Researchers Pilot New Methods in Study of Brandt’s Cormorant Diets on Alcatraz

To learn about what the cormorants were eating, the researchers collected and counted indigestible structures like these fish otoliths, or ear bones, from the birds' nests and pellets. Otoliths are unique to different species of fish.

NPS

October 2019 - A new paper just published in the latest issue of Waterbirds looks at sampling methods of seabird diets on Alcatraz Island. The three-year study took place between 2014 and 2016 and evaluated best methods for determining diets of Brandt’s cormorants nesting on the Island. Diets are typically analyzed after the nesting season through the collection of regurgitated pellets containing undigested prey. But because pellets are susceptible to being blown away by the wind or disintegrating over time, they may only represent cormorant diets towards the end of the breeding season.

For this study, researchers used a new method and collected entire nests, in addition to pellets. Cormorant nests are large and robust, and the researchers hypothesised that they would accumulate remains of prey regurgitated by the birds throughout their months-long breeding period. Upon comparing the dietary information contained in the nests and the pellets, the researchers concluded that while there was a lot of overlap in the prey species they contained, whole nests contained a greater diversity of prey items and different amounts of individual prey species. The researchers also found that Brandt’s cormorants on Alcatraz ate 25 different species of prey, dominated by northern anchovy, speckled sanddab, and plainfin midshipman.

Check out the full article, or contact Tori Seher to learn more.

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