Air movement affects insulatory values of nests constructed by Old World Warblers

Avian nests provide a location for incubation, and in many cases rearing of nestlings. These energetically demanding activities have meant that nest insulation has been the subject of many studies but few to date have dealt with how variation among species in nest construction materials could affect nest insulation. This study investigated the insulation of nests constructed by three species of Old Word warblers (Sylviidae), which vary in size and composition. Insulatory values, i.e. difference in cooling rate of temperature loggers placed inside and outside a nest, and internal cooling rates within the nest cup were determined using temperature loggers under still-air and moving-air conditions. Insulatory values determined in still-air conditions of a laboratory were significantly different among nests of the different species but not when the values were determined within the smaller volume of a wind tunnel. Moving-air increased insulatory values by an order of magnitude but also increased internal cooling rates in all species. Insulatory values were positively correlated with the nest base thickness. Moving-air increased the cooling rate of the external temperature logger much more than the internal logger, which inflated the insulatory value of a nest wall in moving-air. Reasons for these results may reflect the thermal properties of the materials individually or in combination. Future testing of nest insulation should be under standard conditions that limit air movement but the role of the nest location in situ should be investigated in future research.