“Monogamy” refers to different components of pair exclusiveness: the social pair, sexual partners, and the genetic outcome of sexual encounters. Avian monogamy is usually defined socially or genetically, while quantifications of sexual behavior remain scarce. Jackdaws (Corvus monedula) are considered a rare example of strict monogamy in songbirds, with lifelong pair bonds and little genetic evidence for extra-pair offspring. Yet jackdaw copulations, although accompanied by loud copulation calls, are rarely observed, since they occur visually concealed inside nest cavities. Using full-day nest-box video surveillance and on-bird acoustic bio-logging, we directly observed jackdaw sexual behavior and compared it to the corresponding genetic outcome obtained via molecular parentage analysis. In the video-observed nests, we found genetic monogamy, but frequently detected forced extra-pair sexual behavior, accompanied by characteristic male copulation calls. We thus challenge the long-held notion of strict jackdaw monogamy at the sexual level. Our data suggest that male mate-guarding and frequent intra-pair copulations during the female fertile phase, as well as the forced nature of the copulations could explain the absence of extra-pair offspring. Since extra-pair copulation behavior appeared to be costly for both sexes, we suggest that immediate fitness benefits are an unlikely explanation for its prevalence. Instead, sexual conflict and dominance effects could interact to shape the spatio-temporal pattern of extra-pair sexual behavior in this species. Our results call for larger-scale investigations of jackdaw sexual behavior and parentage, and highlight the importance of combining social, sexual and genetic datasets for a more complete understanding of mating systems.

*Breeding data were collected via nest checks and observations.

**Behavioural data were collected using nest-box video surveillance, acoustic bio-logging (microphone backpacks) and observations of banded individuals.

***Information on genetic monogamy was obtained via molecular parentage analysis on chick (buccal swabs) and adult (blood samples) DNA.