SGER: Dispersal Decisions and Social Reorganization of American Crows Following a West Nile Virus "Removal Experiment"

西尼罗病毒“去除实验”后美国乌鸦的扩散决定和社会重组对大多数动物来说，成年雄性和雌性在卵子受精后不会相互联系，雌性产卵孵化的幼鸟没有得到父母的照顾；父母与子女永不相见。某些类型的动物，如鸟类、哺乳动物和某些鱼类，父母一方或双方确实提供亲代照顾——它们可能孵蛋、喂养和/或保护幼崽——但在大多数情况下，后代一独立就离开“家”；没有多少动物是家庭生活的。然而，2-3%的鸟类（和哺乳动物）的后代“延迟分散”——它们与父母呆在家里的时间更长，而不是分散到自己的地方生活——形成了家庭群体。由于延迟扩散的后代往往有助于其父母随后的繁殖尝试，因此发生这种情况的物种被称为“合作繁殖”。在合作繁殖的物种中，大多数选择推迟扩散的个体也选择了推迟繁殖，这激发了许多理论，认为这种看似不适应的行为策略的进化可能与因素有关。成熟的鸟类个体必须评估所有行为选择的后果——分散，尝试繁殖，或者不繁殖——然后以最大限度地提高其适应性的方式行事。当有可接受的繁殖机会时，个体应该分散，当独立繁殖的选择质量差（独立繁殖在生态上受到“限制”）时，个体不应该分散，特别是如果延迟繁殖的成本可以通过作为“辅助”留在家中的利益来补偿。在这种条件下，合作育种群体应该是不稳定的；当缺乏可接受的繁殖机会时，个体应该作为辅助，当机会出现时，它们应该分散到自己的繁殖中。乌鸦的社会组织在同一种群中是可变的，可能相当复杂；美国东部乌鸦（Corvus brachyrhynchos brachyrhynchos）的种群是已知最复杂的种群之一。这个种群中的乌鸦做出了高度多变的分散决定，许多雌雄乌鸦都推迟了数年的分散时间，许多乌鸦在试图独立繁殖之前与其他家庭同居。最近研究的一个重点是试图通过多年来标记和跟踪单个乌鸦来确定与不同分散策略相关的各种成本和收益。2002年9月，西尼罗河病毒传入斯蒂尔沃特，两个月内40%的人口死亡或失踪。无论男女、年龄和地位阶层的人都消失了；9名饲养员丧偶，4个家庭失踪。突然之间，对于这个种群的成员来说，繁殖的机会可能是敞开的。随着2003年天气变暖，西尼罗河病毒周期再次开始，将继续有更多的机会。记录幸存者的扩散反应，以及由此产生的适应性结果，将使“生态约束”在推迟种群繁殖和扩散决策中所起的作用得到检验。在西尼罗河病毒“清除实验”之后的第一个繁殖季节，将与过去几年所做的扩散决定及其后果进行比较。研究这些决定与个体乌鸦的许多特征（包括种群成员之间的遗传相关性）之间的潜在关系，将有助于阐明由种群中乌鸦做出的高度可变的分散决定所导致的高度不寻常的社会组织的进化机制。

Dispersal Decisions and Social Reorganization of American Crows following a West Nile Virus "Removal Experiment"Carolee CaffreyFor most animals, adult males and females do not associate with each other after egg fertilization, and females lay eggs from which young hatch that do not receive parental care; parents and offspring never meet. One or both parents in some types of animals, e.g., birds, mammals, and some fish, do provide parental care - they may incubate eggs, feed and/or guard young - but in most of these cases the offspring leave "home" as soon as they are independent; not many animals live in families. Yet the offspring of 2-3% of bird (and mammal) species "delay dispersal" - they remain home with parents for extended periods rather than disperse to live on their own - creating family groups. Because offspring that have delayed dispersal often contribute to the subsequent breeding attempts of their parents, species in which this occurs are said to "breed cooperatively." Across cooperatively breeding species, most of the individuals choosing to delay dispersal have also chosen to delay breeding, which has stimulated much theorizing as to the possible factors associated with the evolution of this seemingly nonadaptive behavioral strategy.Maturing individual birds must assess the consequences of all behavioral options - to disperse, to attempt to breed, or not - and subsequently behave in ways that maximize their fitnesses. Individuals should disperse when acceptable breeding opportunities are available, and should not disperse when options for independent reproduction are of poor quality (independent breeding is "constrained" ecologically), especially if the cost of delaying breeding is compensated via benefits associated with remaining home as an "auxiliary." Under such conditions, cooperative breeding groups should be unstable; individuals should remain as auxiliaries when acceptable breeding opportunities are lacking, and they should disperse to breed on their own as opportunities open up. Crow social organization is variable across conspecific populations and can be quite complex; that of a population of Eastern American Crows (Corvus brachyrhynchos brachyrhynchos) in Stillwater, OK, is one of the most complex known. Crows in this population have made highly variable dispersal decisions, with many individuals of both sexes delaying dispersal for years, and many individuals moving in with other families before attempting to breed independently. One focus of recent research has been attempting to identify the various costs and benefits associated with different dispersal strategies through marking and following individual crows over years.In September 2002, West Nile virus arrived in Stillwater, and within two months 40% of the population was dead or missing. Individuals of both sexes and all age and status classes are gone; nine breeders were widowed, and four entire families disappeared. All of a sudden, for the members of this population, breeding opportunities are presumably wide open. As the weather warms in 2003 and the West Nile virus cycle begins again, more opportunities will continue to become available. Documenting the dispersal responses of the survivors, and the fitness consequences thereof, will enable the role played by "ecological constraints" on decisions to delay breeding and dispersal in this population to be examined. Dispersal decisions, and their consequences, in this first breeding season following the WNV "removal experiment" will be compared to those made in past years. Examination of potential relationships between those decisions and the many characteristics that have been measured for individual crows (including genetic relatedness among population members), will help to elucidate the evolutionary mechanisms underlying the highly unusual social organization resulting from the highly variable dispersal decisions being made by crows in this population.