Breeding Dispersal by Birds in a Dynamic Urban Ecosystem

Changes in land cover during urbanization profoundly affect the diversity of bird communities, but the demographic mechanisms affecting diversity are poorly known. We advance such understanding by documenting how urbanization influences breeding dispersal—the annual movement of territorial adults—of six songbird species in the Seattle, WA, USA metropolitan area. We color-banded adults and mapped the centers of their annual breeding activities from 2000–2010 to obtain 504 consecutive movements by 337 adults. By comparing movements, annual reproduction, and mate fidelity among 10 developed, 5 reserved, and 11 changing (areas cleared and developed during our study) landscapes, we determined that adaptive breeding dispersal of sensitive forest species (Swainson’s Thrush and Pacific wren), which involves shifting territory and mate after reproductive failure, was constrained by development. In changing lands, sensitive forest specialists dispersed from active development to nearby forested areas, but in so doing suffered low annual reproduction. Species tolerant of suburban lands (song sparrow, spotted towhee, dark-eyed junco, and Bewick’s wren) dispersed adaptively in changing landscapes. Site fidelity ranged from 0% (Pacific wren in changing landscape) to 83% (Bewick’s wren in forest reserve). Mate fidelity ranged from 25% (dark-eyed junco) to 100% (Bewick’s wren). Variation in fidelity to mate and territory was consistent with theories positing an influence of territory quality, asynchronous return from migration, prior productivity, and reproductive benefits of retaining a familiar territory. Costly breeding dispersal, as well as reduced reproductive success and lowered survival cause some birds to decline in the face of urbanization. In contrast, the ability of species that utilize edges and early successional habitats to breed successfully, disperse to improve reproductive success after failure, and survive throughout the urban ecosystem enables them to maintain or increase population size.城市化过程中的土地覆被变化会深刻影响鸟类群落多样性，但目前学界对影响该多样性的种群动态机制仍知之甚少。本研究通过记录美国华盛顿州西雅图都会区内6种鸣禽的繁殖扩散（breeding dispersal）行为——即具有领域性的成鸟的年度移动行为——推进了相关认知。2000年至2010年间，我们为成鸟佩戴彩色脚环，并绘制其年度繁殖活动中心位置，最终获得337只成鸟的504次连续移动记录。我们对比了10处已开发景观、5处保留景观以及11处动态变化景观（即在本研究期间被清理并开发的区域）中成鸟的移动行为、年度繁殖情况与配偶忠诚性（mate fidelity），结果发现：对生境敏感的森林物种（斯温氏鸫（Swainson’s Thrush）和太平洋鹪鹩（Pacific wren））的适应性繁殖扩散行为，即繁殖失败后更换领域与配偶，会受到开发活动的制约。在动态变化的区域中，专性森林敏感物种会从正在开发的区域扩散至周边林区，但在此过程中其年度繁殖率较低。耐受城郊生境的物种（歌带鹀（song sparrow）、斑唧鹀（spotted towhee）、暗眼灯草鹀（dark-eyed junco）与贝威克鹪鹩（Bewick’s Wren））则会在动态变化的景观中进行适应性扩散。栖地保真度（site fidelity）范围为0%（动态变化景观中的太平洋鹪鹩）至83%（林区保留地中的贝威克鹪鹩）。配偶忠诚性范围为25%（暗眼灯草鹀）至100%（贝威克鹪鹩）。配偶与栖地保真度的变化规律，与相关理论假说一致——这些假说认为领域质量、迁徙后异步归返、前期繁殖表现以及保留熟悉领域的繁殖收益，均会对保真度产生影响。代价高昂的繁殖扩散，加之繁殖成功率下降与存活率降低，导致部分鸟类在城市化进程中种群数量下降。与之相反，那些利用生境边缘与早期演替生境的物种，能够成功繁殖、在繁殖失败后通过扩散提升繁殖成功率，并在整个城市生态系统中存活，这使得它们的种群数量得以维持甚至增长。