Gull populations, as largely flexible opportunistic individuals, have been increasingly breeding in many cities around the world, but is still unclear whether urban habitats are of equal or higher quality than the traditional natural habitats; or if they represent an ecological trap with immediate reproductive benefits, but with longer-term detrimental consequences to health. Here we present a study of breeding parameters (nest density, egg dimensions, clutch size, hatching success and adult body condition), and physiological parameters (erythrocyte sedimentation rate, heterophils/lymphocytes ratio, haemoglobin concentration and measurements of oxidative stress), as indicators of the general health condition of Yellow-legged Gull Larus michahellis adults and chicks from natural and urban colonies. Yellow-legged Gulls in the largest urban area (Porto) laid smaller eggs and clutches, showed a significantly lower occurrence of inflammatory processes in chicks, and a slower early chick growth than in the natural colony of Deserta. This suggests urban gulls might be facing important trade-offs between the advantages of breeding in lower density urban colonies, with fewer intraspecific interactions and a lower disease transmission probability; and the disadvantages of having an anthropogenic diet usually lower in nutritional value.