Paretic syndrome in gulls

Credits: SGB

Gaviota, gaviota, vals del equilibrio
Cadencia increíble, llamada en el hombro
Gaviota, gaviota, blancura de lirio
Aire y bailarina, gaviota de asombro
¿A dónde te marchas, canción de la brisa
Tan rápida, tan detenida?

La gaviota (Silvio Rodríguez. «Unicornio», 1982)

I live in Vigo. On a ninth floor. Close to the sea. On the rooftop of my neighbors, nex to a chimney, a pair of yellow-legged gulls raises their chicks every spring. I had never watched it. It’s a touching and curious spectacle.

Seagulls. Almost no one will tell you anything good about them. They soil your car, mug your son’s sandwich on the beach, scour for food in garbage bins, wake you up squawking over the weekend…I did not value their presence either: they’re there every day.

Yellow-legged gull (Larus michahellis) in Cíes Islands. Credits: Gerardo Fdez. Carrera

You don’t appreciate what you have until you lose it. Last year we noticed for the first time in my house that something was missing. The habitual fuss of gulls vanished although the usual couple didn’t miss their rendezvous.

In July I found a dead seagull in the street, at my door. A Colombian visitor in our laboratory asked me surprised by the dead seagulls in Vigo.

On June 26, 2019, the Galician Society of Ornithology (SOG) published a press release warning of the collapse in the populations of yellow-legged gulls.

The Cies islands no longer host the largest colony in Europe after a 78% decrease between 2006-2015 (from 16,035 to 3,520 couples…and in 1996 there were 19,000).

Yellow-legged gull in América beach (Nigrán). Credits: Santi Fraga.

Their decline is generalized in the National Park of the Atlantic Islands, located in the southern Rías Baixas (Cíes, Óns and Sálvora Is.) but also further north, in the Costa da Morte (Sisargas Is.).

In Vigo the calls to remove nests from the rooftops have also been greatly reduced, suggesting a significant decrease in the city.

Many will say: «there were too many.» Right. But how far do their populations should fall to be alarmed and take action?

Seagulls have descended globally in Europe. For example, herring gulls (Larus argentatus) -previously considered a pest- are now included in the UK’s red list of conservation concern.

According to the SGO there must be several causes behind the situation in Galicia: the recent closure of landfills, less food through fishing discards…but there’s something else. I literally quote the SGO:

[…] SGO’s expert ornithologists have for many years detected a large number of seagulls in their nesting areas that at the time of reproduction seem to be affected by some type of poisoning that in a few days, or even just a few hours, weakens them and culminating in full paralysis until death […]
Often, seagulls affected by this syndrome die on the coast, in ports and on beaches commonly used by citizens, and even on the streets.

Less and less seagulls in Galicia («Cada vez menos gaivotas en Galicia». Álvaro Barros, SGO, 2019)

Last year I spoke with Alberto Velando (Animal Ecology Group, University of Vigo), who’s been studying seabirds in the area for many years and explained to me that these deaths have been going on for a decade. For more information, he told me to contact Salvador García Barcelona (IEO Málaga).

Days later I sent him an email and what Salva answered left me stunned:

[…] I’ve been collecting gulls with paralysis syndrome on the coast of Malaga since 2006. I retrieve them at home and then release them with PVC rings to keep track of them in the future.

I don’t know what the cause is, I communicated it several times to the local authorities in case it was convenient to do some kind of analysis. But they answered me that the seagulls were not precisely a priority because they weren’t protected.

So for a few years I freeze all the bodies that I collect and those that end up dying at home in case anyone ever wants to look for the cause of that syndrome. There will be almost two hundred frozen birds of various species, and they keep coming, but not in the form of mass deaths but rather as a trickle.

Lesser black-backed gull (Larus fuscus) with the paretic syndrome. Credits: SGB.

[…] Since 2006 I’ve collected about 500 seagulls by myself, most of them released alive after keeping them at home for a couple of weeks ingesting whey and fish. Fortunately there are other people in the province who collaborate in the collection and care of these gulls, mostly yellow-legged and lesser black-backed ones.

Some are even veterinarians and offer the full potential of their clinics for their care. Our ability to continue collecting gulls is very high since a few months ago we managed to centralize their collection in several veterinary clinics and two canine residences.

