Summary1. The developmental environment plays a key role in determining offspring phenotype, and the parents' behaviour and physiology often dictates developmental conditions. Despite the plethora of studies documenting the importance of incubation temperature on offspring phenotype in reptiles, very few studies have examined such relationships in birds. 2. Because nearly all birds physically incubate their eggs, altering the nest environment may be an important but previously overlooked way parents can influence their offspring's phenotype. Here, we tested the hypothesis that incubation temperature would affect thermoregulation in wood duck (Aix sponsa) hatchlings. 3. We show that a reduction in <1°C in incubation temperature affects the metabolic costs of thermoregulation in offspring of a non-domesticated bird, resulting in 27-40% greater increases in oxygen consumption of ducklings incubated at the lowest temperature relative to ducklings incubated at higher temperatures.4. Because we demonstrate that incubation temperature affects hatchling phenotypic quality, our findings provide novel support for newly proposed frameworks that highlight the importance of incubation temperature to the evolution of clutch size in birds.

Summary1. Thermoregulation and pathogen resistance are two energetically demanding processes that co-occur during seasonal epidemics for many endothermic vertebrates. The ability of hosts to cope with these processes simultaneously may influence population-level disease dynamics. 2. In North American house finches (Carpodacus mexicanus), outbreaks of the bacterium Mycoplasma gallisepticum occur during fall and winter, when ambient temperatures across the host's range are often below thermoneutrality. Here, we examined how ambient temperature influences host energetics and susceptibility to this naturally occurring seasonal pathogen by experimentally infecting wild-caught house finches with M. gallisepticum at either thermoneutral or subthermoneutral temperatures in the laboratory. We quantified the metabolic costs of infection, measures of body condition, two components of the acute-phase response, disease expression and pathogen loads under both temperature regimes. 3. The metabolic costs of simultaneous infection with M. gallisepticum and thermoregulation were additive and significant (combined costs of 4AE71 kJ per night; within the range of the daily energy requirements of passerine moult). Contrary to our predictions, house finches at subthermoneutral temperatures had lower disease expression and higher circulating levels of the cytokine interleukin-6 in response to experimental infection with M. gallisepticum than finches at thermoneutral. However, pathogen loads did not differ between the two temperature treatments. Finches from both treatments expressed fever in response to infection, but the magnitude of fever did not vary with ambient temperature. 4. Despite the significant energy costs of infection and thermoregulation, house finches from both temperature treatments maintained body mass and pectoral muscle condition, suggesting that birds housed at subthermoneutral consumed more food to maintain energy balance. In the field, competition for finite resources would be expected to exacerbate the effects found here and force infected birds to spend more time at feeders where M. gallisepticum is transmitted. 5. Overall, our results indicate that moderate cold stress alters house finch immunity, energetics and disease pathology, but does not alter infectiousness as measured by pathogen load. The effects of ambient temperature on host response and energy demands could directly or indirectly contribute to seasonal and geographical variation in disease dynamics in free-living house finches. More broadly, our results suggest that even subtle changes in abiotic factors such as temperature can alter host disease expression, with broad implications for disease dynamics.