February 13, 2013
Flipping The Bird On Intruders – Song Sparrows Show Their Aggressive Side
Alan McStravick for redOrbit.com - Your Universe Online
A new study out of the University of Washington (UW) seems to build on a previous study reported on here at redOrbit only last month. The previous study, out of Duke University, relied upon a robotic ℠Frankensparrow´ to monitor sparrow behavior leading up to and during territorial brawls. The UW team wanted to learn about the threat signals given off by the defensive sparrow to a potential invader.
As it turns out, song sparrows typically employ an increasingly threatening signal strategy aimed at running off trespassing rivals. The first signal is the early warning where the defending sparrow will match the song of the intruder. This is followed by a show of wing waving. Wing waving is the quick flutter of one wing and the other, meant to intimidate the intruder. The team likens the act of wing waving as the sparrow´s version of “flipping the bird” to a rival avian. If these two symbols go unheeded by the trespassing offender, the likelihood of a full-on attack becomes all but imminent.
According to lead author ÃaÄlar AkÃ§ay, who undertook the research while a UW graduate student, “This is one of the most complicated communication systems outside of human language.” The hierarchical warning scheme, therefore, adds a bit more nuance to a communication system that has long been used as a model to study how people use and learn language. AkÃ§ay is now a postdoctoral researcher at Cornell University.
“Here we find that if a sparrow matches the intruder´s song as the intruder invades his territory, this almost always predicts that he will eventually attack the intruder,” AkÃ§ay said.
“We succeeded here because we recognized that song-matching is an early warning signal,” said co-author Michael Beecher, a UW professor of psychology. “We designed our experiment to simulate an escalating intrusion by another song sparrow, so that our subject would begin with low-level threat signals before switching to higher-level threat signals.”
Beecher, in pointing out the team´s recognition of song-matching, helped write a study published online this month in the journal Proceedings of the Royal Society B which presents the first evidence that song-matching is used as an early warning signal. Previous studies have alluded to the possibility of this behavior representing an overtly aggressive posturing on the part of the defensive sparrow, but that connection had never been clearly established.
The male song sparrow (Melospiza melodia) has been known to defend his home turf against any male song sparrow that intrudes. This sparrow has a collection of eight to 10 songs used to attract a potential mate, post his territory, communicate with neighbors, and, as in this most recent study, threaten an intruder.
The UW research team conducted a fair amount of their research in Discovery Park in Seattle. It was here they identified and recorded songs from 48 different sparrows. The team also had a stuffed song sparrow decoy they used to represent the possible intruder. The stuffed sparrow was placed either in a bush or tree while the team played back a recorded song.
The first playing of a recorded song was done just outside of the subject sparrow´s territory. With the recorded song coming so close to their own territory, the subject sparrow would respond with song while making an approach to the speaker from which the song was emanating.
Of course, the second playing of the song by the researchers was then played at a distance of approximately 60 feet within the subject´s immediate territory. In this experiment, the decoy stuffed sparrow was uncovered and placed directly on top of the speaker. With its beak exposed, the decoy appeared as if it were actually performing the song the subject was hearing.
The live subject sparrows responded somewhat predictably. Most of the subject sparrows responded with a matching song. When the stuffed intruder moved into the territory and persisted in singing, the defending sparrow progressed to higher-level warnings that included both soft songs and wing waves.
As the stuffed sparrow is inanimate, it was unable to react to the elevated activity of the higher-level warning signs presented by the subject sparrows. This ultimately led to an attack by the live bird on the research team´s faux feathered friend.
“Birds generally do all this signaling,” AkÃ§ay said, “because it´s usually beneficial to avoid getting into a fight if it can be avoided. There are less costly ways to persuade an aggressor to back down.”
The aggression of this species was noted as being particularly high. Some 31 of the 48 birds observed in the study eventually engaged in an attack. Those birds that engaged in song matching were found to be the most likely to attack.
This means, however, not all of the birds exhibited the same aggressive stance toward the intruder. Some of the birds, termed “bluffers” by the team, would match the trespasser´s song but then not follow through with an attack. On the opposite side of the spectrum were those termed “under-signalers.” This subset of the group would often attack without having provided a soft song or wing wave.
Of note to the team was how these behavioral subtypes, which they are now studying, appear to mimic humans in that they have personalities that shape their behavior in distinctive ways.
“These kinds of field studies provide context for laboratory research that uses bird song learning as an animal model for exploring the brain mechanisms of learning,” Beecher said. “Using bird song as a model system without understanding its natural social context would be like studying the neural basis of language without any idea of what humans use their language for.”
Other co-authors are Mari Tom, a recent UW graduate, and Elizabeth Campbell, a research technician at the UW. The National Science Foundation funded the study.