Los Farallones

Dispatches from Point Blue’s field station on the Farallon Islands National Wildlife Refuge

The Diet of Emerging Chicks

Newly hatched western gull chick.

Two week old Cassin’s auklet chick.

It’s happening. All over the island chicks are busting free from their shells after weeks of incubation. In order to provision these new balls of down during the brooding phase, parent birds have begun the task of collecting prey from the ocean, or in the case of the western gulls, from regional dumps and local supermarkets back on the mainland. Scott Shaffer, a researcher with San Jose State University, and his grad student Emma Kelsey recently deployed GPS tags on several birds out here on SEFI, in an effort to provide insights into the foraging ranges of individual birds. Gulls are professional opportunists. We’ve watched them regurgitate a wide range of prey from fish and squid, eggs and chicks from neighboring nests, to chicken breasts and plastic action figures. As you can see from the tracks below, foraging patterns vary widely amongst individuals; some apparently prefer the bounties of the continental shelf break while others prefer the bounties of the city.    

Once a successful foraging trip has been made, different species of birds have adopted different strategies for food delivery. In response researchers have developed various approaches to determine what exactly parent birds are feeding their young. Western gulls for example barf up ingested prey to their chicks, which from a distance has the appearance of, well, barf. Thus a barf sample collected from a parent bird is necessary to key out individual prey items. This same technique is also used for determining Cassin’s auklet and cormorant diet. Birds like common murres or pigeon guillemots require a far less messy approach. These two species of Alcids bring back a single fish or invertebrate to their awaiting chick after each foraging bout. Parent birds will fly in from the sea carrying prey items in their bill, visible enough to be identified with binoculars from a distance. Other Alcids, such as rhinoceros auklets, delivery food only after the sun sets when visual observation is impossible. These birds have to be netted at dusk to obtain samples of fish they bring in. Rhinoceros auklets carry multiple fish of varying species in a single bill load back to their chicks, some samples with as many as 10 prey items.

The gang from 2012 during a pigeon guillemot diet watch.

Energetically, not all prey sources are equal. Young of the year salmon, for example, are far more beneficial to a growing fluff ball than Pacific Saury. Though at times rather invasive, determining what chicks are eating out here is crucial for understanding the overall health and status of seabird populations on the Farallones, as well as the overall health of the ocean where these birds forage and make a living. Seabirds are excellent indicators of changing oceanic conditions, during a time when our oceans are experiencing change at an unprecedented rate.

Measuring salmon brought in by a rhinoceros auklet, with bags of Cassin’s auklet barf samples (mostly krill)  on the left.