Brood sex ratio is a fundamental aspect of evolutionary biology, with profound implications for life-history traits, parental investment, sex allocation, offspring rearing, and ultimately, reproductive success, population dynamics, and viability. Previous research on birds' brood sex ratio bias has demonstrated associations with various parameters, including environmental and reproductive conditions, the social environment, and parental quality. Identifying and understanding the parameters that influence brood sex ratio can significantly advance our knowledge of sex allocation in ecological and evolutionary contexts. The Black-crowned Night Heron Nycticorax nycticorax, a colonial species in the family Ardeidae, is sexually monomorphic, making it an ideal biological model for sex allocation studies. The present study, conducted in 2023, assessed the relationships between the brood sex ratio and several variables: nest-site selection parameters (nests = 18), breeding timing, brood size, and nestling body condition. Research was carried out at the Zaghmarz Forest and Rangeland Research Station, an anthropogenic ecosystem dedicated to silvicultural research in eastern Mazandaran Province, northern Iran. Blood samples were collected from nestlings (n = 54)  to determine the sex of each individual. The best-supported model, based on ΔAIC, indicated that only nest diameter and nest position explained variation in the brood sex ratio of Black-crowned Night Herons. The findings revealed that none of the investigated parameters—nest-site selection, timing of breeding, brood size, or nestling body condition—had a significant relationship with the brood sex ratio. These results suggest that the brood sex ratio in Black-crowned Night Herons may be primarily controlled by genetic mechanisms or other unmeasured physiological and environmental parameters. This underscores the need for further research into the mechanisms regulating brood sex ratio in this species.