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Bird nests represent an extended phenotype of individuals expressed during reproduction and so exhibit variability in composition, structure and function. Descriptions of nests based on qualitative observations suggest that there is interspecific variation in size and composition but there are very few species in which this has been confirmed. For these species, data of the amounts of different materials indicate that nest construction behaviour is plastic and affected by a variety of factors, such as prevailing temperature, geographic location, and availability of materials. The lack of data on nest composition is hampering our understanding of how nests achieve their various functions and how different species solve the problem of building a nest that will accommodate incubation and allow successful hatching of eggs. This study deconstructed nests of four species of the Turdidae, four species of the Muscicapidae, and six species of the Fringillidae and quantified the size of the nests and their composition. These data were used to test: (1) whether nest size correlated with adult bird mass; (2) whether it was possible to distinguish between species on the basis of their nest composition; and (3) whether, within a species, it was possible to distinguish between the cup lining and the rest of the nest based on composition. Most but not all nest dimensions correlated with bird mass. Principal component analysis revealed species differences based on nest composition and discriminant analysis could distinguish cup lining from the outer nest based on material composition. Intraspecific variation in composition varied among species and in general fewer types of material were found in the cup lining than the outer nest. These data provide insight into how nests are constructed by the different species and in conjunction with studies of the mechanical, thermal and hydrological properties of a nest, will begin to reveal how and why individual species select particular combinations of materials to build a nest.

The various roles that birds’ nests play are widely discussed and there is increasing interest in developing a better understanding of how they achieve these roles. The insulatory properties of nests have been investigated in a variety of songbird species and can vary with temperature and latitude. However, data are only available for a limited number of species. Here, we study the variations of the insulatory properties of European Robin ( Erithacus rubecula) nests in conjunction with the morphology of the nests and composition of the materials used in construction. As well as quantifying the materials used to construct these nests for the first time, we test the hypothesis that there is a latitudinal effect on nest insulation. Robin nests have a cup structure that is quite distinct, and easily separated, from the nest's base. Although the cup and nest's base shared similar construction materials discriminant analysis showed that they can be distinguished on the basis of their differing proportions of leaves. Insulatory values correlated with the mass of leaves in the cup but not latitude. Unlike other songbirds of comparable size that nest in nestboxes, European Robins appear to be able to use plant-derived materials rather than animal-derived materials to effectively insulate their nests.

Animals that use materials to build nest structures have long since fascinated biologists and engineers alike. Avian nests are generally composed of collected materials brought together into a cup-like structure in which the bird sits to incubate eggs and, in many cases, it is where chicks are reared. Hence, the materials in a nest can be presumed to be loaded in compression, but relatively few studies have investigated the mechanical role of the nest elements and their position within the structure. However, recent studies on avian and primate nests have used engineering tests to quantify the structural properties of the nest elements to provide insight into how animals arrange the materials they collect and their mechanical role within the nest. In two studies, we deconstructed nests from Common Blackbirds (Turdus merula) and Bullfinches (Pyrrhula pyrrhula) into their component parts. We then used three-point bending tests to investigate regional variation in the structural properties of these elements within the nest. Results indicate that the birds use materials with different physical characteristics in various parts of the nest, thereby providing an indirect indication of nest-building behaviour in these species. We also used a novel cutting test, previously developed to measure the mechanical anisotropy of keratin, to investigate the mechanical behaviour of the mud-cup structure found in Common Blackbird and Song Thrush (Turdus philomelos) nests. This allowed us to determine whether these species utilise and arrange mineralised materials in a particular way to improve the toughness of their nests. The applicability of similar and other mechanical testing methods are discussed in relation to future investigations of the architecture and function of songbird nests.

Several methods have been used to determine the thermal properties of nests and in the last few years small iButton® temperature loggers have been used to determine an insulatory value. A recent study on Common Bullfinch ( Pyrrhula pyrrhula) nests used a different method, namely infrared (IR) thermography, to study nest wall insulation because pushing temperature loggers into the nest wall materials would have destroyed their structural integrity and probably adversely affected the insulatory value. Here, we compared the insulatory values of Common Linnet ( Carduelis cannabina) nests determined using temperature loggers (to record cooling rates in °C 20 s−1) with the temperature differences (ΔT in °C) across the nest wall as measured with IR thermography when the inside of the nest was heated under a steady state. Insulatory values were more variable than ΔT but there was no correlation between these two proxy measures of insulation. Despite being within the same family, Common Linnets build nests with very different materials from Common Bullfinches but ΔT values measured using IR thermography suggested that the nest wall of Common Linnet nests offered better insulation than the walls of Bullfinch nests. Our results suggest that comparative studies need to use the same methodologies but more research is needed to better understand the limitations of different methods in determining nest wall insulation.

