Factors Affecting Thermal Insulation of Songbird Nests as Measured Using Temperature Loggers.

AbstractBird nests represent an extended phenotype that is variable among and within species in terms of nest location and construction materials. It has also been suggested that nests indicate niche construction, although empirical evidence to support this is lacking. The nest wall is often considered to confer some insulation because this would help minimize the energetic expenditure by the incubating adult. However, it has been previously suggested that nests are constructed primarily for their structural role rather than for insulation, but to date, studies have used a variety of techniques to investigate the insulation of nest walls but only for relatively few species. This study used temperature loggers to determine insulatory values of nests for 16 previously undescribed species and to replicate values for eight previously described species. In addition, data for nest wall insulation that were determined using temperature loggers have been collated for a total of 32 different passerine species. The effects of nest dimensions, mass, and composition on these values were examined. Base thickness, but not wall thickness, and nest mass significantly positively affected insulatory values. This study found that the proportions of feathers and moss in the nest wall significantly positively correlated with insulatory values. This suggests that there is a key role for nest materials in determining insulation, which provides empirical evidence that nest building constitutes niche construction. The data will also help us interpret data from future studies of the thermal properties of nests. Ultimately, we need to develop our understanding of the role of insulatory values in the evolution of nest function.

Infrared thermography provides insight into the thermal properties of bird nests.

Thermal properties of nests have been investigated using a variety of techniques. Infrared (IR) thermography has the advantage of being a non-invasive technique allowing the integrity of the nest wall to be retained during measurement. This study investigated the insulative properties of nests of the Eurasian Bullfinch (Pyrrhula pyrrhula), the Common Blackbird (Turdus merula) and the Song Thrush (Turdus philomelos) using IR thermography. Nests were inverted over a heat source and the temperature of the external nest surface was recorded. Bullfinch nests were less insulated than thrush nests. Including foil inside the nest cup decreased the amount of convection through the open walls of Bullfinch nests. Removal of the outer nest and cup lining of thrush nests only slightly decreased the degree of insulation offered by the nest indicating an important insulative role for the substantial 'mud cup' in these nests. The results suggested that the nest wall is not sealed and convection currents may a play a significant role in nest insulation. In conjunction with a steady-state heat source IR thermography is useful in assessing the insulative properties of bird nests.