Department of Biology, Indiana State University, Terre Haute, IN, 47809, USA,
Oecologia. 2014 Mar;174(3):713-21. doi: 10.1007/s00442-013-2804-7. Epub 2013 Oct 17.
Wintering birds can gain significant thermal benefits by foraging in direct sunlight. However, exposure to bright sunlight might make birds easier to detect by predators and may also cause visual glare that can reduce a bird's ability to monitor the environment. Thus, birds likely experience a trade-off between the thermal benefits and predation-related costs of foraging in direct sunlight. To examine this possible thermoregulation-predation trade-off, we monitored the behavior of mixed-species flocks of wintering emberizid sparrows foraging in alternating strips of sunlight and shade. On average, these sparrows routinely preferred to forage in the shade, despite midday air temperatures as much as 30 °C below their thermoneutral zone. This preference for shade was strongest at relatively high temperatures when the thermal benefits of foraging in sunlight were reduced, suggesting a thermoregulation-predation trade-off. Glare could be reduced if birds faced away from the sun while feeding in direct sunlight, but we found that foraging birds tended to face southward (the direction of the sun). We speculate that other factors, such as the likely direction of predator approach, may explain this southerly orientation, particularly if predators use solar glare to their advantage during an attack. This interpretation is supported by the fact that birds had the weakest southerly orientation on cloudy days. Wintering birds may generally avoid foraging in direct sunlight to minimize their risk of predation. However, given the thermal benefits of sunshine, such birds may benefit from foraging in habitats that provide a mosaic of sunlit and shaded microhabitats.
候鸟在阳光直射下觅食可以获得显著的热量增益。然而,暴露在明亮的阳光下可能会使鸟类更容易被捕食者发现,也可能导致视觉眩光,从而降低鸟类监测环境的能力。因此,鸟类在阳光直射下觅食时可能会在热增益和与捕食相关的成本之间进行权衡。为了检验这种可能的体温调节与捕食之间的权衡,我们监测了冬季混合物种雀形目雀科鸟类在阳光和阴影交替地带觅食的行为。平均而言,尽管正午的空气温度比它们的热中性区低 30°C,但这些麻雀经常更喜欢在阴凉处觅食。当阳光觅食的热增益降低时,这种对阴凉处的偏好最强,这表明存在体温调节与捕食之间的权衡。如果鸟类在阳光直射下觅食时背对太阳,眩光可能会减少,但我们发现觅食的鸟类往往面向南方(太阳的方向)。我们推测,如果捕食者在攻击时利用太阳眩光,那么其他因素,例如捕食者可能的接近方向,可能会解释这种向南的方向,特别是如果捕食者在攻击时利用太阳眩光。事实上,阴天时鸟类的南向方向最弱,这支持了这种解释。候鸟通常可能会避免在阳光直射下觅食,以最大程度地降低被捕食的风险。然而,考虑到阳光的热量增益,这些鸟类可能会受益于在提供阳光照射和阴影微生境镶嵌的栖息地觅食。