The Effects of Nestbox Thermal Environment on Fledging Success and Haematocrit in Tree Swallows

Abstract

Nest microclimate, particularly temperature, can affect energy balance of both parents and offspring. Here I investigate patterns of correlation between internal temperatures of artificial nestboxes and fitness in Tree Swallows (Tachycineta bicolor). Nests with cold overnight temperatures were less likely to fledge offspring. Conversely, nests with high daytime temperatures were also less likely to fledge offspring. In particular, nest success declined when internal nestbox temperatures exceeded 35°C. To investigate a potential physiological cause for reduced fitness, I tested whether haematocrit, a rough indicator of blood volume, was related to warmer nest temperatures. Whilst variable, nests with higher temperatures had higher average haematocrit of nestlings on day 13. This may indicate greater evaporative heat loss reflected in lower plasma volumes. These results as a whole indicate that temperatures outside the thermo-neutral zone may influence energy balance of both parents and offspring and may have an important effect on fitness.