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Lesser Black-backed Gulls Made Up 73% of Paresis Cases in Lisbon

Adult lesser black-backed gull perched beside water in Lisbon, Portugal
Research · Other research

A retrospective study of gull admissions to Lisbon’s Wildlife Rehabilitation Center found that lesser black-backed gulls (Larus fuscus) made up 62% of gull admissions and 73% of gulls showing paresis, or partial paralysis. Records covered birds originating in Lisbon and Setúbal districts from 2016 through 2025. Admissions peaked in spring and autumn, coinciding with departure and return migration in this species.

Yellow-legged gulls (Larus michahellis), a resident species, made up 21% of admissions and 16% of the gulls with paresis. Fish and shellfish eaten by seabirds can contain marine biotoxins produced by certain microalgae. The researchers therefore compared weekly admissions of paretic gulls with the seasonal occurrence of neurotoxins associated with amnesic shellfish poisoning and paralytic shellfish poisoning.

The toxin patterns did not match the recurrent admission peaks. Amnesic shellfish-poisoning toxins were brief and rarely exceeded the human safety threshold, while paralytic shellfish-poisoning toxins exceeded that threshold only sporadically, in winter 2016/17 and autumn 2018. The authors found no evidence that the strong contamination in the Lisbon area in 2018 explained the recurring autumn pattern.

Paresis incidence rose mainly with the number of lesser black-backed gull admissions and with ambient air temperature, and mortality among paretic gulls was higher in autumn than in spring. The authors suggest that botulism may be involved, as previously shown in gulls from southern Portugal, and propose that the energy demands of migration could lead birds to readily available leftovers that may be spoiled.

Sources and image credit

  1. Vale, P., Bogalho, V. F., & Palma, E. (2026). Retrospective study of seagull poisonings in central Portugal: can marine phycotoxins be involved? Proceedings of The 3rd International Online Conference on Toxics, 9–11 September 2026. MDPI Sciforum.Primary conference proceedings abstract published 4 September 2026. It analyses gull admissions to Lisbon’s Wildlife Rehabilitation Center from 2016–2025 and compares recurrent paresis admissions with marine phycotoxin occurrence.

Image: Luiz Lapa / Wikimedia Commons — lesser black-backed gull at Parque Tejo, Lisbon, Portugal. Representative image for this story; not presented as an animal or scene from the reported study or event. · CC BY 2.0 · Source

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