Insulation of Common Chaffinch Fringilla coelebs Nests is Largely Driven by Animal-Derived Materials in the Cup Lining

Abstract. Incubation of eggs and nestlings is energetically costly and may be affected by the amount of thermal insulation the surrounding nest provides. In many small passerine species, animal-derived materials, i.e., feathers and hair, are often used in the nest lining where, presumably, their lower thermal conductance offers better insulation. A previous study that involved partial deconstruction of thrush nests revealed that different parts of the nest confer different levels of insulation. The present study examined the insulatory values of Common Chaffinch Fringilla coelebs nests because of the high proportion of animal-derived materials in their cup lining but not the outer nest. Insulatory values and cooling rates of temperature loggers inside the cup were determined for whole nests and then only the cup lining. There were no significant relationships between nest measurements and measures of thermal insulation. However, removal of the outer nest wall reduced the insulatory value by around 10% despite the cup lining being less than half the thickness of the complete nest wall and the cup lining being only a third of the total nest mass. Differential placement of animal-derived materials within the cup lining of Common Chaffinch nests means that the birds seem to be able to confer a high level of insulation without expending too much energy searching for a large mass of particular nest materials.