Comparison of Two Methods for Determination of the Insulation of Passerine Nest Walls

Abstract

Several methods have been used to determine the thermal properties of nests and in the last few years small iButton® temperature loggers have been used to determine an insulatory value. A recent study on Common Bullfinch (Pyrrhula pyrrhula) nests used a different method, namely infrared (IR) thermography, to study nest wall insulation because pushing temperature loggers into the nest wall materials would have destroyed their structural integrity and probably adversely affected the insulatory value. Here, we compared the insulatory values of Common Linnet (Carduelis cannabina) nests determined using temperature loggers (to record cooling rates in °C 20 s⁻¹) with the temperature differences (ΔT in °C) across the nest wall as measured with IR thermography when the inside of the nest was heated under a steady state. Insulatory values were more variable than ΔT but there was no correlation between these two proxy measures of insulation. Despite being within the same family, Common Linnets build nests with very different materials from Common Bullfinches but ΔT values measured using IR thermography suggested that the nest wall of Common Linnet nests offered better insulation than the walls of Bullfinch nests. Our results suggest that comparative studies need to use the same methodologies but more research is needed to better understand the limitations of different methods in determining nest wall insulation.