Wednesday, May 9, 2012

Why "The 'Yeow' Call", you ask?

I was stewing away in the library, sitting by the window and studying Organic Chemistry for my final exam tomorrow, when I looked up from the mind-boggling hexagonal shapes for a split second to take a sip of coffee and, at that exact moment, the setting sun broke through the heavy bank of purple clouds that had lined the sky all day. It flashed bright orange, the light reflecting off of wet walkways on the Arts Quad and I just had to lay aside all useless cramming of information that I don't want nor will ever need and instead employ my time in doing something more productive. Hence, this.

Also, a from-a-year-ago-but-inspired-by-the-sun-today photo of sunset on Appledore

The countdown stands at nine days and the research project I'm going to be working on at Shoals has started to take a more definite form. The island is host to mainly two gull species – the Great Black-backed Gull (Larus marinus) and the Herring Gull (Larus argentatus). These gulls breed on the island during the warm(er) summer months in dense mixed-species colonies, coming in close contact with each other. A number of social interactions ensue – from fighting over territories and eating each others' eggs to dutifully feeding their chicks and dive-bombing intruders; a gull colony is never a dull place. These gregarious birds also have a number of difference vocalizations, each used for a specific purpose. Common ones include the long-drawn courtship called the 'mew' call, the shriller, trumpeted, territorial 'long' call, the 'kek kek kek' given upon disturbance and finally, the call I'm interested in, which is also used to signal a potential threat, the infamous 'yeow' call.

The call consists of a clear, sharp, (comparatively) high-pitched note of descending frequency that is given either singly or repeated continuously. Other gulls have been observed to react to this call by becoming alert and looking around as well as, at times, chiming in with the originally calling bird. Additionally, both Herring and Great Black-backed Gulls respond similarly to calls given by individuals of their own species and those of the other species. And, even more additionally, it sounds like the nature of the call depends on the level of threat – calls given in response to a high threat-level sound different than those elicited at low threat-levels.

My question, then, is what exactly do the birds change in their calls to signal the threat-level; that is to say, is the call given in response to a high threat-level of a higher frequency? Or are there, perhaps, shorter intervals between successive calls? Or, perhaps, each individual call is longer? There are countless possibilities, countless variations a gull can make to a simple 'yeow' call. Going through a bunch of recordings (coursey of Sarah MacLean, one of the interns last year), however, I have been able to identify a couple of parameters that change significantly between the two threat-levels (high and low). This summer (correction: in nine days) I will attempt to answer my question by artificially changing these parameters on recorded calls and then playing them back to gulls and recording their responses. By my prediction, the gulls' level of agitation will be higher in response to the calls manipulated to signal a high-threat level, thus showing that the two parameters are indeed varied by the gulls themselves to signal threat-level, and that other gulls pick up on these variations and understand their meaning.

Screenshot of a Herring Gull 'yeow' call being analyzed in RavenPro 1.4

The experiment will involve a lot of setting speakers near gull nests and then hiding in bushes that may or may not contain poison ivy to observing the gulls and note their response. The second, more uncomfortable, element may be eliminated by using a video camera but the idea of leaving a piece of expensive equipment unattended near a gull nest isn't the most compelling and remains to be deliberated. Either way, it sounds like a promising or, at the very least, fun project and I hope to faithfully report each and every misadventure and guano-hit that befalls me along the way!

Today, however, I leave you with a panoramic view of part of the island from Kiggins Commons, the dining hall cum place to hang out if you want wireless.

View from Kiggins -- a part of the northern half of Appledore

No comments:

Post a Comment