Internal Architecture Differentially Affects Thermal Insulation in the Walls of Common Blackbird Turdus merula Nests

Abstract. Nest construction may result in niche creation by the bird, i.e., modification of the immediate environment in terms of temperature or humidity. Here thermal insulation of nests of the Common Blackbird Turdus merula was investigated using a heat source placed inside the cup. The temperature profile of the nest wall was recorded using thermistors placed at different positions in the nest wall. Temperatures were affected by the layers in the wall with the grass cup lining and mud cup offering more insulation than the outer wall. Thermal conductance of the wall of Common Blackbird nests was relatively high compared to other reports for this variable in other passerine species from Australia. Variation among Common Blackbird nests in terms of temperature differences across layers, and thermal conductance overall, correlated with the thickness of the grass cup lining. Thermal insulation provided by a nest wall is dependent on the thicknesses and types of materials used in each part of the wall. Previous studies using nest deconstruction have demonstrated that the various layers in a wall have differing thermal characteristics. This is the first report of clear differences in the temperature profile across the different parts of the intact nest wall. Whether this pattern is also observed in nests of other species requires further investigation. The placement of different materials within the nest wall by the bird is not a random process and the results support the idea that the avian nest is involved in niche creation.