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.

Egg discrimination by hosts and obligate brood parasites:a historical perspective and new synthesis

With the knowledge that cuckoos and cowbirds lay their eggs parasitically,and that some hosts eject parasitic eggs,ornithologists began to ponder the question of how host females discriminate between a foreign egg and their own eggs,wondering how hosts 'know' which egg to remove.Results of one of the rst uncontrolled experiments were inappropriately interpreted to imply ejection was based on discordancy,with hosts simply ejecting the egg in the minority,or the 'odd-looking' egg.Controlled experiments eventually revealed that hosts rst learn the appearance of own their eggs and discriminate between them and any odd egg in their nest,regardless of which egg type is in the minority.Recent work has shown that discordancy may play a role in discrimination by males mated successively with females that lay polymorphic eggs.We examine the details of the early experiments,in light of recent advances in studies of egg recognition.An ability to recognize eggs also has been extended,implicitly,to include obligate brood parasites,as it underlies several hypotheses in explanation of the behavior of parasites toward their hosts.Egg recognition in parasites,however,has not been experimentally con rmed,nor has a mechanism been identi ed by which parasites could discriminate between their own eggs and the other eggs in a nest.We review hypotheses(parasite competition,egg removal and multiple parasitism,ma a,farming) that require the ability of obligate brood parasites to discriminate eggs at di erent levels and the potential mechanisms used by parasites to recognize their own eggs and suggest experiments to test for egg discrimination.An assessment of the egg recognition ability of parasites is germane to our understanding of how parasites counteract defenses of hosts.