joaquin valleys and to coastal valleys south of the San Francisco Bay in California. In the Sacramento Valley, its range is characterized by Hot Summer Mediterranean Type Climate (Linsdale 1937). MATERIALS AND METHODS