Novel vs. Traditional Approaches to Sampling Seabird Diet: Nest and Pellet Analysis of Brandt's Cormorants (Phalacrocorax penicillatus)

Abstract

Diet of the Brandt's Cormorant (Phalacrocorax penicillatus) breeding population on Alcatraz Island, California, USA was determined for 2014-2016 using two sampling methods. Regurgitated pellets and complete nests were collected, once all chicks had left colonies, to reduce disturbance to nesting birds. Results identified 25 prey taxa including 23 teleost fishes and two cephalopods. Diet was dominated by northern anchovy (Engraulis mordax) and speckled sanddab (Citharichthys stigmaeus), with plainfin midshipman (Porichthys notatus), sole spp. (Pleuronectidae), and sculpin spp. (Cottidae) rounding out the top five abundant species. Comparison of diet by sample type yielded similar results in terms of prey species composition, but total diet differed between the two samples due to rare prey items being more prevalent in nest samples. Determining cormorant diet using a combination of pellet and nest samples is a novel approach with the potential to provide a better representation of prey consumed during the full breeding period (up to 6 months for cormorants), and similar methods may be informative for other seabirds that produce pellets. In contrast, diet determined from pellets alone may only represent a short time period just prior to collection, as pellets are often blown away by strong winds or disintegrate through trampling and/or exposure to the elements in open seabird breeding colonies.