How does the bird-nest incubation unit work?

The bird-nest incubation unit was proposed as a means of explaining the functionality of incubation in most avian species where the egg is in direct contact with the incubator. Subsequent work resulted in revision of this idea to suggest that large birds act as incubators and the nest is a location for incubation; by contrast, in small species the interaction between the bird and its nest played a key role in ensuring efficiency of incubation. This paper briefly reviews what we know about the incubation environment within nests and considers how small birds can contact-incubate eggs whilst ensuring appropriate levels of respiratory gas exchange. A hypothesis is proposed that the incubating bird does not 'sit tight' on its nest to maintain egg temperature at a minimal energy expenditure but, rather, it allows warmed air to escape from the nest cup. This convection flow draws air into the cup from the nest walls, providing oxygen for developing eggs and diluting carbon dioxide that they produce. Evidence supporting this idea of an 'open' nest during incubation is presented. Further experimentation is required to understand the environment within the nest cup better and this paper presents a hypothesis that can be tested in future studies. Investigations of how nests interact with their immediate environment should no longer be biased towards the effects of temperature on nest dimensions or composition. Instead, they should address all of the environmental factors that could affect the composition and function of a bird nest.