Capsule For nest construction by Blue Tits, but not Great Tits, first‐egg date (FED) and air temperature significantly affected the mass of the nest as a whole and some of its component parts. Aims To test the hypothesis that use of nest materials is influenced by prevailing climatic conditions during nest construction. Methods Nests used in the study were built by Blue Tits Cyanistes caeruleus and Great Tits Parus major in nestboxes at a site in Lincolnshire, England during the 2008 and 2009 breeding seasons. Nests were dissected into their component parts and then weighed. Results Stepwise discriminant analysis showed that the masses of grasses, feathers and bark were significantly affected by species (all higher in Blue Tits) and year significantly affected the mass of wool and dust in the nests. anova showed that total mass of the nest was not significantly affected by year of construction or species. By contrast, species, but not year, did significantly influence the masses of animal‐ and plant‐derived materials in the nest. In Blue Tit nests there were significant correlations between FED and the mass of animal‐derived material in 2008, but with plant‐derived material in 2009. There were significant correlations between mean air temperature recorded during the seven days up to FED and the mass of the nests and their plant‐derived materials. No significant correlations were observed between FED and nest components for Great Tits. Conclusion Nest construction is potentially affected by a variety of environmental factors, which may impact upon how nests function. A better understanding of how nest variability affects its function may allow better assessment of how climate change may impact upon the reproductive performance of birds.

A sexually mature individual of Darwinopterus preserved together with an egg from the Jurassic of China provides direct evidence of gender in pterosaurs and insights into the reproductive biology of these extinct fliers. This new find and several other examples of Darwinopterus demonstrate that males of this pterosaur had a relatively small pelvis and a large cranial crest, whereas females had a relatively large pelvis and no crest. The ratio of egg mass to adult mass is relatively low, as in extant reptiles, and is comparable to values for squamates. A parchment-like eggshell points to burial and significant uptake of water after oviposition. This evidence for low parental investment contradicts the widespread assumption that reproduction in pterosaurs was like that of birds and shows that it was essentially like that of reptiles.

Recent fossil finds in China and Argentina have provided startling new insights into the reproductive biology and embryology of pterosaurs, Mesozoic flying reptiles. Nineteen embryos distributed among four species representing three distinct clades have been described and all are assumed to be at, or near, term. We show here how the application of four contrasting quantitative approaches allows a more precise identification of the developmental status of embryos revealing, for the first time to our knowledge, the presence of middle and late developmental stages as well as individuals that were at term. We also identify a predicted relationship between egg size and shape and the developmental stage of embryos contained within. Small elongate eggs contain embryos at an earlier stage of development than larger rounder eggs which contain more fully developed embryos. Changes in egg shape and size probably reflect the uptake of water, consistent with a pliable shell reported for several pterosaurs. Early ossification of the vertebral column, limb girdles and principal limb bones involved some heterochronic shifts in appearance times, most notably of manus digit IV, and facilitated full development of the flight apparatus prior to hatching. This is consistent with a super-precocial flight ability and, while not excluding the possibility of parental care in pterosaurs, suggests that it was not an absolute requirement.

Incubation of alligator eggs at 30°C produces 100% females, at 33°C 100% males; temperatures in between produce varying sex ratios. Wild nests of Alligator mississippiensis show similar effects and the populations are biased towards females. The incidence and patterns of temperature dependent sex determination (TSD) in other crocodilians are reviewed. Temperature also affects hatchling size and pigmentation patterns, post-hatching growth rates and thermoregulation by juvenile crocodilians. The significance of temperature sensitive periods defined by temperature shift experiments is questioned in relation to a hypothesis to explain the mechanism of TSD in crocodilians. It is postulated that there is an initial sex differentiation mechanism which involves a quantum period of time and a threshold for a dose of a male determining factor. The conditions for induction of males are precise but exhibit variation between individuals within the population. Females develop by default. The hypothalamus may have an important role in a later sex differentiation mechanism. The hypothesis is used to explain the late temperature sensitive periods defined by high to low temperature shift experiments, why cooler temperatures are more effective at determining sex, how intermediate temperatures can produce both sexes, the differences in the pattern between turtles and crocodilians and geographical similarities in the pattern of TSD within crocodilians despite differing climates. The phylogenetic advantages of TSD in crocodilians are concerned with the overall reproductive strategy of the animals. Those crocodilians which are incubated and grow to maturity under optimal environmental conditions will be both large and male. Larger males are more likely to produce more offspring. A review of the effects of the environment on sex determination in amphibians and fish suggests that there is a general relationship between size and sex in vertebrates