Kleptoparasites follow Brown Pelicans
October 19, 2019
Early each morning from the top of Lighthouse Hill, Point Blue biologists count Brown Pelicans roosting on the South Farallon Islands before they leave to forage for the day. For the past month, pelican counts have been consistently high, with numbers frequently topping 1000 individuals. On October 17th, 3301 pelicans were observed, the highest number of individuals sighted since 2006, when 3443 were seen on August 30th, and 3799 were observed on September 1st.
Along with the pelicans, Heermann’s Gulls numbers have also been increasing. Throughout September and early October, no more than three individuals were sighted in a day. On October 13th, 30 arrived, followed by 111 individuals on the 14th. Heermann’s Gulls breed in colonies primarily on one small islet, Isla Rasa, in the Gulf of California, Mexico. In the 1990’s, a small population established the only breeding colony in the U.S. on artificial islands in Robert’s Lake in Seaside, California. In recent years these islands have eroded away, and the birds have begun attempting to nest on the rooftops of commercial buildings surrounding the lagoon, with limited success. After the breeding season, Heermann’s Gulls disperse northward along the Pacific Coast to take advantage of more abundant food resources in the California Current.
The Brown Pelicans that we see on the Farallones also dispersed northward from their more southerly breeding grounds. After breeding on rocky islands off the coast of southern California and Mexico, they move north along the Pacific Coast during the summer and fall months. Both species are regularly observed as far north as British Columbia during their post-breeding dispersal. Heermann’s Gulls frequently kleptoparasitize Brown Pelicans, meaning that the gulls follow plunge-diving pelicans and surround them as they come up from the water with fish in their gular pouch. The gulls attempt to grab fish out of the pelican’s bill, even going so far as to land on a pelican’s head to force them to regurgitate fish. The high number of pelicans present on the Farallones may explain the arrival of so many Heermann’s Gulls. In 2006, high numbers of Heermann’s Gulls also coincided with high pelican counts.
One adult Brown Pelican wearing a blue leg band has recently been observed on Southeast Farallon Island. The band has been traced back to International Bird Rescue, indicating that this bird was probably injured or ill at some point, then treated and rehabilitated before making the flight to the Farallones. More information can be found here: https://www.bird-rescue.org/our-work/research-and-education/banding-program.aspx