School of BioSciences - Theses
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ItemAdaptations for temperature control in bird eggs: the role of reflectance( 2021)Early life stages are highly vulnerable in many animals. In birds, egg temperature must be maintained within an optimal range for successful development and survival. Chilling and especially overheating can lead to abnormal embryos and even be lethal. Bird embryos are effectively ectotherms because heat from incubation by parents is essential for embryo development. Therefore, parental behaviours are crucial for egg temperature regulation during the incubation period. Parents, however, face tradeoffs between incubation and self-maintenance. There are often periods of time when bird parents will leave the nest for foraging or territory defense etc., and eggs will be then left unattended. During this time, egg temperature will not be regulated by parents and will be affected by both extrinsic factors such as environmental conditions (air temperature, solar radiation and humidity) and the architecture of the nest; and intrinsic factors such as eggshell reflectivity and egg size. In this thesis I focused on studying the relationship between these factors and egg thermoregulation. Eggshell reflectivity is one of the most important intrinsic properties that may affect egg temperature. Wisocki et al. (2020) and Gomez et al. (2018) provided evidence that eggs tend to be darker in colder climates and higher latitudes. They suggested that darker colours are adaptive for eggs to retain heat in colder environments for thermoregulation. These studies, however, looked only at ultraviolet-visible wavelengths (300 – 700 nm). These wavelengths correspond to egg colour patterns and may play a role in camouflage, egg recognition, protection from DNA damaging light, in addition to temperature regulation. Due to the multiple potential functions of egg coloration, it can be difficult to isolate the role of specific selective pressures. By contrast, reflectance of near infrared (NIR) wavelengths (700 – 2500 nm) primarily affects temperature. NIR wavelengths account for more than half of the solar energy. In 1978 Bakken et al. (1978) found a uniformly high near infrared reflectance among 25 species (19 of them are ground nesting birds with cryptic eggs) and pointed out the potential importance of NIR reflectance for egg thermoregulation. However, since then there have been no subsequent studies examining the importance of NIR reflectance on bird eggs on a broader scale. In this thesis, I first aimed to study the macro-evolutionary relationships between eggshell reflectivity, climate and nest type in birds. We predicted that eggs would have higher eggshell reflectivity in hot climates, and species with eggs that are exposed would have higher eggshell reflectivity. Using a phylogenetic comparative approach among 186 species of Australian birds we found that eggs tend to have higher total reflectivity in hotter climates with higher solar radiation. Moreover, species with open nests (i.e. with potentially higher exposure to direct sunlight) had higher NIR reflectivity than species with closed nests. This suggests that eggshell reflectivity, especially NIR reflectivity, may play an important role in egg thermoregulation. Second, we investigated how egg size, reflectivity and exposure interact to affect egg temperature. We predicted that high eggshell reflectivity and shade can protect eggs from reaching lethal temperature compared to eggs with low eggshell reflectivity and without shade. We used a biophysical model to predict egg temperatures in a hot environment (Kimberley, Western Australia). The results showed that eggs with higher reflectivity exceeded the critical thermal limit for less time and did not reach temperatures as high as eggs with lower reflectivity. The amount of sunlight eggs are exposed to (shade levels in the model) also strongly affected egg temperature. These results indicate that eggshell reflectivity and exposure to sunlight are likely to be important for egg temperature regulation during incubation, particularly in hot environments. Reflectivity had a greater effect on the temperature reached by large eggs than small eggs, even though small eggs had a higher heating rate. Together, this thesis highlights the importance of eggshell reflectivity on egg thermal control and contributes to our understanding of the macro-evolution of reflectivity in bird eggs.