All birds construct nests in which to lay eggs and/or raise offspring. Traditionally, it was thought that natural selection and the requirement to minimize the risk of predation determined the design of completed nests. However, it is becoming increasingly apparent that sexual selection also influences nest design. This is an important development as while species such as bowerbirds build structures that are extended phenotypic signals whose sole purpose is to attract a mate, nests contain eggs and/or offspring, thereby suggesting a direct trade-off between the conflicting requirements of natural and sexual selection. Nest design also varies adaptively in order to both minimize the detrimental effects of parasites and to create a suitable microclimate for parents and developing offspring in relation to predictable variation in environmental conditions. Our understanding of the design and function of birds' nests has increased considerably in recent years, and the evidence suggests that nests have four nonmutually exclusive functions. Consequently, we conclude that the design of birds' nests is far more sophisticated than previously realized and that nests are multifunctional structures that have important fitness consequences for the builder/s.

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The reproductive biology of dinosaurs is of great interest, particularly in light of the many fossil eggs assigned to this group. The ultrastructural characteristics of dinosaur eggshells are examined in order to calculate water vapour conductance, which indicates the nesting environment. Data were mainly derived from the literature but new values are also presented. Allometric analyses were carried out on a variety of shell parameters against predicted egg mass, and comparison was made with allometric equations for bird eggs. Shell thickness was generally larger than seen for extant birds. Total pore number and pores per unit area were similar to values predicted from bird eggs. Total pore area showed an isometric increase with egg mass, parallel to the relationship for birds, but the constant value was an order to magnitude higher than the bird values. Pore radius was unaffected by egg mass. Water vapour conductance showed an allometric increase with egg mass, parallel to the bird values, but for any given egg mass values for dinosaurs were an order of magnitude higher. Mass‐specific water vapour conductance was unaffected by egg mass but was an order of magnitude higher than the bird values. Water vapour conductance per pore showed an allometric decrease with egg mass but again the predicted values were an order of magnitude higher than for bird eggs. The ultrastructural characteristics of dinosaur eggshells indicate that the nesting environment had to be saturated with water vapour and that dinosaur eggs had to be fully buried in a substrate. In this sense, therefore, dinosaur eggs resemble more those of modern reptiles than those of birds. As a consequence, maintenance of incubation conditions would have depended on the prevailing environment.

Aim The laying of eggs and the building of a nest structure to accommodate them are two of the defining characteristics of members of the class Aves. Nest structures vary considerably across avian taxa and for many species the design of the completed nest can have important consequences for both parents and their offspring. While nest characteristics are expected to vary adaptively in response to environmental conditions, large-scale spatial variation in nest characteristics has been largely overlooked. Here, we examined the effects of latitudinal variation in spring temperatures on nest characteristics, including insulatory properties, and reproductive success of blue tits, Cyanistes caeruleus, and great tits, Parus major. \ud \ud Location Great Britain. \ud \ud Methods Nests and reproductive data were collected from seven study sites, spread over five degrees of latitude. Then, the nest insulatory properties were determined before the nests were separated into either nest base material or cup lining material. \ud \ud Results As spring temperatures increased with decreasing latitude, the mass of the nest base material did not vary in either species, whilst the mass of the cup lining material and nest insulatory properties decreased in both species. This suggests that in response to increasing temperatures the breeding female reduces the mass of the cup lining material thereby maintaining an appropriate microclimate for incubating and brooding. The mean laying dates of both species advanced with decreasing latitude and increasing spring temperatures, although reproductive success did not vary. \ud \ud Main conclusions This is the first study to demonstrate that birds are systematically adjusting their nest structure in response to variation in ambient temperatures across large spatial scales. Therefore, nest composition reliably indicates environmental conditions and we suggest that studies of nest structure may be sentinels for the early signs of rapid climate change