Four species of songbird, Great Tits Parus major, Blue Tits Cyanistes caeruleus, Pied Flycatchers Ficedula hypoleuca and Wood Warblers Phylloscopus sibilatrix, have nests that have similar insulatory properties, but are made of widely differing materials. This study used a novel technique to investigate the role of air-gaps between component materials of the nest wall on the insulatory properties of the whole nest. Nests of four passerine species were investigated by measuring the rate of warming of a temperature logger within the wall materials before and after vacuum-packing, which removed the air and compressed the nest. After vaccum-packing nest volume was estimated to have decreased by 90% and the measured insulatory values decreased by an average of 20% irrespective of the nest materials. Nest maintenance behaviors that may serve to ensure that air-gaps are maintained within the nest wall, which perhaps maximises its insulatory properties, are discussed.