Exploratory analyses of sexual size dimorphism (SSD) and sexual dichromatism (SD) in free-living purple herons (Ardea purpurea)

Ardeidae tend to exhibit low sexual dimorphism, both in size and plumage coloration, making sex attribution in the field challenging for both birdwatchers and ringers. Here, we assessed whether biometrics and plumage patterns are a good proxy for sex assignment in the Purple Heron (Ardea purpurea). We based our work on 27 molecularly-sexed free-living adults captured in Italy and Romania during the breeding season from 2018 to 2021. We found significant sexual size dimorphism in the beak, which resulted longer in males, but not in weight, wing, tarsus or sternum length. Birds with darker plumages, stronger contrast between reddish ornamental feathers and blackish mantle were classified as males, allowing for correct sex attribution in 70% of the males and 73 % of the females. On the one hand, we concluded that molecular sexing is the most reliable approach for sex attribution in this species. On the other hand, we also found that trained ringers or expert observers may achieve satisfactory sex attribution rates either based on biometrics or accurate plumage observation, possibly even from a distance.