They also give us notices from the cleaning services of some beaches and some patrols of the local police of Málaga. And once they reach us in the points we have (mainly to the Protective Society of Animals and Plants of Málaga) they remain in captivity for a few days, are ringed with PVC and released somewhere near predictable food sources, such as the fishing port.

If some gull dies, they keep the bodies and freeze them and then do the autopsies. From post-mortem exams we get a lot of information about the birds (sex, stomach content, molt…). The pity is that there’s no way to carry out analyses to know the cause of these poisonings as there’s no funding or specific projects in progress.

What are the causes of this syndrome? The only work I’ve found in the Iberian Peninsula on the causes of this syndrome was published in 2014 at the University of Lisbon: the doctoral thesis of Susana Patrícia Veloso Soares: «PARETIC SYNDROME IN GULLS (LARIDAE) IN THE SOUTH OF PORTUGAL». The author studied 148 seagulls received at a wildlife rehabilitation center in Algarve (RIAS) between 2009-2012.

The most common symptoms were muscular weakness, wing-drop, ataxia, diarrhoea (cloaca distended), depressed mental status and poor general condition (feathers soiled, thin and dehydrated specimens). Only half of these survived but the analyses failed to reveal the possible causes for this syndrome.

In her work Susana examined, indeed, a lot of them: aspergillosis, salmonellosis, sarcocystosis, botulism, metal poisoning (lead, copper, mercury), pesticides, lack of vitamin B1 (thiamine) and…microalgal toxins.

Harmful algae can seriously intoxicate all kinds of marine fauna through the bioaccumulation of toxins in the food chain (or by other ways). In this blog I’ve cited examples of that in several regions of the world (1,2,3,4,5), but in the Iberian Peninsula there are hardly any confirmed cases of fatal poisoning by biotoxins in seabirds.

Lucía Soliño (IPMA) told me two of them. First, the death of several dozen pelagic seabirds on the coast of Catalonia in 2007 (mainly Mediterranean shearwater, Puffinus yelkouan) associated with amnesic toxins (domoic acid) produced by diatoms of the genus Pseudo-nitzschia.

And secondly, the death of fish and several specimens of waterfowl (common coots and black-headed gulls, among others) in the Doñana National Park (Huelva) in 2004, in this case by microcystins produced by cyanobacteria (Microcystis aeruginosa).

Cory’s shearwater (Calonectrys borealis), one of the species studied by Soliño et al. (2019). Credits: Niclas Ahlberg. Source: niclasahlberg.se

Last year Soliño et al. also published a study on the presence of amnesic toxins (domoic acid) in pelagic seabirds of the Canary and Balearic Islands, detecting these toxins in almost half of the analyzed individuals.

The birds showed no symptoms of intoxication, but this study demonstrates a chronic exposure to domoic acid, worrying due to the possible increase in HAB’s of the genus Pseudo-nitzschia, associated with global warming.

A few years ago, the Parque Nacional das Illas Atlánticas sent us samples of four seagulls with paralysis syndrome from Cíes. But the biotoxin results were negative for the compounds studied (PSP and ASP toxins).

Other possible causes. In northern Europe (Baltic Sea) Balk et al. (2009) associated deaths with symptoms of paralysis in water and land birds in recent years with a deficiency of vitamin B1 (thiamine). But the conclusions of that study were then questioned.

In 1964, residues of various chlorinated hydrocarbon insecticides were discovered in terns and spatulas, dying or already dead, on the island of Texel (​​Wadden Sea, The Netherlands). The symptoms -tremors and convulsions- were compatible with poisoning, probably due to neurotoxic substances.

Fishermen cleaning fish in Essaouira (Morocco). Credits: F. Rodríguez

The usual suspects of gull paralysis syndrome, based on what’s been reported in other European countries, are the botulinum toxins. Although, as we shall see, in the case of the Iberian Peninsula, the suspect is far from being «behind bars».

Avian botulism is usually due to forms of botulinum toxin C / D, distinct to those that affect people by consuming, e.g., tinned food in poor condition.

It affects waterfowl all around the world. Its impact in Europe is greater in the north (Denmark, Sweden, Holland), but also in the United Kingdom and France.

Clostridium botulinum. Source: fineartamerica

It can harm birds in lakes, marshes and public parks that feed on invertebrates (crustaceans, mollusks) that concentrate these toxins.
In fact, avian botulism was first registered in the US in 1910 as «western duck sickness«.

Clostridium botulinum spores are present in terrestrial and aquatic soils, as well as in decomposing organic matter that gulls scavenge in landfills.

