An adult brown pelican can spread its wings more than six feet, but that size offered little protection when a river otter attacked one at Abbotts Lagoon on California’s Point Reyes peninsula. In October 2021, Terence Carroll and Megan Isadore, co-founders of the River Otter Ecology Project, watched as the otter forced the bird underwater and drowned it.

Carroll and Isadore already knew that otters were attacking pelicans. They had observed and documented evidence at the lagoon since fall 2018, while visitors contributed reports, photographs and videos. What no one had measured was how frequently the attacks occurred.

To find out, the pair began regularly surveying the lagoon’s shoreline. Across 19 visits in 2023, they documented 73 pelican carcasses linked to river otters.

Their study, published in the IUCN Otter Specialist Group Bulletin, records repeated predation between two animals recovering in Northern California. It also includes the first known reports of brown pelicans moving away when a river otter approached.

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River Otter Surveys Find 73 Brown Pelican Carcasses

In September 2022, Carroll and Isadore watched a group of otters feeding on a freshly killed pelican they had dragged ashore.

Otters commonly severed the pelicans’ heads and necks, apparently to reach the flesh around the breast. That signature allowed Carroll and Isadore to identify likely kills even when nobody witnessed the attack.

From June through November 2023, Carroll and Isadore surveyed the shoreline every 7 to 10 days. They mapped each carcass and marked its bones to prevent double-counting.

They found new carcasses on 17 of 19 visits, with as many as 11 in a single survey. The count was probably low because parts of the shoreline remained inaccessible and scavengers may have carried away remains.

During the 2023 surveys, the attacks did not appear to drive pelicans away. They continued resting in the same general area, even as new carcasses appeared along the shoreline. More carcasses generally turned up during weeks when eBird users reported more pelicans, perhaps because larger flocks left isolated birds around their edges.

Why River Otters May Target Such Large Birds

River otters already eat fish and waterbirds year-round at Abbotts Lagoon. An earlier report from Rodeo Lagoon proposed that poor water quality had reduced fish stocks, pushing otters toward pelicans as a substitute food source. The same environmental condition does not exist at Abbotts Lagoon.

Carroll and Isadore therefore think the attacks likely extend the otters’ usual hunting of waterbirds. River otters expend considerable energy and lose body heat while hunting in water, so they tend to pursue larger, less agile prey. A seven- to nine-pound pelican offers a substantial meal that an otter can drag ashore and eat without remaining in the water.

River otters and brown pelicans are both recovering in Northern California. Otters have returned to parts of the region, while pelicans were removed from federal endangered species protection in 2009. At Abbotts Lagoon, one recovering species is now preying on the other.

Brown Pelicans May Be Learning to Avoid River Otters

During several visits in late summer 2024, Carroll and Isadore found three carcasses in an area where they had counted about 20 during the same period in 2023. Because their 2024 visits did not follow the same regular schedule, they cannot yet say whether otters were killing fewer pelicans.

They also watched resting pelicans take flight when an otter approached. Another observer reported the same response at Rodeo Lagoon that August. These were the first known accounts of brown pelicans avoiding river otters.

Two encounters cannot show that the birds have learned to recognize otters as predators. But after years of attacks at Abbotts Lagoon, some pelicans had begun to leave when an otter came near.

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Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in the IUCN Otter Specialist Group Bulletin: NORTH AMERICAN RIVER OTTER (Lontra canadensis) PREDATION ON BROWN PELICANS (PELECANUS OCCIDENTALIS) AT ABBOTTS LAGOON, POINT REYES NATIONAL SEASHORE, CALIFORNIA, USA