Heat dissipation behaviour of birds in seasonally hot, arid-zones: are there global patterns?

Quantifying organismal sensitivity to heat stress provides one means for predicting vulnerability to climate change. Birds are ideal for investigating this approach, as they display quantifiable fitness consequences associated with behavioural and physiological responses to heat stress. We used a recently developed method that examines correlations between readily-observable behaviours and air temperature (Tair) to investigate interspecific variation in avian responses to heat stress in seasonally hot, arid regions on three continents: the southwestern United States, the Kalahari Desert of southern Africa and the Gascoyne region of Western Australia. We found substantial interspecific variation in heat dissipation behaviours (wing-drooping, panting, activity-reduction, shade-seeking) across all three regions. However, pooling the data revealed that little of this interspecific variation was systematically explained by organismal traits (foraging guild, diet, drinking dependency, body mass, or activity levels) at the scale we tested. After accounting for phylogeny, we found that larger birds engaged in wing-drooping behaviour at lower Tair and had lower activity levels at high Tair compared to smaller birds, indicating an effect of body mass on heat dissipation behaviour (HDB). In the Kalahari, reliance on drinking was correlated with significantly lower Tair at which panting commenced, suggesting a key role of water acquisition in HDB in that region. Birds also tended to retreat to shade at relatively lower Tair when more active, suggesting a behavioural trade-off between activity, heat load, and microsite selection. Our results imply that the causes underlying interspecific variation in heat dissipation behaviours are complex. While the variation we observed was not systematically explained by the broad scale organismal traits we considered, we predict that the indices themselves will still reflect vulnerability to potential fitness costs of high air temperatures. Further research is needed on a species-specific basis to establish the functional significance of these indices.

量化生物个体对热胁迫的敏感性，是预测气候变化脆弱性的有效途径之一。鸟类是探究该研究方法的理想类群，因为其在响应热胁迫时的行为与生理变化会带来可量化的适合度后果。我们采用一种新近开发的方法，通过检测易观测行为与气温（Tair）之间的相关性，探究了三大洲季节性炎热干旱区域内鸟类热胁迫响应的种间变异：这些区域分别为美国西南部、非洲南部喀拉哈里沙漠，以及澳大利亚西部加斯科因地区。我们在三个研究区域均发现，鸟类的散热行为（垂翅、喘气、活动减少、遮阴行为）存在显著的种间变异。然而，整合所有研究数据后发现，在我们的测试尺度下，这类种间变异几乎无法通过生物性状（觅食类群、食性、饮水依赖度、体重或活动水平）进行系统性解释。在校正系统发育效应后，我们发现相较于体型更小的鸟类，体型更大的个体将在更低气温下表现出垂翅行为，且在高温环境下的活动水平更低，这表明体重对散热行为（heat dissipation behaviour, HDB）存在调控作用。在喀拉哈里沙漠区域，物种的饮水依赖度与喘气行为起始时的气温显著负相关，这表明在该区域，水分获取对HDB具有关键作用。当鸟类活动水平更高时，它们往往会在相对更低的气温下退回遮阴区域，这暗示活动水平、热负荷与微生境选择之间存在行为权衡。我们的研究结果表明，鸟类散热行为种间变异的成因十分复杂。尽管我们观测到的变异无法通过本次研究考量的广谱生物性状进行系统性解释，但我们预测，这些行为指标本身仍可反映高气温环境下潜在适合度代价的脆弱性。未来仍需开展物种特异性研究，以明确这些行为指标的功能意义。