Brood parasitism leads to zero recruitment in the globally endangered Yellow Cardinal Gubernatrix cristata

Summary The Yellow Cardinal Gubernatrix cristata is an ‘Endangered’ passerine from southern South America. For the past three years a management plan for Yellow Cardinals has been implemented in Argentina, in which rescued individuals from the illegal cage bird trade were released back into suitable habitats within their population of origin. We studied the reproductive success of a mixed population of released and wild Yellow Cardinals in La Pampa province, Argentina, during the reproductive season of 2019. The population was highly parasitized by the brood parasitic Shiny Cowbird Molothrus bonariensis. The frequency of parasitism was 100% and the intensity of parasitism was 4.5 ± 3.4 (mean ± SD) eggs per parasitized nest (range 1–13). No Yellow Cardinal chicks were recruited in the monitored nests, mainly as a consequence of Shiny Cowbird parasitism. The unusually high rate of parasitism led to a poor outcome of the reintroduction programme and indicates the need to update the conservation actions that have been carried out for the Yellow Cardinal so far. Shiny Cowbird abundance varies within the distribution of Yellow Cardinals, related to habitat modification and farming activities. Thus, further research on habitat suitability and assessment of Shiny Cowbird abundance should be incorporated into future strategies for the conservation of the Yellow Cardinal.

Egg rejection behavior does not explain the lack of cowbird parasitism on an eastern North American population of Red-winged Blackbirds

Background 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. 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. Methods We 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. Results Red-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%. Conclusions Egg 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.

Age-dependent costs of cowbird parasitism in Yellow Warblers (Setophaga petechia)

Brood parasitism by Brown-headed Cowbirds (Molothrus ater (Boddaert, 1783)) often reduces the reproductive success of their hosts. We examined whether the ability of females to avoid or mitigate the costs of brood parasitism improved with age in a population of Yellow Warblers (Setophaga petechia (L., 1766)) breeding near Revelstoke, British Columbia, between 2004 and 2011. Cowbirds parasitized 18% of Yellow Warbler nesting attempts and females rejected 24% of parasitized nests, principally by deserting the nest and initiating a new breeding attempt. We found no evidence that older females were better at avoiding parasitism or more likely to reject parasitized nests than yearlings. On average, brood parasitism reduced clutch sizes by 0.8 eggs, had no effect on nest success, but reduced the number of young fledged from successful nests by 1.3 offspring. Despite age-related improvement in some measures of breeding performance, the costs of brood parasitism at each period of the breeding cycle did not vary with age. There was, however, some evidence, that brood parasitism reduced the annual productivity (total number of young fledged) of older females less than the annual productivity of yearlings suggesting that the cumulative costs of brood parasitism varied with age.