Glucocorticoid responses to capture, captivity and high environmental temperatures among southern African birds

Abstract

Analyses of steroid hormones such as glucocorticoids (GC) are widely used to understand how animals respond to environmental perturbations. Circulating GCs increase in response to a wide range of stressful stimuli in birds. The magnitude and duration of GC elevations can be used to determine the severity of a given stressor. My thesis focuses on GC responses to activities associated with the welfare of birds in research i.e. capture and confinement in captivity, to gain insights into how the welfare of birds involved in ornithological studies can be improved. I also evaluated a method for quantifying avian GC responses to high environmental temperatures in desert birds, in the context of research on how species are likely to be affected by global heating. To evaluate GC responses to capture in mist nets among southern African birds, I measured GC concentrations in five common bird species representing three orders: dark-capped bulbul (Pycnonotus tricolor), southern masked-weaver (Ploceus velatus), Karoo thrush (Turdus smithi), laughing dove (Spilopelia senegalensis), and speckled mousebird (Colius striatus) during the austral summer. Three of the species were also sampled in winter to determine whether GC concentrations vary with season. Both baseline and stress-induced (subjected to the capture-restraint protocol) GC concentrations were measured in blood samples collected from the brachial vein of each sampled bird. I found significant interspecific differences in GC concentrations among my study species. However, season had no influence on GC concentrations in this study. Given the invasive nature of blood sample collection, bird droppings are often used as a matrix for quantifying chronic (long-term) GC responses. However, the species-specific nature of GC metabolism necessitates the need to validate this method initially. I conducted ACTH challenge tests on three species: laughing dove, fork-tailed drongo (Dicrurus adsimilis) and white-browed sparrow-weaver (Plocepasser mahali) to determine whether changes in fGCM concentrations are correlated with changes in circulating GC and identified enzyme-immunoassays (EIAs) suitable for measuring fGCM concentrations in these species. A tetrahydrocorticosterone EIA, utilizing antibodies against 5ß-pregnane-3α, 11ß, 21-triol-20-one metabolites emerged as suitable for both passerine species, whereas a 5α-pregnane-3β, 11β, 21-triol-20-one EIA measuring 3β,11β-diol-cortisol metabolites emerged as suitable for fGCM analysis in the doves. Using the validated EIAs, I evaluated the effect of housing condition on fGCM concentrations in laughing doves by measuring fGCMs in birds housed in outdoor aviaries versus indoor cages. Birds housed in aviaries had significantly lower fGCM concentrations compared to birds in cages. In my last data chapter, I evaluated the potential of using captive birds to quantify fGCM responses to environmental temperatures. Maximum daily air temperatures (Tmax) at 32, 37 and 42 °C did not influence fGCM concentrations in both white-browed sparrow-weavers and southern pied babblers (Turdoides bicolor) exposed to experimentally manipulated thermal environments. This unexpected observation may have been influenced by several factors including food and water availability in captivity and responses to capture and confinement in captivity. In conclusion, I found significant interspecific differences in GC concentrations in a subset of southern African birds. My observations on captive birds suggest that stress-related studies on birds are best conducted in free-living individuals. However, given their elusive nature, it is difficult to collect serial samples from free-living birds. Thus, more studies evaluating the feasibility of non-invasive sampling methods are necessary to improve the welfare of research birds in captivity.