Avian nests are crucial to reproduction as they are the site for incubation of the clutch of eggs and, in many cases, it is the location for the rearing of chicks hatched. Despite many years of reports of the characteristics of nests from many species our understanding of their functional properties has been very poor. Over the past few years, interest in the functional properties of nests has increased and this review brings together information regarding how materials used to construct the nest contribute to various functional properties. The review starts with a brief consideration of the various functions of nests but thereafter the focus is on the key roles that it plays during incubation. Details of the variety of materials used in nest construction emphasises the importance of quantitative data on the types and amounts of materials used in nests and how this information allows analysis that can improve our understanding of functional properties. Subsequent sections explore structural and thermal properties of nests in more detail. The roles that a nest can play in maintaining nest humidity and in weather-proofing the contents are also explored. The concept that the nest is an extended phenotype that has a role in niche creation is explored before the review concludes with a consideration of future directions for research.