Direct effects of increasing temperatures on birds

Abstract

Increasing air and sea temperatures are driving global ecological change. Here, I explore the direct effects of increasing temperatures on birds, which has thus far been studied less than the indirect effects. In Chapter 1, I review the literature to examine adaptations that enable birds to maintain core body temperatures in response to increasing heat loads, before summarising the effects that exposure to high temperatures can have. I also discuss the avenues for potential research in this field and highlight the importance of incorporating direct effects of increasing temperatures into future prediction models. In Chapter 2 I investigate the impact of increasing temperatures on two closely related pelagic birds, constrained to come to land to breed in areas with differing temperature regimes. Specifically I quantify the proportion of time that northern gannets Morus bassanus (breeding in the boreal north Atlantic) and cape gannets Morus capensis (breeding in the Mediterranean biome of South Africa) spend gular fluttering (a proxy for heat stress), and examine the consequence of this for evaporative stress. I found that whilst gular fluttering did not correlate with humidity, it did increase asymptotically with ambient temperature. However, the slope and intercept vary between different age categories and across species, with the onset of gular fluttering occurring at lower ambient temperatures for younger birds and northern gannets in general. I also find that evaporative water loss increases with gular fluttering for all ages and across species, with higher rates of water loss found for the heavier northern gannets. Our predictive models show that rates of both gular fluttering and evaporative water loss will increase based upon climate change projections and subsequently we can conclude that by 2100 increasing temperatures will have seriously impacted both northern and cape gannets, particularly birds constrained to the nest. Impacts are likely to be more severe for the northern gannets due to the greater change in ambient temperatures predicted at higher latitudes and the bird’s lower gular fluttering onset thresholds and high rates of evaporative water loss. The findings presented here support the theory that increasing temperatures pose a serious and direct threat to birds and that the associated negative impacts may occur more rapidly than indirect effects. They also highlight the need for greater research into spatial and temporal patterns of heat stress, both within and between species. Finally, we propose the incorporation of the direct effects of increasing temperatures into future prediction models.