In the last decade, Common Gulls from the Iberian Peninsula have been observed to suffer from a paralyzing syndrome of unknown origin, affecting mostly adults in the west and south regions. This disease has been attributed to different causes, such as nutritional deficiencies, avian botulism, and exposure to algal toxins. In the quest for the causative agent, we selected twelve individuals of Larus michahellis, L. fuscus, L. ridibundus and Ichthyaetus melanocephalus that succumbed to this illness concomitantly with marine algal blooms in the nearby coasts of Málaga (Spain). Amnesic and paralytic shellfish toxins were analyzed in the gull´s liver, kidney and feces. Furthermore, the neurotoxin β-Methylamino-L-alanine, and its analogues were measured in the liver, brain and blood serum of the same individuals by UPLC-MS/MS. Liver and brain were also used to determine stable isotope (SI) signature as a trophic marker. None of the analyzed neurotoxins were detected in any of the samples, indicating that poisoning due to blooms caused by the algae producers of these neurotoxins was unlikely cause of the bird death. Moreover, despite similar symptoms, the SI signatures revealed significant differences in the feeding grounds between the species. These results point to the causative agent not being related to the food sources of the birds.