<p>Cardiovascular diseases occur frequently in domesticated and aviary birds. Reference data on various heart variables are crucial to obtain a proper diagnosis, but the literature on wild birds is extremely sparse or non-existent. Here, we present heart and body morphometric traits of a medium-sized gull, the Black-headed Gull <i>Chroicocephalus ridibundus</i>, in relation to age and sex. The majority of the studied variables increased with age, reflecting the organism’s growth and development. However, heart proportions were usually the highest in chicks and similar in fledglings and adults. In adults, all external biometrical variables were higher in males than in females, except for body mass and total body length. Adult females had significantly higher values of numerous heart size parameters than males. Among all studied external biometric traits, only total head length and wing span were significantly related to fresh heart mass and increased with increasing heart mass. This study provides the first comprehensive reference dataset to support veterinarians in diagnosing pathological changes and anomalies in Black-headed Gull heart development.</p>