Abstract
BackgroundRed-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Although hosts of Old World cuckoos (Cuculidae) often show geographic variation in egg rejection behavior, cowbird hosts typically exhibit uniform responses of all acceptance or all rejection of cowbird eggs. Thus, geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. In this study, we tested whether egg rejection behavior may explain the lack of parasitism observed in our eastern red-wing population, which may provide insight into low parasitism levels across eastern North America.MethodsWe parasitized red-wing nests with model cowbird eggs to determine their response to parasitism. Nests were tested across three nest stages and compared to control nests with no manipulations. Because rejection differed significantly by stage, we compared responses separately for each nest stage. We also monitored other songbird nests to identify parasitism frequencies on all potential hosts.ResultsRed-wings showed significantly more rejections during the building stage, but not for the laying and incubation stages. Rejections during nest building involved mostly egg burials, which likely represent a continuation of the nest building process rather than true rejection of the cowbird egg. Excluding these responses, red-wings rejected 15% of cowbird eggs, which is similar to rejection levels from other studies and populations. The overall parasitism frequency on 11 species surveyed in our study area was only 7.4%.ConclusionsEgg rejection behavior does not explain the lack of parasitism on red-wings in our eastern population. Alternatively, we suggest that cowbird preference for other hosts and the low abundance of cowbirds in the east might explain the lack of parasitism. Future research should also explore cowbird and host density and the makeup of the host community to explain the low levels of parasitism on red-wings across eastern North America because egg rejection alone is unlikely to explain this broad geographic trend.
Highlights
Red-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania
The fitness costs of obligate brood parasitism put a strong selection pressure on host species to evolve defenses against parasitism (Rothstein 1990). Frontline defenses, such as cryptic nest placement and aggressive nest defense, may prevent nests from being parasitized (Feeney 2017), whereas egg rejection behavior is the most effective defense to mitigate the costs of parasitism after it has occurred (Rothstein 1975; Ruiz-Raya and Soler 2017)
Most of the rejections during the building stage were burials (44% of rejections), but all three methods of rejection were recorded during this stage (Table 2). These rejections may not be a direct response to parasitism, this demonstrates that all rejection behaviors are present in our population, including ejection
Summary
Red-winged Blackbirds (Agelaius phoeniceus), hereafter red-wings, are much less frequently parasitized by Brown-headed Cowbirds (Molothrus ater) in eastern North America than in central North America and had not been recorded as hosts in our study area in southeastern Pennsylvania. Geographic variation in cowbird parasitism frequencies might reflect a different behavioral response to parasitism by hosts where only some populations reject parasitism. The fitness costs of obligate brood parasitism put a strong selection pressure on host species to evolve defenses against parasitism (Rothstein 1990) Frontline defenses, such as cryptic nest placement and aggressive nest defense, may prevent nests from being parasitized (Feeney 2017), whereas egg rejection behavior is the most effective defense to mitigate the costs of parasitism after it has occurred (Rothstein 1975; Ruiz-Raya and Soler 2017). The egg rejection response of cuckoo hosts can vary with the risk of parasitism over time (Soler et al 1998; Thorogood and Davies 2013) and over large geographic areas (Liang et al 2016). Frequencies of observed cowbird parasitism on Warbling Vireos in different geographic areas reflect this different rejection response (Gardali and Ballard 2000; Ortega and Ortega 2003)
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