Data from: Thermoregulatory windows in Darwin's finches

1. Darwin’s finches have been the focus of intense study demonstrating how climatic fluctuations coupled with resource competition drive the evolution of a variety of bill sizes and shapes. The bill, as other peripheral surfaces, also plays an important role in thermoregulation in numerous bird species. The avian bill is vascularized, while limbs have specialized vasculature that facilitate heat loss or heat conservation (i.e., they are thermoregulatory windows). 2. The Galápagos Islands, home to Darwin’s finches, have a hot and relatively dry climate for approximately half of the year, during which thermoregulatory windows (i.e. surfaces) could be important for thermoregulation and the linked challenge of water balance. 3. We hypothesized that Darwin’s finch bills have evolved in part for their role in thermoregulation, possibly co-opted, following adaptation for other functions, such as foraging. We predicted that bills of Darwin’s finches are effective thermoregulatory windows, and that species differences in bill morphology, along with physiology and behavior, lead to differences in thermoregulatory function. 4. To test these hypotheses, we conducted a field study to assess heat exchange and microclimate use in three ground finch species and sympatric cactus finch (Geospiza spp.). We collected thermal images of free-living birds during a hot and dry season and recorded microclimate data for each observation. We used individual thermographic data to model the contribution of bills, legs, and bodies to overall heat balance and compared surface temperatures to those from dead birds to test physiological control of heat loss from these surfaces. We derived and compared species-specific threshold environmental temperatures, which are indicative of a species’ thermally neutral temperature. 5. In all species, the bill surface was an effective heat dissipater during naturally occurring warm temperatures. As expected, we found that finches controlled surface temperatures through physiology and that young birds had higher surface temperatures than adults. Larger bills contributed proportionally more to overall heat loss than smaller bills. 6. We demonstrate here that related, sympatric species with different bill sizes exhibit different patterns in the use of these thermoregulatory structures, supporting a role for thermoregulation in the evolution and ecology of Darwin’s finch morphology.

1. 达尔文雀（Darwin’s finches）一直是学界广泛研究的对象，相关研究揭示了气候波动与资源竞争共同作用如何推动多种喙部（bill）尺寸与形态的演化。与其他外周体表一样，喙在众多鸟类的体温调节（thermoregulation）过程中发挥着重要作用。鸟类的喙具有血管分布，而四肢则拥有特化的脉管系统，可促进散热或保温（即体温调节窗口（thermoregulatory windows））。
2. 作为达尔文雀的栖息地，加拉帕戈斯群岛（Galápagos Islands）每年约有半年时间气候炎热且相对干燥，在此期间体温调节窗口（thermoregulatory windows）对体温调节以及与之相关的水平衡维持而言至关重要。
3. 我们提出假说：达尔文雀的喙部演化在一定程度上服务于体温调节功能，这一功能可能是在适应觅食等其他功能之后被征募复用而来。我们预测，达尔文雀的喙部是有效的体温调节窗口，且不同物种的喙部形态（morphology）、生理学（physiology）与行为学（behavior）特征差异会导致体温调节功能的差异。
4. 为验证上述假说，我们开展了一项野外研究，以评估3种地雀物种以及同域分布的仙人掌地雀（Geospiza spp.）的热交换与微气候利用情况。我们在炎热干燥的季节采集了野生鸟类的热成像图像，并为每一次观测记录了微气候数据。我们利用个体热成像数据，建模分析喙部、四肢与躯体对整体热平衡的贡献，并将活体体表温度与死鸟的体表温度进行对比，以验证这些体表散热的生理调控机制。我们推导并比较了物种特异性的环境温度阈值，该阈值可反映物种的热中性温度（thermally neutral temperature）。
5. 在所有研究物种中，喙部体表在自然升温环境下均为有效的散热结构。正如预期，我们发现雀类通过生理调控维持体表温度，且幼鸟的体表温度高于成鸟。相较于较小的喙，较大的喙对整体散热的贡献比例更高。
6. 本研究证明，喙部尺寸各异的近缘同域分布物种，在体温调节结构的使用模式上存在差异，这支持了体温调节在达尔文雀喙部形态的演化与生态适应中发挥着重要作用这一结论。