Yellow-legged gulls: from the sea to the landfills?

The yellow-legged gull (Larus michahellis) is one of the most familiar and widespread seabirds in the Mediterranean region. Bold, vocal, and highly adaptable, this species has long thrived along rocky coastlines and offshore islands, including the iconic Medes Islands in Catalonia. But unlike many other seabirds that remain tightly linked to the marine environment, the yellow-legged gull has proven exceptionally versatile—capable of exploiting both natural and artificial food sources. Over the past few decades, this adaptability has raised new ecological questions: how much have they come to rely on human-altered landscapes? From garbage dumps to slaughterhouses, the line between wild and urban ecosystems is becoming increasingly blurred—and the yellow-legged gull might be one of its most emblematic examples.

Despite their ubiquity and generalist nature, recent studies suggest that their populations are quietly declining across parts of their range. The causes behind this downward trend remain unclear, making it more urgent than ever to understand how these gulls interact with the changing environments that now sustain them. 

Since 2018, researchers from the University of Barcelona and the Institut de Recerca de la Biodiversitat (IRBio) have been investigating the feeding ecology of yellow-legged gulls breeding in the Medes Islands. Their main tool? A combination of cutting-edge GPS/GSM tracking devices attached to adult gulls during the nesting season. These lightweight, solar-powered tags allow the team to monitor the birds' movements with high precision. These lightweight tags are fitted to the birds using a small, custom-designed harness and include a solar panel that extends their operational lifespan significantly, allowing for months—or even years—of continuous data collection. 

 The GPS (Global Positioning System) records the gulls’ exact location at regular intervals, while GSM (Global System for Mobile Communications) technology transmits that data via the mobile phone network—meaning researchers don’t need to recapture the birds to retrieve the information. This system gives unprecedented insight into where these seabirds are flying, how far they travel, and which areas they rely on to find food during this critical time of year.  This year, our team, liderated by Raül Ramos (associated proffessor), Céline Van Weelden (REDUCE technician), Núria Patiño (REDUCE PhD) and Jécar Rodríguez (Master student),  we came to Medes to tag and track some more yellow-legged gulls. 

Over the course of several years, the tracking data have revealed a consistent and surprising pattern: many of the marked gulls regularly abandon the nearby marine environment to forage deep inland. Rather than relying primarily on fish or other sea-based resources, these gulls are targeting landfills and slaughterhouses scattered throughout the province of Girona. Some individuals undertake repeated journeys to specific waste sites located tens of kilometers from the coast, showing clear and deliberate routes that suggest learned behavior and site fidelity. One particularly striking finding is the number of gulls flying all the way to Beuda, a rural area in the Garrotxa region, quite far from the sea. Others take off in the opposite direction, heading south toward other known sources of anthropogenic food. 

This reliance on human-provided resources, especially those far removed from the sea, has major ecological implications. It not only challenges our understanding of the yellow-legged gull’s niche as a coastal predator and scavenger, but also raises questions about how human waste and agricultural by-products are reshaping the behavior, distribution, and even population dynamics of generalist species like this one. 

These findings remind us that even the most adaptable and widespread species, like the yellow-legged gull, are deeply influenced by the landscapes we create. As human activity continues to reshape natural ecosystems, the behaviors, diets, and survival strategies of wildlife are also shifting in response. Understanding how these birds navigate a world increasingly dominated by our presence is essential—not just for their conservation, but for recognizing the broader ecological consequences of our growing footprint.