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

大多数幼小的动物在发育过程中需要父母的广泛保护和养育。对于雏鸟来说尤其如此，因为早期的发育是在蛋里进行的。如果蛋要存活下来，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，这些蛋就越有可能存活、孵化并为下一代做出贡献。然而，雌性也必须觅食以获得维持健康所需的食物。这种活动通常需要离开巢穴，从而使蛋暴露在冷却和可能的捕食中。然而，在许多物种中，雌性通过雄性的孵化喂养来缓解这种权衡——雄性给雌性带来食物的行为，从而延长了雌性在巢穴里的时间。研究人员主要考虑了两个可能控制孵化模式的因素：巢穴捕食的风险和巢穴周围的气候。在潜在的捕食者很常见的地方，雌性应该减少往返巢穴的次数，并尽可能长时间地呆在巢穴里。然而，由于雄性的孵化喂养会将巢穴的位置暴露给捕食者，因此孵化喂养应该尽量少。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里，只离开很短的时间，并严重依赖孵化喂养。然而，还有第三个尚未被考虑的因素，它可能会导致孵化行为的巨大变化：幼虫寄生虫（如褐头牛鹂）将卵取出。牛郎鸟和其他寄生物在其他物种的巢中产卵，留下这些“寄主”抚养它们的后代。养牛郎鸟的鸟通常会少养自己的孩子，部分原因是牛郎鸟经常在把自己的蛋放入巢中之前或之后从宿主的巢中取出蛋。当成鸟不占巢时，取蛋的可能性更大，因此，如果雌性认为一只牛鹂知道她的巢的位置，那么它应该花更多的时间在巢上，从而可能会取走她的一个蛋。事实上，在之前的工作中，pi发现被寄生的雌性比未被寄生的雌性花更多的时间在巢穴上。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，以应对牛鹂取卵的威胁。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将吸引牛鹂到巢中，在那里它们将与黄莺互动，然后pi将实验性地寄生在这些巢中，用一个不育的牛鹂蛋代替一个黄莺蛋。然后，pi将再次拍摄巢穴，看看它们是否改变了孵化行为，以保护巢穴免受被移除卵子的威胁。为了进行比较，在另一个随机的巢群中，在初始拍摄后，pi将播放猫鸟的歌声，猫鸟对巢没有威胁，并模仿实验寄生造成的干扰，但pi不会在巢中放置牛鸟蛋。在这些巢穴中，pi将再次拍摄这些巢穴，以确定鸟类是否对干扰做出反应，而不是寄生本身。最后，pi还将拍摄自然被寄生的巢穴，比较人工被寄生的巢穴的孵化行为。这个实验将允许pi提供第一个关于幼虫寄生作为孵化行为决定因素的重要性的测试。

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.