Research Analyzes Meaning of Bird Song
LEXINGTON, Va., Sept. 20, 2013 – Summer research in VMI’s biology department is often an adventure. That was especially true of a summer project that delved into the complexities of avian communication.
Col. Dick Rowe, professor of biology, and Alyssa Ford ’14 worked throughout the summer to observe and analyze the behavior of tree swallows nesting at Sky Farm, Vista Links Golf Course, and area residences. Parental defense in response to pre-recorded alarm calls varied from guarding the entrance to dive-bombing the researchers.
“It has been pretty exciting,” said Ford. “Being dive-bombed by a tree swallow is a one-of-a-kind experience. It’s weird, scary, and terrifying, but it’s also pretty cool.”
The research builds upon work done last summer by Rowe and another cadet, Sarah Hunziker ’13. That work produced recordings of tree swallows responding to a variety of stimuli, including a recording of the alarm call generated when bluebirds attempted to capture a tree swallow’s nest box.
“Tree swallows’ natural born enemies are bluebirds,” said Ford. “They always fight for any cavity space whether it’s in the wild or in our nest boxes.”
By isolating the tree swallow’s alarm call from the circumstances that elicited it, researchers can start to understand what kinds of information, if any, their calls convey.
“To find out what these vocalizations mean, you play them in the absence of the stimulus,” said Rowe. “The question is: do they exhibit guarding or protective behaviors as if there’s a bluebird trying to take their nest?”
Observing the birds’ behavior involved exhaustive field research at each site, which took place around sunrise each morning. Each day’s observations included setting up a tripod near a nest, playing the alarm call, noting the swallows’ behavior, and peeking in the nest to check the status of the eggs or nestlings. This was repeated for each nest box.
“We collected a ton of data. We started our data collection three days after graduation and kept going until July,” said Ford. “I was waking up before 6:30 every day. Birds don’t have weekends, so I was out there on weekends too.”
At one point during the summer, when a predator attack on the nest boxes at Sky Farm ended data collection there, alternate sites had to be found. Thanks to the hospitality of the Vista Links Golf Course and area residents who offered their own properties as research sites, the research continued.
The recordings did indeed generate defensive behaviors, including perching on the nest box, guarding the entrance, and circling above the nest box, and male and female tree swallows exhibited distinct differences in behavior.
“It’s a nice, compact study. We got solid data, and we can explain it based on what we know about the bird’s behavior,” said Rowe.
In addition to playing the bluebird tape, the two played a predator tape on a few occasions that elicited more aggressive responses.
“On the way out to the Buena Vista golf course, we saw a lot of tree swallows on the power lines. Once we started playing the call we were in a vortex of like 70 birds flying around us. They were all tree swallows,” said Ford, noting that the swallows departed as quickly as they arrived. “It was funny; when the tape stopped they all just went away.”
Though there’s not enough data to make conclusions about the tree swallows’ responses to the predator tape, these initial observations led Ford and Rowe to believe that there’s a significant difference in the messages of these two alarm calls.
Knowing that these calls carry meaning, future research will attempt to zero in on what information the birds are conveying through their calls.
Rowe is looking to continue the research next summer to gather more data on the predator call. The nest boxes at Sky Farm have been rebuilt with predator-deterrent measures, and the Vista Links Golf Course will continue to support the research. –John Robertson IV