These places favor the development of the bacillus and the birds themselves can transport spores from other areas (in their feathers and excrements), setting the basis for further intoxications.

Botulism produces weakness and sagging: individuals cannot fly or barely walk. The potency of these toxins produces fast death by suffocation, drowning (because they can’t hold their heads out of the water), or even if they survive for a while, they eventually perish from starvation or thirst.

Very sad and worrying. These symptoms reminiscent of botulism are those exhibited by the affected seagulls in Portugal and Spain (Andalusia and Galicia).

Celia Sánchez Sánchez, who was a veterinarian at the Wildlife Recovery Center of El Acebuche (Doñana), quotes that a few years ago registered spastic paralysis in herons and some gulls. They named it the syndrome «of His Highness«: the individuals remained rigid, like cardboard, gradually weakening until they died from starvation.

They also frequently received seagulls with paralysis syndrome. It happened not only in the coastal provinces, so one of their hypotheses was the intoxication at landfills. However, the diagnoses through histopathological studies did not elicit the causes of these symptoms.

Although botulism has already been cited in local media such as La Voz de Galicia («Avian botulism, origin of the descent of the seagulls, 20-VIII-2019»), the tests performed in the «Parque das Illas Atlánticas» have been negative.

The only certainty is that there are hardly any studies and much less conclusive evidence to establish the agent (or agents) involved in the syndrome of paresis/paralysis in the Iberian Peninsula.

If they were another animal with greater sympathy it would be easier. That’s why public and private initiatives (from natural parks, research organizations, associations and individuals such as Salvador García [I recommend his twitter @salvagaviotas]) are so valuable for rehabilitating birds, collecting data and biological material for further research on the reasons of this syndrome.

Common guillemot. Credits: blogdeaves

This summer I attended a talk by Pablo Pereira Sánchez, in the visitor center of the National Park das Illas Atlánticas. He mentioned that Galician ornithologists have a hole in the heart in the shape of a guillemot.

During his talk (which fascinated me), I learned that species change their distribution areas for complex reasons (natural and/or anthropogenic ones). It’s not always in our hands to prevent them from leaving, nor is it that new ones arrive.

But it’s indeed our duty to put into action the resources to conserve and showcase a natural heritage that we must protect from our activities and impact on the climate and ecosystems.

Acknowledgments: for the information, review of the text and pictures to Salvador García Barcelona, ​​Lucía Soliño Alonso, Celia Sánchez Sánchez, Pablo Pereira Sánchez, Alberto Velando Rodríguez, Pilar Riobó Agulla, Santiago Fraga Rivas and Gerardo Fernández Carrera. Thank you very much everyone for your comments and collaboration, which contributed so much to elaborate the final version of this post.


  • Balk L. et al. Wild birds of declining European species are dying from a thyamine deficiency syndrome. PNAS 101:12001-12006 (2009).
  • Gutiérrez R. et al. Toxic algae in the Western Mediterranean: a new threat for the critically endangered Balearic Shearwater Puffinus mauretanicus. Waterbird Society Meeting (2007). DOI:10.13140/RG.2.2.22752.12807
  • Koeman J.H. et al. Residues of chlorinated hydrocarbon insecticides in the North Sea environment. Helgol. wiss. Meeresunters. 17:375-380 (1968).
  • López-Rodas V. et al. Mass wildlife mortality due to cyanobacteria in the Doñana National Park, Spain. Veterinary Record 162:317-318 (2008).
  • Ortiz N.E. & Smith G.R. Landfill sites, botulism and gulls. Epidemiol. Infect. 112:385-391 (1994).
  • Rocke T.E. & Barker I. Proposed link between paralytic syndrome and thiamine deficiency in Swedish gulls not substantiated. PNAS 107: E14 (2010).
  • Sociedade Galega de Ornitoloxía. Cada vez menos gaivotas en Galicia. Enlace web: SGO.
  • Soliño L. et al. Are pelagic seabirds exposed to amnesic shellfish poisoning toxins? Harmful Algae 84:172-180 (2019).
  • Veloso Soares S.P. Paretic syndrome in gulls (Laridae) in the South of Portugal. Universidade de Lisboa, 180 pp. (2014).
  • Web sources: Birds of Conservation Concern 4. & In defense of seagulls: they’re smart and they co-parent, 50/50 all the way (NY Times, 23-VIII-2019).