By David Spector Gazette Contributing Writer
On a bird trip I once led for the Hitchcock Center for the Environment I noticed a northern harrier (also known as marsh hawk) hunting over a field in Hadley. This lovely raptor, found over the fields and marshes of the northern hemisphere, rarely breeds in western Massachusetts but occurs here regularly as a migrant and in small numbers as a winter resident. I pointed out the bird and described the features that I used to distinguish this species from other hawks: proportionately long, narrow wings held above the horizontal; a proportionately long, tubular body; and a relatively flat, owl-like face.
After I explained how I identified the bird, someone asked, somewhat hesitantly, “Does that bird have a white rump?ˮ The answer was yes, indeed, the bird had a white patch above the base of the tail, as do most northern harriers. The question and answer brought out another field mark that helped identify the individual’s species. For me, as an environmental educator, it also revealed much about how people see, learn and teach.
The person asking the question wanted confirmation that the white rump was really there. Observers, especially beginners, sometimes need external affirmation to have confidence in what they see. Other observers, both beginning and more experienced, can be overconfident in an impression that might have been distorted by an odd angle of observation or difficult lighting conditions. An effective teacher may agree with an observation, correct an observation or, sometimes, simply point again and let a learner draw an obvious conclusion.
The question taught me some lessons. I had neglected to mention the white rump, often the first characteristic a beginning birdwatcher learns for identifying northern harriers. Instead I had discussed how I identified the bird. Key words in my comments were “proportionatelyˮ and “relatively.ˮ For an experienced observer the proportions of the wings and tail and the shape of the face can be useful field marks. On the other hand, those shapes do not have a meaningful frame of reference for someone who has looked critically at few, if any, hawks. It would have been more useful to mention the beginner’s field mark first and then to describe how attention can shift to other aspects of the bird with experience.
My focus on the face reflects not only my experience with hawk identification but also my familiarity with research on this species. In a classic study of how a wild bird finds food, William Rice, who was then at Oregon State University, showed that a northern harrier could locate its prey by sound alone in dense grass. Rice used an elegant experimental design that allowed him to test the hearing of wild birds in a natural setting. In fields where northern harriers hunted, Rice put short lengths of conduit into the ground with a small speaker at the bottom of each tube and a sheet of thin plastic at the top. When a hunting harrier came near one of the tubes, either a sound was played like that of a mouse or there was no sound. If a sound was played, the bird attacked with accuracy, demonstrated by talon punctures in the plastic sheet (nine strikes and one near miss in 10 trials with five birds).
The relatively flat face of a northern harrier serves to focus sound to the bird’s ears and is an adaptation for hunting by sound. The harrier’s facial shape is a case of convergence (i.e., an independently evolved, similar solution to the same biological problem) with the facial discs of owls and the large external ears of bats—other animals that also hunt by ear.
My attention to a northern harrier’s facial shape reflects several of my interests: features that help me identify birds, the hunting behaviors of birds, the interactions of biological form and function, and the ways humans learn about biology. Learning from interactions with beginning birdwatchers, such as the incident I described at the start of this essay, can make me better at sharing these interests with students and trip participants.
David Spector is a former board president of the Hitchcock Center for the Environment and teaches biology at Central Connecticut State University.
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