From Scavengers to Killers: The Transformation of Kelp Gulls

Kelp gulls (Larus dominicanus) are a seabird species native to the Southern Hemisphere and currently under protection. These gulls nest and hunt along coastlines and islands throughout the region. Like other gull species, kelp gulls exhibit an opportunistic feeding behavior, consuming a wide variety of prey, including fish, coastal reptiles, and even some larger mammals. Some kelp gulls have also gained notoriety for their kleptoparasitic tendencies, stealing food from other predators.

Initially regarded as a conventional gull species, kelp gulls have demonstrated an extraordinary behavioral adaptation over the past half-century. As indicated in the title, these birds were originally scavenging on whale carcasses in addition to their regular hunting practices. However, a pivotal shift occurred when they discovered that their feeding strategy did not necessarily require whales to be deceased. A single inquisitive gull altered their feeding behavior, transforming them into aggressive predators.

Abstract

Kelp gulls have developed a novel feeding strategy by targeting live whales surfacing for air, consuming their skin and fat. This behavior has become habitual, prompting adaptive responses from whale mothers and their calves. In response to these persistent attacks, adult whales have modified swimming speed, resting postures, and overall behavior. While adult whales can endure such attacks, calves, whose skin has not yet thickened, suffer severe injuries, often resulting in mortality. Additionally, due to their higher breathing frequency, calves surface more frequently than adults, making them more vulnerable to these predatory interactions.

Since the mid-1990s, kelp gull-induced injuries have significantly increased. By 2003, unusually high mortality rates among calves were observed in certain regions, with growing evidence suggesting that gull harassment was a primary contributing factor. The survival rates of harassed calves during long migrations declined drastically within their first year. Consequently, scientists conducted extensive observations of various whale populations from 1974 to 2017. Their findings indicate that gull harassment has had a measurable impact on whale population dynamics.

Source: Ivan Stecko on Pexels.com

The Onset of the Phenomenon

Each winter, southern right whales (Eubalaena australis) migrate to the coastlines of South America to nurture their calves. While the exact timeline remains uncertain, a particularly opportunistic kelp gull (Larus dominicanus) deviated from scavenging on beached or deceased whales and instead took a bite from a surfacing whale. Observing that this action could be performed without repercussions, the gull continued the behavior, which eventually spread throughout the population.

This phenomenon was first documented in the early 1970s on Golfo San José Island. By the 1980s, it had been classified as a parasitic interaction and subsequently expanded to the neighboring Golfo Nuevo by the 1990s. In the early 2000s, the frequency of these attacks and harassment increased rapidly. The proportion of whales exhibiting injuries inflicted by kelp gulls surged from 2% to 99% compared to the initial reports. Initially, adult whales were the primary targets, but by the mid-1990s, calves became the primary victims, suffering from persistent harassment and severe injuries inflicted by the gulls.

Source:Macarena Agrelo(a1), Rodrigo A. Martínez Calatalán (a2–a4) and Fredrik Christiansen (c).

These events had profound negative effects on the psychological well-being and overall health of the whales. Between 2003 and 2013, a total of 672 whale deaths were recorded, with 91% of the deceased calves being younger than three months old—a mortality rate far exceeding expected levels. A recent study established a positive correlation between calf mortality rates and kelp gull harassment, further reinforcing the severity of the issue.

How Did Whales Respond?

In response to increasing attacks, mother whales developed various defensive strategies. The initial response was to swim away quickly and distance themselves from the shores. However, this proved highly energy-consuming and unsustainable, as calves could not match their mothers’ speed.

As a secondary adaptation, whales attempted a new surfacing technique, exposing only their tails and heads while keeping their backs submerged. Although initially effective, kelp gulls quickly adapted by diving underwater to continue tearing flesh from the whales’ backs. Over time, these once opportunistic scavengers evolved into relentless predators, consuming live calves while they surfaced to breathe.

This gruesome cycle persisted for years—until whales devised an original and effective strategy. By tilting their heads at a specific angle while surfacing for air, they made it impossible for the gulls to attack, as their heads were covered in thick callosities that the birds could not penetrate. This new behavioral adaptation ultimately proved successful, allowing whales to counteract the persistent gull harassment.

Conclusion

Kelp gulls have truly broken the mold with this behavior. We don’t know exactly when it all started, but based on the data we have, this has been going on for at least fifty years. There are plenty of parasitic and mutualistic relationships in nature, but I’ve never seen anything quitelike this. Nature is full of surprises and wonders.

References:

• Agrelo, M., Marón, C. F., Daura-Jorge, F. G., Rowntree, V. J., Sironi, M., Hammond, P. S., Ingram, S. N., Vilches, F. O., Seger, J., & Simões-Lopes, P. C. (2023). Effect of kelp gull harassment on southern right whale calf survival: a long-term capture–recapture analysis. Biology Letters, 19(6). https://doi.org/10.1098/rsbl.2023.0119
• Oceana. (2023, May 18). Kelp Gull | Oceana. https://oceana.org/marine-life/kelp-gull/