DISSERTATION RESEARCH: An Experimental Test of a New Hypothesis-Brood Parasitism as an Influence on Incubation Strategies

大多数幼年动物在发育过程中需要父母的广泛保护和哺育。对于幼鸟来说尤其如此，因为早期发育发生在一个蛋内。如果卵要存活，父母(通常是雌性)必须提供保护和温暖。雌鸟坐在蛋上的时间越长，这些蛋存活、孵化并为下一代做出贡献的机会就越大。然而，雌性也必须觅食以获得她们持续健康所需的食物。这种活动通常需要离开鸟巢，从而使卵暴露在降温和可能的捕食之下。然而，在许多物种中，雌性通过雄性孵化进食来缓解这种权衡--这是一种雄性给雌性带来食物的行为，从而延长了雌性在巢上花费的时间。研究人员主要考虑了两个可能控制孵化模式的因素：鸟巢被捕食的风险和鸟巢周围的气候。在潜在捕食者常见的地方，雌性应该减少往返巢的次数，并尽可能长时间地呆在巢中。然而，由于雄性孵化进食会将巢的位置泄露给捕食者，孵化进食应该是最少的。相比之下，在气候非常寒冷的地方，雌性应该尽可能多地呆在巢中，只在短时间内下车，并严重依赖孵化进食。然而，还有第三个因素尚未被考虑，这可能会导致孵化行为的巨大变化：孵化寄生虫，如褐头牛鸟，清除卵子。牛头鸟和其他幼虫在其他物种的巢中产卵，留下这些“宿主”来抚养他们的幼崽。饲养牛鸟的鸟类通常较少饲养自己的幼鸟，部分原因是牛鸟经常在将自己的蛋放入巢中之前或之后从宿主的巢中取出蛋。当成年鸟不占据鸟巢时，取卵的可能性更大，因此，如果雌性牛头鸟知道它的巢所在位置，因此很可能会取出它的一个蛋，它应该花更多的时间在巢上。事实上，在之前的工作中，PI发现被寄生的雌性在巢上花费的时间比未被寄生的雌性多得多。PI将进行测试，以确定黄莺是否真的改变了它们的孵化行为，以应对牛鸟取卵的威胁。PI将使用摄像机记录未被寄生的黄莺的孵化行为。在一个随机的巢子集中，PI将把牛鸟吸引到它们将与黄莺互动的巢中，然后PI将通过用不育的牛鸟蛋取代一个黄莺蛋来实验性地寄生这些巢。然后，PI将再次拍摄鸟巢，看看它们是否改变了孵化行为，以保护鸟巢免受感知到的蛋移除的威胁。作为比较，在另一个随机的鸟巢子集中，在最初的拍摄之后，PI将播放一只不会对鸟巢构成威胁的鸟类的歌声，并模仿实验寄生造成的干扰，但PI不会在鸟巢中放置牛鸟蛋。在这些巢中，PI将再次拍摄这些巢，以确定鸟类是否对干扰做出反应，而不是寄生本身。最后，PI还将对自然寄生的巢进行拍摄，以比较人工寄生的巢与自然寄生的巢的孵化行为。这项实验将允许PI首次测试幼虫寄生作为孵化行为决定因素的重要性。

Most young animals require extensive protection and nurturing by their parents while they develop. This is particularly true for young birds, because early development takes place within an egg. The parents (usually the female) must provide protection and warmth if the egg is to survive. The more time a female bird can spend sitting on her eggs, the better chance those eggs have of surviving, hatching and contributing to the next generation. However, females must also forage to get the food they need for continued health. This activity typically requires getting off the nest, thus exposing the eggs to cooling and possible predation. However, in many species, the females alleviate this tradeoff through male incubation feeding - a behavior in which the male brings food to the female, thus extending the time she can spend on the nest. Researchers have primarily considered two factors that might control patterns of incubation: the risk of nest predation, and the climate around the nest. Where potential predators are common, females should reduce the number of times they go to and from the nest, and stay on the nest as long as possible. However, because incubation feeding by males can give away the location of the nest to predators, incubation feeding should be minimal. In contrast, where climates are very cold, females should spend as much time as possible on the nest, get off only for short periods, and rely heavily on incubation feeding. However, there is a third factor that has not yet been considered, and which may cause large shifts in incubation behavior: egg removal by brood parasites, such as the brown-headed cowbird. Cowbirds and other brood parasites lay their eggs in the nests of other species, leaving these "hosts" to raise their young. Birds raising cowbirds typically raise fewer of their own young, partially because cowbirds often remove eggs from the nest of the host before or after they place their own egg in the nest. Egg removal may be more likely when an adult does not occupy the nest, thus the female should spend more time on the nest if she believes a cowbird knows the location of her nest and is thus likely to remove one of her eggs. Indeed, in previous work, the PIs found that parasitized females spend much more time on the nest than unparasitized females. The PIs will conduct tests to determine if Yellow Warblers actually shift their incubation behavior in response to the threat of egg removal by cowbirds. The PIs will use video cameras to record the incubation behaviors of unparasitized Yellow Warblers. In one random subset of nests, the PIs will attract cowbirds to the nests where they will interact with the Yellow Warblers, and then the PIs will experimentally parasitize these nests by replacing one yellow warbler egg with an infertile cowbird egg. The PIs will then film nests again to see if they have changed their incubation behaviors to protect the nest from the perceived threat of egg removal. For comparison, in another random subset of nests, after initial filming, the PIs will play the song of a Catbird, a bird which represents no threat to the nest, and mimic the disturbance caused by experimental parasitism, but the PIs will not place a cowbird egg in the nest. In these nests, the PIs will again film these nests, to determine if birds are reacting to the disturbance, rather than the parasitism itself. Finally, the PIs will also film naturally parasitized nests to compare incubation behaviors with the artificially parasitized nests. This experiment will allow the PIs to provide the first test of the importance of brood parasitism as a determinant of incubation behavior.