Seasonal Variation in Cold Tolerance, Basal Metabolic Rate, and Maximal Capacity for Thermogenesis in White-Breasted Nuthatches Sitta carolinensis and Downy Woodpeckers Picoides pubescens, Two Unrelated Arboreal Temperate Residents

White-breasted Nuthatches Sitta carolinensis and Downy Woodpeckers Picoides pubescens are permanent residents in areas with cold temperate climates and are ecologically similar, but belong to different taxonomic orders. We measured basal metabolic rate (BMR) and peak metabolic rate (PMR = peak rate of cold-induced thermogenesis) for wild, free-living summer and winter acclimatised White-breasted Nuthatches and Downy Woodpeckers to determine seasonal and interspecific variation in cold tolerance and thermogenic ability. Both species were more cold tolerant and had significantly higher BMR and PMR in winter than in summer. Furthermore, during midsummer (early July to mid-August), nuthatches exhibited a significant decrement in cold tolerance and PMR relative to other seasons. A winter increment of metabolism, coupled with only minor seasonal variation in body mass and fat scores in both species, indicates that both species rely more on metabolic adjustments than gross morphological adjustments for winter acclimatisation. Woodpeckers and nuthatches exhibited roughly similar BMR and PMR at both seasons. The only significant interspecific differences in metabolic rates involved per-bird PMR in winter and midsummer, per-bird BMR in winter and summer (woodpeckers > nuthatches in both cases), and mass-specific PMR in summer (early/ late summer nuthatches > summer woodpeckers). Interspecific differences in cold tolerance and metabolic rates are probably explained, at least in part, by differences in body mass, as woodpeckers were significantly larger than nuthatches at all seasons. These data indicate that White-breasted Nuthatches and Downy Woodpeckers exhibit similar thermogenic performance and major phylogenetic effects on thermoregulation are not apparent.