A challenge of warmer temperate winters: long-term exposure to rain in cold temperatures drives higher thermogenic investment in a captive songbird

Abstract Long-term cold exposure induces many endotherms to invest in metabolic heat production, but how precipitation impacts thermogenic capacity in the context of different thermal conditions is largely undescribed in animals. In this study, songbirds (red crossbills; Loxia curvirostra) were held in warm (21°C) or cold (6°C) temperatures and either dry or rain precipitation treatments while experiencing a more northern or southern latitude photoperiod. We hypothesized that individuals experiencing winter rain would increase investment in thermogenic capacity, metabolic machinery and catabolic enzymes to cope with increased thermogenic demands. Food intake and activity were monitored weekly, summit (Msum) and basal metabolic rates were measured in November, January and February, and tissues were collected at the end of the study. Individuals held in cold rain treatments achieved the highest Msum and had higher food intake, subcutaneous fat, heart mass and metabolic enzyme activities. Furthermore, birds in dry conditions showed slight disinvestment of Msum in late winter, while birds in rain treatments maintained thermogenic investment. Our results suggest that cold rain induces increased investment in thermogenic capacity across the winter season. Thus, rain may offset the potential thermal benefits that warming winters would otherwise provide to small-bodied endotherms.