In recent years, highly pathogenic avian influenza (HPAI) A (H5N1) clade 2.3.4.4b viruses have caused mass mortality events in seabirds worldwide, raising concern for long-lived species with low reproductive rates. Using 28 years of ringing data together with individual-level data from the 2022 mass mortality event in northwestern European Sandwich terns (Thalasseus sandvicensis), we show that mortality was biased towards older individuals, while no sex bias was observed. HPAI-related mortality increased from approximately 5-10% in young breeders to more than 40% in the oldest individuals. This age-biased loss likely removed the most experienced individuals from the population. Our findings highlight a previously underappreciated mechanism through which HPAI outbreaks may impair the resilience of long-lived avian populations.