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.

量化生物对热胁迫的敏感性，是预测物种受气候变化影响脆弱性的有效途径之一。鸟类是研究该方法的理想类群，因为其热胁迫相关的行为与生理响应可带来可量化的适合度后果。我们采用近期开发的方法——即分析易观测行为与气温（Tair）之间的相关性，以探究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：美国西南部、非洲南部的喀拉哈里沙漠，以及澳大利亚西部的加斯科因地区。我们在上述三个区域均发现，鸟类的散热行为（包括垂翅、喘息、活动减少、遮阴躲避）存在显著的种间变异。不过，整合所有数据后发现，在我们的测试尺度下，这类种间变异几乎无法通过所考量的宽泛生物性状（觅食类群、食性、饮水依赖性、体重或活动水平）进行系统性解释。在纳入系统发育因素后，我们发现相较于体型较小的鸟类，体型更大的鸟类会在更低的气温（Tair）下出现垂翅行为，且在高温环境下的活动水平更低，这表明体重对散热行为（HDB）存在影响。在喀拉哈里沙漠区域，物种的饮水依赖性与喘息起始时的较低气温显著相关，这暗示水资源获取在该区域的散热行为（HDB）中扮演关键角色。此外，当鸟类活动水平更高时，它们往往会在相对更低的气温下躲避到遮阴处，这表明活动、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所考量的宽泛生物性状进行系统性解释，但我们预测，这些散热行为指标本身仍可反映高气温环境下潜在适合度成本带来的脆弱性。未来仍需开展物种特异性研究，以明确这些指标的功能意义。