Dataset for: Relative abundance trends of bird populations in high intensity croplands in the Central US

Declining bird populations across the US have been noted in a number of studies. Although multiple explanations have been proposed as causes of these declines, agricultural intensification has often been suggested as a significant driver of bird population dynamics. We examined this relationship by comparing bird count data from the Breeding Bird Survey collected from 1995-2016 across 13 states in the central US, to corresponding categorical changes in land cover within a 2 km radius of each survey transect using spatially explicit USDA-NASS Cropland Data Layer. This approach allowed us to compare the slopes of counts for 31 species of birds between grassland- and cropland-dominated landscapes, and against increasing levels of cropland (all types combined) and pooled corn/soybean land cover types. Nearly all birds demonstrated significant responses to land cover changes. In all cases, the number of species exhibiting positive or negative responses was comparable and median differences in percent change per year ranged from -0.5 to 0.7 percent. Species that responded either positively or negatively did not appear to fall into any particular foraging guild. If changes in agricultural practices are a major cause of declines, we would expect to see it across the spatial scale studied and across the majority of species. While these results do not rule out that potential agricultural impacts, such as toxicity resultant from pesticide exposure, which may have species specific or localized impacts, a variety of factors related to habitat are likely the most significant contributor overall. Given these results over a large spatial scale basis (multi-state) and across numerous bird species, there is not a broad general trend of greater decline in crop intensive areas.

多项研究均已证实，美国全境鸟类种群数量呈下降趋势。尽管已有多项假说解释此类种群下降的成因，但农业集约化常被认为是驱动鸟类种群动态变化的关键因素之一。本研究通过对比1995-2016年间美国中部13个州的繁殖鸟类调查（Breeding Bird Survey, BBS）所得鸟类计数数据，结合空间显式的美国农业部国家农业统计服务处（USDA-NASS）耕地数据图层（Cropland Data Layer, CDL），分析各调查样带2公里半径范围内土地覆盖类别的对应变化，以此探究二者的关联。该分析框架使得我们能够对比31种鸟类在草原主导与耕地主导景观中的种群数量变化斜率，并分析其随耕地（所有类型合并）及玉米/大豆合并土地覆盖类型占比提升的响应趋势。几乎所有鸟类均对土地覆盖变化表现出显著响应。在所有分析场景中，呈现正向或负向响应的鸟类物种数量相当，年百分比变化的中位数差异介于-0.5%至0.7%之间。呈现正向或负向响应的鸟类物种，并未集中归属于某一特定觅食类群。若农业生产方式变革是鸟类种群下降的主要诱因，则在本次研究的空间尺度下，多数鸟类物种均应表现出相应响应。尽管本研究结果并未排除农业活动的潜在影响——例如农药暴露引发的毒性效应可能存在物种特异性或局域性影响——但总体而言，与栖息地相关的多种因素或许是驱动种群变化的最关键因素。结合本次研究在大空间尺度（多州范围）及众多鸟类物种上得到的结果，并未发现耕地集约化区域存在普遍且显著的鸟类种群加速下降趋势。