Unveiling the diet of the Kelp Gulls that nest in Andean Patagonia – Journal of Zoology Blog

Rosciano, N.G., Ojeda, V., Polito, M.J. and Yorio, P. (2025). I know what you ate last breeding season: anthropogenic food subsidies as a driver of the trophic ecology of nesting Kelp Gulls in a freshwater ecosystem in Patagonia, Argentina. Journal of Zoology, vol. 327, pp. 349-362. https://doi.org/10.1111/jzo.70060

Gulls are seabirds with the most varied reputation: admired by some but often considered a problem in many parts of the world. They nest on rooftops, gather around landfills, and even cause problems near airports. This reputation is often associated with their incredible ability to adapt and take advantage of almost any food source available (typical opportunist birds). Kelp Gulls (Larus dominicanus) are a widespread species in the Southern Hemisphere, and in Argentina, they mainly breed in marine environments. Their generalist opportunistic behavior has been of great interest to study along the Argentine Patagonian coast. Studies have shown a frequent association of Kelp Gulls with urbanized environments, taking advantage of predictable anthropogenic food resources. However, Kelp Gulls also inhabit freshwater environments in Patagonia, where their relationship with adjacent ecosystems and human activities remains poorly studied.

I was not much of a gull fan, I should say… but my supervisor convinced me of the importance of conducting a study on this Kelp Gull population breeding at the Nahuel Huapi Lake. And that is how this story started, pursuing a post-doctoral fellowship on the Kelp Gull Foraging Ecology. In a world where human-derived food sources are increasingly available, particularly from landfills and fishery discards, it is not surprising that species with such flexible feeding habits are thriving. However, this adaptation can come at a cost, both to the birds and to the ecosystems they inhabit, including the people immersed in these ecosystems.

In the Nahuel Huapi National Park, located in north-west Patagonia (Argentina), Kelp Gull colonies are found on small islands within the lake that gives the park its name. Between 2019 and 2020, we studied two of the largest colonies to understand what these gulls were eating during the breeding season, and to what extent their diet relied on food of anthropogenic origin.

Multiple challenges had to be faced, since the beginning of the breeding season of the Kelp Gulls coincided with the windy spring in the city of Bariloche… and therefore, crossing to islands with a small boat was usually quite a bumpy and wet ride, especially in the afternoon when returning (ask my colleagues and field assistants about their memories, and this will surely come up!). Fieldwork involved early morning trips to the islands to collect pellets, regurgitates, and blood samples. Working in 2020, in the middle of the COVID-19 pandemic, added an unexpected challenge. Getting out to the field and coordinating logistics under all the protocols was quite an experience. Despite everything, I am very grateful to all the people who trusted me as a driver of the little boat on the furious Nahuel Huapi Lake, and also to everyone that worked either at the nesting area and/or in the lab. This work would not have been possible without them.

The results of this study revealed that the Kelp Gulls that nest at the Nahuel Huapi Lake fed on terrestrial and aquatic prey, and also garbage. The proximity to the Bariloche and Villa La Angostura garbage dumps, located about 20 and 24 km from the colonies, respectively, likely favored this opportunistic feeding behavior.

Feeding on garbage, however, can have negative consequences. Birds that consume waste are exposed to pathogens, toxic substances, and non-edible materials such as plastics which can cause severe internal injuries. We even found plastic and glass fragments in samples from both adults and chicks, showing the risks of feeding in human-influenced environments. In addition, gulls that move between natural and urban areas can act as carriers of pathogens and contaminants, a concern that has grown in recent years with the spread of highly pathogenic avian influenza (HPAI H5N1).

Still, the most unexpected finding from this study came in 2019: a year when Kelp Gulls consumed large numbers of rodents. This unusual behavior was linked to a massive flowering event of the native Bamboo Chusquea culeou, which triggered a population explosion of native rodents such as Oligoryzomys longicaudatus and Abrothrix species. Along the northern shores of the Nahuel Huapi Lake, a large number of dead rodents accumulated, providing a sudden, abundant food source. Seemingly, the gulls did not hesitate to take advantage of it. Opportunistic feeding on rodents during bamboo masting events had been reported once before, in southern Chile in 1995, but it was striking to see it again during this study, when even chicks were being fed this prey type.

Our first encounter with rodent remains came while on the first fieldwork trip. We were marking nests and collecting pellets from the incubation period. We were surprised to see that most of them were like small fur balls, similar to pellets that can be found under owl nests for example. Later in the season we continued to find similar pellets, but we also found whole or parts of rodents in stomach contents from chicks. This was absolutely new to us, and we were all astonished.

This study shows the remarkable flexibility of Kelp Gulls in using whatever resources become available, whether natural or human-related. Such adaptability can benefit individuals by improving survival and breeding success, but it may also bring costs due to increased exposure to pollutants, pathogens, or plastic ingestion.

Kelp Gulls’ movements between urban areas, garbage dumps, and natural habitats make them excellent models to study how wildlife adjust to human-altered landscapes. They also remind us how closely connected our lives are to those of other species, even the ones we tend to overlook.

Kelp Gulls are extremely intelligent birds and have fascinating behaviors that make them an interesting study subject. In particular, this study of the trophic ecology of the Kelp Gulls in the Nahuel Huapi Lake provides essential baseline information for future assessments of the gulls’ demographic behavior and their role within the urban-lake-terrestrial ecosystem. Further studies are also needed to understand the use of anthropogenic food resources during the non-breeding season, since these supplementary food sources may be especially relevant for Kelp Gulls during the winter months, when weather conditions in the area can be severe. Understanding the diet composition of Kelp Gulls throughout the annual cycle will also help reveal the potential population-level consequences of changes in anthropogenic food availability resulting from changes in garbage management.

Indeed, I believe that one of the most important take-home messages is that our actions matter: what we throw into the garbage does not simply disappear, but it can reach nature and harm wildlife. And therefore, being conscious about how we dispose our waste and demanding good waste management for municipalities can protect wildlife … and also us!

Natalia Rosciano