Capsule
During spring and summer 2023, the highly pathogenic avian influenza (HPAI) H5N1 virus caused mass mortality among colony-breeding gulls and terns in Poland.

Aims
To document the national impact of the 2023 HPAI outbreak on scarcer species of gulls and terns in breeding colonies across Poland.

Methods
Surveys were primarily focused within breeding colonies of Black-headed Gulls Chroicocephalus ridibundus, an ‘umbrella species’ whose breeding colonies serve as nesting sites for other scarce gull and tern species.

Results
In total, during the survey period we recorded the deaths of at least 46 Common Gulls Larus canus, 21 Mediterranean Gulls Ichthyaetus melanocephalus, nine Caspian Gulls Larus cachinnans, two Herring Gulls Larus argentatus, 1,369 Common Terns Sterna hirundo, 42 Little Terns Sternula albifrons, three Black Terns Chlidonias niger, two Whiskered Terns Chlidonias hybrida and 35 Sandwich Terns Thalasseus sandvicensis. The most affected populations were those of Mediterranean Gulls (12% of the Polish breeding population) and Common Terns (16% of the Polish breeding population).

Conclusions
Common Terns and Common Gulls were considered the most vulnerable species to longer-term negative population impacts. It is strongly recommended to monitor mortality and productivity of all gull and tern species during future breeding seasons, to determine the ongoing impact of HPAI.