Birds have fascinated Sarah MacLean ’13, natural resources, since early adolescence. Following her love of ornithology, last summer MacLean conducted a research project at the Shoals Marine Lab. There, she examined the gradient of reactions displayed by the gulls of Appledore Island towards threatening and non-threatening signals.
MacLean concluded that the gulls on Appledore are able to recognize threats by judging different types of auditory and visual cues and responding correctly according to the threat level. A higher level of danger corresponded to a more noticeable and hostile reaction.
She found that the human presence disturbed the gulls more so than any other stimulus causing them to move away from the nest, take to the sky or become aggressive. “The gulls would fly up, dive bomb your head, pooh and screech. And the chicks tend to regurgitate when they’re agitated.” MacLean said, “My summer was so full of getting horrible sea bird related fluids on me.”
In addition to proving the human stimulus, MacLean presented different types of auditory and visual signals to more than sixty nesting gulls on the island. She then observed and recorded the birds’ reactions to the cues, which changed based on how threatening the gulls determined the stimuli to be.
“Anti-predator behavior is so energy costly, so the birds have to be able to distinguish between varying levels of threat and respond appropriately,” she said. MacLean’s work, which combined many different stimuli and gull reactions observed in other studies, synthesized for the first time a variety of information that had previously been looked at separately.
“I tested the gulls’ response to each of the auditory cues and observed this whole gradient of what was basically non-threatening all the way up to most threatening, a visual human standing right next to their nest,” MacLean said. To quantify the gulls’ gradient of reactions, MacLean assigned a 0-7 categorical scale where each number specified a specific set of behaviors. No reaction constituted a 0; other responses like low levels of caution; higher levels of caution; displaying alarm calls; getting up from the nest warranted; moving from the nest; and flying away from the nest made up the gradient.
At the non-threatening end of the stimuli scale was the sound of the Song Sparrow, a small songbird that makes nests across the island. The gulls typically hear the sparrows sing, and as a result they showed virtually no reaction to this non-threatening species, MacLean found.
Next, MacLean introduced the novel birdcall of the Western Scrub-Jay. She had anticipated this noise to be relatively non-threatening to the gulls, but instead the birds reacted with more surprise than they did to the Song Sparrow chirps. According to MacLean, the gulls probably gave such excited responses to this sound because they had never heard the Western Scrub-Jay call before.
“This was kind of the wild card of the stimuli. I wasn’t sure what the gulls’ reaction would be,” she said. “But it turned out to be between their complete non-reaction to the Song Sparrow and their heightened reaction to the alarm calls of other gulls on the island.”
MacLean also analyzed the reactions that Appledore’s two gull species, the small Herring Gull and the aggressive Black Backed Gull, exhibited towards each other. “We saw that the Black Backed Gulls don’t pay as much attention to the Herring Gull alarm call as they do to other Black Backed alarm calls. On the other hand, the Herrings give equal attention to both calls,” MacLean said.
In another test, MacLean introduced the call of the Bald Eagle, a potential gull predator. The gulls felt more threatend by this call than by most other bird calls.
MacLean described her time and research with the gulls as rewarding. “Implementing this study that I put a lot of time into designing and then seeing the results was really satisfying,” she said.
“I started out as a bird feeder watching birds and sending in pictures to Cornell’s Project FeederWatch program in middle school. If you stick with it, eventually you will have the chance to participate in these fulfilling and exciting projects. My experience at Shoals was fantastic,” she said.