Thermoregulation in breeding crowned plovers (Vanellus coronatus)
Ground-nesting birds nesting in the open have to cope with an extreme thermal environment. Their nests egos and chicks are often located within the boundary layer and are exposed to, direct solar radiation. In preventing eggs and chicks from overheating, adult incubating birds expose themselves to severe heat loads. This study looked at two aspects of this system. Firstly, we examined the role of behaviour in the thermoregulation of breeding crowned plovers (Vanellus coronatus). We then examined the development of independent thermoregulation in hatchling crowned plovers. This study assessed the role of shading behaviour in the thermoregulation of incubating crowned plovers (Vanellus coronatus). Shading behaviour was shown to have no direct benefit for eggs, at times even causing eggs to rise close to lethal levels. Instead, shading behaviour played an important role in maintaining incubating bird temperatures at a constant level. We therefore suggest changing the term "shading" to "standing" behaviour. In addition, core body temperatures of free-ranging adult incubating birds and their eggs were examined on both a daily and a seasonal scale. A range of core bird temperature of 8.8° C was measured during the course of the study. Both daily and seasonal differences in core bird temperature occurred. Daily differences are a result of daily circadian rhythms while seasonal differences are explained by changes in ambient temperature. Both daily and seasonal differences in egg temperatures also occurred. The concept of optimum incubation temperature for a species is therefore reconsidered. Differences between and within species may not be as a result of phylogenetic differences as previously reported, but may be as a result of relatively short-term changes in response to ambient temperature changes. The validity of using taxidermic models in thermoregulatory studies was assessed by comparing data collected using both real and model birds. No direct correlations were found between real and model eggs and birds. This suggests that the patterns seen in models may not adequately reflect short term changes that occur in the real system, thus reducing the ability to use such data to make broad generalizations about thermoregulation in general. The models are, however, useful in providing insight into the heat load an animal carries under different environmental conditions, and in estimating the overall, long-term effects of metabolic heat production in a real bird and egg. Secondly, we looked at the development of thermoregulation in a ground- nesting shorebird. An issue of general importance with regard to the ontogeny of precocial chicks is the timing of the onset of independent thermoregulation. We wanted to determine the mass at which crowned plover (Vanellus coronatus) chicks are able to thermoregulate independently in both the laboratory and the field, and to compare this to predicted values previously reported. Laboratory reared chicks attained independent thermoregulation at a mass of 65 g, while chicks in the field did so at a mass of 55 g. This is 33 - 39% of adult body mass, or approximately twice that predicted for a 167 g shorebird chick. Development of homeothermy represents a trade-off between growth rate and development of thermoregulation. Efficient use of behavioural thermoregulation, coupled with predation risks, allows more energy to be channelled into growth early on in the development of crowned plover chicks. Development of homeothermy in anyone taxonomic group may not, therefore, always be described by a single function. Instead , differences reflect a balance between growth rate, development of physiological thermoregulation, use of behavioural thermoregulation, predation risk, and environmental factors.