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

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Abstract
The yellow-billed magpie (P. nuttalli) and black-billed magpie (P. pica) are closely related and morphologically similar species. Because of their allopatric distributions, they experience different climatic regimes. Climatic adaptation in the thermoregulatory responses of these species was studied by measuring their body temperature, O2 consumption and evaporative water loss at stable air temperatures between -10.degree. and 40.degree. C. At temperatures below the thermoneutral zone (TNZ), both species had similar metabolic rates, thermal conductances and body temperatures. Within the TNZ, metabolism of black-billed magpies averaged 7.52 mW g-1, which is 22% lower than that of yellow-billed 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 O2 consumption with increasing temperature and were more heat tolerant than their congeners. 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.