Intra-specific nest parasitism in Black-headed Gull Larus ridibundus

Intra-specific nest parasitism in Black-headed Gull is reported based on several lines of indirect evidence. The within-clutch egg size asymmetry seems to be a component of protective mechanism against parasitism. Results of our observations and analyses presented in this paper suggest that intra-specific nest parasitism does exist in Black-headed Gulls. The phenomenon seems more common and important than suggested by studies using traditional methods, molecular studies are needed to assess its actual intensity.