Juv-Adult Pairwise Capture and Relatedness data for 2022/23

Obligate brood parasitic birds provide ontogenetic and life history complications for members of their own species, especially in the context of social development. Accordingly, Brown-headed cowbirds (Molothrus ater) exhibit complex social dynamics, with juveniles depending on early adult interactions for their successful social development. Our study examines these brood parasites’ juvenile-adult interactions and the potential role of genetic relatedness in cowbird ontogeny by investigating co-capture patterns and DNA data collected across three years (2022–2024) at a single study site in East Central Illinois, USA. Through the use of microsatellite genotyping, parentage analysis, and Queller-Goodnight relatedness estimates (QGM), we examined the genetic relationships of 122 adults and 7 juveniles and compared these with co-capture occurrences. Of the 478 potential adult-juvenile pairings, only three pairings amongst two juveniles showed QGM relatedness values suggestive of parent-offspring relationships, and parentage analysis confirmed that only two of our juveniles had putative parents within our sampled adult cowbird population. Juveniles were seldom co-captured with adults with whom they had higher than expected relatedness and were never co-caught with putative parents. Generalized linear mixed models produced no evidence supporting an effect of relatedness on juvenile-adult co-capture rates in contrast to earlier findings. We did, however, find strong evidence suggesting that juveniles are more likely to associate with adult females than males (p = 0.001), supporting prior behavioral studies which suggest juvenile preferences are driven by self-referent phenotype matching and attraction to adult female plumages and vocalizations, respectively. Our findings challenge previous assumptions that relatedness underpins juvenile-adult interactions across diverse species of obligate brood parasites and highlight the importance of behavioral cues and phenotypic matching over genetic ties. Lastly, this study underscores the need for further research on social-ontogenetic mechanisms in cowbirds and other obligate brood parasites.