Avian urate spheres :a non-invasive method to biomonitor environmental pollution and stress in birds

Abstract

Birds are commonly used as biomonitors of environmental pollution, with most tests involving invasive or destructive sampling techniques. The need to develop and validate non-invasive techniques has long been recognised. From blood, eggs, feathers or guano, the last shows most promise in this field. However it constitutes both faecal and urinary excretions. The faecal component has serious analytical drawbacks from digestive processes and being comprised of both bio-available and unabsorbed components. In contrast the typically white urine part of guano represents substances emanating entirely from within the bird. Despite the analysis of urine (urinalysis) being widely and successfully used in mammals, its limited application to date in birds is at best misguided because it disregards the nature of avian urine. This thesis endeavours to show how the analysis of the (normally discarded) solid component of avian urine may provide a quantifiable measure of both environmental pollutant exposure and endogenous stress hormone concentrations in birds. The literature is reviewed with regard to birds as biomonitors of the environment and the use of non invasive sampling techniques, especially excreta collected from wild animals including birds. Avian renal physiology and urine composition is described with specific reference to current avian urinalysis methods and how these compare with the proposed use of avian urate spheres (AUS) for biomonitoring. It is also shown how the biomineralisation process of AUS formation is relevant to their collection, extraction and chemical analysis from bird guano. To investigate if AUS contents could be used as a measure of a bird’s environmental pollution exposure, concentrations of lead, copper and zinc, were determined in urate spheres from domestic chickens (Gallus domesticus) exposed to a soil contaminated by these metals. Furthermore an attempt was made to compare metal concentrations in AUS with eggs, feathers and whole guano from the same birds. The results suggested AUS contained higher levels of the contaminating metals in exposed birds compared to control birds. However the aim to show the utility of AUS for biomonitoring the birds’ metal exposure was not achieved because of experimental design limitations. A similar investigation was carried out into the suspected exposure of nestling seabirds to elevated metal concentrations in their fish diet. Metal concentrations in urate spheres from the seabirds were measured along with those in various body tissues of their young. This metal analysis, although limited by small sample size, provided no evidence of an elevated exposure when compared with values reported in the literature. Subsequent reanalysis of earlier tested fish samples showed normal metal concentrations, suggesting the earlier reported fish data had been incorrect. To determine if AUS can be used to measure biologically relevant levels of the avian stress hormone corticosterone, a series of experiments is described using captive great tits (Parus major). These involved the ELISA detection of excreted corticosterone in AUS extracts. The suppressive response to dexamethasone administration was measurable in AUS from these birds, suggesting a physiological validation. However many issues have still to be resolved concerning this method of measuring corticosterone levels in birds. The overall finding of this thesis is that the analysis of AUS may have potential value as a noninvasive sampling method to biomonitor environmental pollution and stress in birds.