Interacting effects of temperature and humidity on physiological and behavioural heat dissipation among birds in mesic eastern South Africa

Abstract

Heat-related wildlife mass mortalities are increasingly reported as extreme heat events become more common globally. Existing research on the proximal causes of these mortalities (e.g. whether animals die of hyperthermia or lethal dehydration) has so far failed to explore the role of humidity. During one such mass mortality event affecting birds and bats in northeastern South Africa in late 2020, nearly half the documented avian fatalities belonged to the blue waxbill (Uraeginthus angolensis). Blue waxbills’ apparent sensitivity to the hot, humid conditions prevailing on the day suggest this as a useful model species to investigate how interactions between temperature and humidity may influence avian thermal tolerance limits. In chapter 1, I quantified blue waxbill body temperature, resting metabolic rate and evaporative water loss at air temperatures (Tair) approaching and exceeding normothermic Tb in dry (1.1 ± 0.9 g H2O m-3) or humid (21.3 ± 0.4 g H2O m-3) air. The humid treatment was associated with significant reductions in evaporative cooling capacity and overall heat tolerance, with maximum tolerable Tair ~2 ⁰C lower in the humid (45.7 ⁰C) vs the dry air treatment (47.9 ⁰C). These results support the view that lethal hyperthermia was the cause of blue waxbill mortalities during the 2020 heat event and reveal how humidity increases the likelihood of avian mortality during extreme heat events. The heat event additionally identified that species within a community may have variable sensitivities to temperature and humidity. Apart from physiological responses, birds can also use behavioural heat dissipation behaviours such as panting, shade-seeking and wing-drooping to assist the regulation of body temperature during periods of high air temperatures (Tair). In chapter 2, I aimed to: (1) assess interspecific variation in panting, shade-seeking and activity behaviours (hereafter referred to as heat dissipation behaviours; HDB) in response to Tair and humidity and (2) identify if blue waxbills are more sensitive relative to other species. Additionally, I aimed to (3) investigate whether HDB indices (pant50, shade50, act50, which are the Tair where 50% of individuals of a species are observed to pant, seek shade and be active respectively) are correlated with diet, drinking habits and body mass. Lastly, (4) waxbills were used as a model species to identify potential costs to foraging efficiency when panting and shade-seeking. The avian community displayed considerable interspecific variation within and across HDB. Although humidity did not consistently increase the rates of HDB across species, the existence of humidity main effects and some significant interactions between Tair and humidity in predicting the occurrence of HDBs suggest that humidity plays an important, species-specific role in behavioural thermoregulation amongst species in this community. I additionally found that HDB indices were not significantly predicted by organismal traits, further demonstrating the complexity of avian behavioural thermoregulation within hot environments. Blue waxbills were not clearly more sensitive to environmental conditions relative to other species but did appear to incur reduced foraging efficiency when exploiting HDB.