Birds’ nests exhibit a considerable amount of plasticity in their size and construction. Indeed, several studies have shown that geographical location can affect this plasticity because local climatic conditions appear to affect nest-construction behaviours. This study examined nests collected in the UK and housed either at the University of Lincoln or at the British National Nest Reference Collection at the Hunterian Museum, University of Glasgow. Our aim was to increase the species diversity and the sample sizes for each species, over and above published studies. Mass and linear dimensions were recorded from nests of five finch species and two thrush species together with longitude and latitude for the nest site locations. An effect of species was shown for all finch nest dimensions measured but only for particular thrush dimensions. Latitude (range from 50.4 to 57.8°N) had no effect on nest measurements taken for thrush or finch species. However, while longitude (range from 6.8°W to 1.1°E) had no effect on thrush nests, it had a significant effect on the nest mass of finches, with nests constructed in the west being lighter in mass than those from the east. Previous studies examining the effects of latitude have used nests collected from different locations and built in a single year, which suggests that climatic conditions will be broadly comparable between locations. We found that museum collections have nests from a range of years and this rather limits their value in studies trying to ascertain the effects of location. Across different years there may be many unknown factors contributing to the patterns observed. We suggest that any future studies of the effects of climate on nest construction will have to rely on materials especially collected rather than using museum collections.

The behaviour of the incubating bird should be linked with the functional characteristics of its nest because they could minimise the incubation period that will reduce the risk of predation. Incubation attentiveness (percentage of daylight hours) in small passerines is a highly variable behaviour that is unrelated to egg mass. The thermal characteristics of nests are known to vary but whether they correlate with attentiveness, i.e. high attentiveness could reflect poorly insulated nests, has yet to be investigated. In this study, insulatory values were determined using temperature loggers for five species of British songbird in order to supplement equivalent data for six species from published sources. Attentiveness values for each species were determined from the literature. There was a significant negative correlation between attentiveness and insulatory values, which was unaffected by phylogeny. By contrast, attentiveness did not correlate with female body mass. That the avian incubation behaviour correlates with the functional properties of bird nests supports the concept of a bird–nest incubation unit, where reproductive success depends on both the bird and the nest structure. Further research should be directed towards a better understanding of this relationship for more species and, given the observed plasticity exhibited in nest composition and function, within species.

Recent studies are documenting the extent to which the mass and construction of bird nests varies between individuals and locations. In the Blue Tit ( Cyanistes caeruleus) and Great Tit ( Parus major), temperatures experienced by females during nest construction are inversely related to nest mass. Moreover, Mainwaring et al. ( Journal of Biogeography, 2012) showed that nests constructed at high latitude are heavier and better insulated than nests built by conspecifics in the south. Although mean spring temperature was used as a proxy for latitude in the Mainwaring et al. study, it remains untested whether individual birds build nests in response to a narrower range of temperatures experienced at the start of the breeding season. Our study showed that irrespective of latitude nest mass, and in particular nest cup mass, of Blue Tits and Great Tits was significantly affected by the temperature experienced by the birds for the seven days preceding clutch initiation. Similar results were seen with the insulatory properties of nests. The potential impact of variation in nest construction and insulation on subsequent incubation and chick-rearing behaviour is discussed.

Nest humidity is a function of how water vapour is retained within the nest. Previous analysis suggested that in general, nest humidity was largely unaffected by ambient humidity but there was no consideration of the type of nest involved. This study examined data collected from the literature to test the hypothesis that the relationship between nest and ambient humidity is a function of the type of nest. Data for water vapour pressure for the egg (PE), nest (PN) and ambient air (PA) were collected for 48 examples of nests from 44 species of bird from a range of Orders. Initial egg mass and water vapour conductance of the eggshell was also recorded for each species. Analysis showed that birds on scrape nests are able to raise PN above PA by on average 5 Torr irrespective of PA. By contrast, PN in cup nests was unaffected by PA, such that at low values nests are well insulated against water vapour loss. The balance between the proportion of water vapour lost from the egg to the ambient air that is controlled by the nest varied with the type of nest and PA. For eggs in scrape nests mass specific water vapour conductance showed a positive relationship with PA. By contrast, for eggs in cup nests the relationship was negative and under conditions of low PA the high PN means that mass specific shell conductance values have to be high so as to ensure the appropriate rate of weight loss from the egg. Whilst birds do not control nest humidity directly the inter-relationship between weight loss from an egg, its initial mass, its shell conductance, and the incubation period can now be extended to include the water vapour conductance of the nest wall.

This review details the six lineages of large flightless birds that evolved in the Late Cretaceous, Tertiary and Quaternary periods of geological time. Estimates of mass for each type of bird suggest that maximal mass is no greater than 500kg with most species attaining only 250–300 kg or less. By contrast, non-avian Archosaurs of the Mesozoic, and many mammal species of the Tertiary, attained great size with many species reaching several tonnes. Size has been limited in flightless birds because of the strength of the eggshell and in the largest species reproduction was only possible if the smaller males incubated. That reproductive characteristics limit mass in flightless birds suggests that truly gigantic non-avian theropods could not contact incubate their eggs and had to rely on environmental sources of heat energy to drive embryonic development. If fossil evidence ever arises to support proper contact incubation in a non-avian theropod then it is predicted that it will only be from a small (