Light and thermal niches of ground-foraging Amazonian insectivorous birds

Insectivores of the tropical rainforest floor are consistently among the most vulnerable birds to forest clearing and fragmentation. Several hypotheses attempt to explain this pattern, including sensitivity to extreme microclimates found near forest borders—particularly brighter and warmer conditions. Importantly, this “microclimate hypothesis” has additional implications for intact forest under global climate change that could be evaluated through direct assessment of the light and temperature environment of terrestrial insectivores. In this study, we harness novel technology to directly quantify the light and thermal niches of 10 species of terrestrial insectivores in undisturbed Amazonian rainforest. Loggers placed on birds (N = 33) and their environment (N = 9) recorded nearly continuous microclimate data from 2017–2019, amassing >5 million measurements. We found that midday light intensity in treefall gaps (~39,000 lux) was >40 times higher than at the ground level of forest interior (950 lux). Light intensity registered by sensors placed on birds averaged 17.4 (range 3.9–41.5) lux, with species using only 4.3 (0.9–10.4) % of available light on the forest floor. Birds thus selected very dark microhabitats—the light environment was >2,200 times brighter in treefall gaps. Bird thermal niche was a function of ambient temperature as well as body temperature, which averaged >40.5 °C but varied among species. Forest floor temperature peaked daily at 27.0 °C, while bird loggers averaged 35.1 (34.5–35.7) °C at midday. The antpitta Myrmothera campanisona and the antthrush Formicarius colma used thermal conditions closest to their body temperatures, whereas leaftossers (Sclerurus spp.) and Myrmornis torquata occupied relatively cool microclimates. We found no general link between abundance trends and variation in species-specific light and thermal niches. Rather, all species occupied remarkably dim and cool microclimates. Because such conditions are rare outside of the interior of primary forest, these results support the microclimate hypothesis in disturbed landscapes. Moreover, strong avoidance to conditions that are becoming more common under climate change highlights the vulnerability of terrestrial insectivores even in the absence of disturbance and may be the reason for enigmatic declines in Amazonia and elsewhere.

Methods
Please see the associated publication for methods on data collection and processing.

热带林地表层食虫鸟类始终是受森林砍伐与片段化影响最严重的鸟类类群之一。已有多项假说试图解释这一现象，其中包括对林缘极端微气候——尤其是光照更强、温度更高的环境——的敏感性。重要的是，这一“微气候假说（microclimate hypothesis）”对全球气候变化下的原生森林具有额外启示，可通过直接评估陆生食虫鸟类的光照与温度环境来验证。本研究借助新兴技术，直接量化了未受干扰的亚马孙雨林中10种陆生食虫鸟类的光照与热生态位。我们在33只鸟类身上以及9个环境点位部署了微型数据记录仪，于2017至2019年间记录了近乎连续的微气候数据，累计获取超500万条观测记录。研究结果显示，林隙正午的光照强度（约39000勒克斯）较林内地表（950勒克斯）高出40倍以上。附着在鸟类身上的传感器记录的平均光照强度为17.4勒克斯（区间3.9~41.5勒克斯），各物种仅利用了林地表层可获得光照的4.3%（区间0.9%~10.4%）。可见此类鸟类偏好极暗的微生境——林隙的光照强度较林地表层高出2200倍以上。鸟类的热生态位受环境温度与自身体温共同影响，其体温平均高于40.5℃，且存在物种间差异。林地表层温度的日峰值为27.0℃，而鸟类佩戴的记录仪正午平均温度为35.1℃（区间34.5~35.7℃）。蚁鸫（Myrmothera campanisona）与蚁鶲（Formicarius colma）所选择的热环境最接近自身体温，而叶鹪鹩（Sclerurus属）以及领蚁鵙（Myrmornis torquata）则栖息于相对凉爽的微气候中。我们未发现种群丰度趋势与物种特异性光照、热生态位变异之间存在普遍关联。相反，所有物种均栖息于极暗且凉爽的微气候环境中。由于此类生境仅在原生林内部较为罕见，本研究结果支持扰动景观中的微气候假说。此外，此类鸟类对气候变化下日益常见的环境表现出强烈回避行为，这表明即使未受人为干扰，陆生食虫鸟类仍面临极高的生存风险，这或许也是亚马孙及全球其他区域种群数量神秘下降的原因。

方法
有关数据采集与处理的详细方法，请参阅相关发表文献。