The common grackle, or Quiscalus quiscula, may seem little more than a pest. However, I found that these birds made a surprisingly good study for some of the basic principles of animal behavior.
My first observations were on a cold, cloudy afternoon in Rice University’s west parking lot. A small group of birds was perched on the roofs of several cars. Grackles look a bit like small, lanky crows, with longer legs and tail. They are sexually dimorphic, meaning that there are distinct differences between the sexes. Male grackles are an iridescent blue-green-purple with hints of copper—rather oddly like a patch of gasoline in a parking lot. Females, as we can expect in birds, are much blander, with dull brown bodies and dark-capped heads. Because of this difference, I could tell that there was a mix of sexes in the group—three males and three females.
Male grackle, Quiscalus quiscula. Photo credit to safariari on flickr, used under Creative Commons license (CC BY-NC-SA 2.0).
As the birds hopped and shifted on top of the cars, I noticed that they made several distinct sounds. Most common was a short, guttural chk-chk-chk. There were also long, loud squawks and, least frequently, an occasional brief, high whistle that birders have likened to a squeaky hinge (Peer and Bollinger 1997). I also noticed that the birds definitely appeared to “talk” to each other—a call never seemed to go unanswered. The grackles didn’t seem to alter their behavior in response to these calls, but they did seem to get more noisy as I approached. I wondered if they were perhaps calling to encourage alertness in the flock, and would have eventually taken off if I had come close enough to seem threatening.
Of course, we know that animals don’t use sounds to form a language in the same way that humans do—but they do communicate. Since there were such a variety of sounds, I could assume that they must have different purposes. I could not find any sources that discussed specifically the sound of grackles in any detail, but bird calls and animal communication in general are well-studied areas. One sound might be to warn of predators, another to attract mates, and another to threaten rivals (Prehn 2010). The ability to make distinct, controlled vocalizations is called vocal specificity (Suzuki 2014). Many birds even encode more detailed information in their call, such as the particular predator type, by altering the rate of calling or number of sound repetitions of a basic call (Suzuki 2014). From this, we can see that birds are capable of recognizing the meanings of certain sounds, and modifying those sounds for different situations—essential for animal communication.
It’s easy to see how this communication might arise in a population. By chance, a grackle male might have a gene that makes him have a call that is particularly attractive to certain females. Offspring of these individuals have the chance to inherit two characteristics—the ability to make the sound (if male), and the tendency to find it attractive (if female). If callers and responders were more likely to mate and produce offspring, the next generation of birds would have a slightly higher percentage of callers and responders. In this way it could spread through the population until all males call, and those who don’t are unfit and do not mate. (We have to assume, though, that one characteristic came first—most likely females were genetically predisposed to calls, and males evolved in response.) The same evolutionary explanation could apply to different sounds for other situations, such as predators or rivals.
I observed grackles a second time outside of West Servery at Rice, on a warmer morning. Three individuals were hopping around at the tables, picking food off the ground and watching people eat, waiting for more food to be dropped. They did not communicate or seem to be a cohesive “flock” like the group in the parking lot. Their behavior struck me as similar to Barnard and Sibley’s (1981) study of house sparrows, where individuals were “scroungers” or “producers.” Producers work to make a resource available, while scroungers use this “behavioral investment” of another to gain the resource for themselves. Here, however, the scrounging was off humans, not other birds.
Like the house sparrows, grackles as a species are most likely are capable of both producing and scrounging—behaviors that have roots in intraspecific competition (meaning, between members of the same species—other grackles). As with house sparrows, grackles may be genetically predisposed to favor either producing or scrounging (Barnard and Sibley 1981). However, it is possible that individuals merely learn to increase their scrounging behavior around humans to exploit that situation. In populations of grackles that live in close quarters with humans, it is possible that scrounging behavior may be selected for, as birds that are good scroungers would be expected to do well in urban environments. We might expect populations of urban grackles to have a higher percentage of inherent scroungers than rural populations—such a study would be interesting. Although communication and foraging may seem like entirely different behaviors, they are both important examples of how natural selection can favor certain behavioral traits and allow them to spread through a population.
Works Cited
Barnard, C. J., and R. M. Sibley. 1981. Producers and scroungers: a general model and its application to captive flocks of house sparrows. Animal Behavior, 29: 543-550.
Peer, Brian D. and Eric K. Bollinger. 1997. Common Grackle (Quiscalus quiscula). In The Birds of North America, No. 271 (A. Poole, Ed.). The Birds of North America Online, Ithaca, New York. Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/271
Prehn, Emily. 2010. Costs and benefits of non-predator eavesdropping in mammal-bird alarm call interactions. Mockingbird Tales. Accessed from Connexions Web site. http://cnx.org/content/m34711/1.3/
Suzuki, Toshitaka N. 2014. Communication about predator type by a bird using discrete, graded and combinatorial variation in alarm calls. Animal Behavior, 87: 59-65.
Tags: birds, communication, foraging