Impact of Cowbird Parasitism on Major Host Species in Ontario

Tthe relative effect of cowbird parasitism on a large number (n = 12) of different host species at a very large spatial scale (the province of Ontario) was compared, using a large sample size of nests (N > 4500). We used data from an unique and underutilized resource, the Ontario Nest Record System (ONRS), and a novel application of log-linear models was designed to fully utilize the ordinal data available from nest cards. We compared the distribution of broods of different sizes in parasitized and unparasitized nests belonging to 5 Neotropical migrant species and 7 short distant migrants and residents. The results highlighted that 3 species (chipping sparrow, eastern phoebe, and northern cardinal) experience particularly severe reduction in hatching success due to parasitism relative to the other species examined (odds ratio test, p < 0.05). We also showed that 4 species (song sparrow, chipping sparrow, eastern phoebe, and northern cardinal) experienced particularly severe reduction in fledging success when parasitized in comparison to other species examined (odds ratio test, p < 0.05). Furthermore, we ruled out host body mass as a factor in the pattern of reduced production that parasitized hosts experienced (P = 0.271). We also found that as a group neotropical migrant songbirds in Ontario were parasitized more often than short distance migrants and residents (X22,1 = 19.97, p < 0.001). However, we found that parasitized neotropical migrant birds did not experience increased loss in reproductive success compared with parasitized short distance migrants and residents. Finally, we found that all host species hatched and fledged fewer young when they were parasitized (X22,44 = 225, P < 0.001; and X22,3 = 59.3, P < 0.001, respectively).

本研究在超大空间尺度（加拿大安大略省）范围内，以超过4500个巢的大样本量，对比了牛鹂寄生对12种不同寄主物种产生的相对影响。我们采用了一项独特且未被充分利用的资源——安大略省巢记录系统（Ontario Nest Record System, ONRS）的数据，并设计了对数线性模型（log-linear models）的全新应用方法，以充分利用巢卡所蕴含的有序数据。本研究针对5种新热带界迁徙鸟类、7种短距离迁徙鸟类与留居鸟类，对比了其被寄生巢与未被寄生巢的不同窝雏数分布情况。
研究结果显示，相较于其余受试物种，3个物种（美洲树雀鹀、东菲比霸鹟、北美主红雀）的孵化成功率因牛鹂寄生受到了尤为严重的抑制（优势比检验，p<0.05）。此外，4个物种（歌带鹀、美洲树雀鹀、东菲比霸鹟、北美主红雀）在被寄生后，其出飞成功率相较其余受试物种出现了尤为显著的下降（优势比检验，p<0.05）。
进一步分析排除了寄主体重作为寄生寄主繁殖产出下降模式的影响因素（P=0.271）。我们还发现，整体而言，安大略省的新热带界迁徙鸣禽相较于短距离迁徙鸟类与留居鸟类，被牛鹂寄生的概率更高（χ²₂,₁=19.97，p<0.001）。但被寄生的新热带界迁徙鸣禽，其繁殖成功率的损失程度并未高于被寄生的短距离迁徙鸟类与留居鸟类。
最后，所有寄主物种在被寄生后，其孵化出的幼鸟数量与成功出飞的幼鸟数量均有所减少（分别对应χ²₂,₄₄=225，P<0.001；χ²₂,₃=59.3，P<0.001）。