Eavesdropping on a bald eagle breeding pair

JoAnn McGee – mcgeej@umn.edu
VA Loma Linda Healthcare System, Loma Linda, CA 92357
Center for Applied and Translational Sensory Science, University of Minnesota, Minneapolis, MN 55455
Peggy B. Nelson – nelso477@umn.edu
Department of Speech-Language-Hearing Sciences and the Center for Applied and Translational Sensory Science, University of Minnesota, Minneapolis, MN 55455
Julia B. Ponder – ponde003@umn.edu
The Raptor Center, College of Veterinary Medicine, University of Minnesota, St. Paul, MN 55108
Christopher Feist – feist020@umn.edu
Christopher Milliren – milli079@umn.edu
St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55414
Edward J. Walsh – ewalsh@umn.edu
VA Loma Linda Healthcare System, Loma Linda, CA 92357
Center for Applied and Translational Sensory Science, University of Minnesota, Minneapolis, MN 55455

Popular version of paper 1pABb6
Presented at the 181st ASA Meeting in Seattle, Washington

One of the many challenges associated with efforts to characterize the acoustic properties of free-ranging bald eagle (Haliaeetus leucocephalus) vocalizations in a behavioral context is the relative inaccessibility of individual, interacting signalers. Here, we take advantage of the opportunity to eavesdrop on vocal exchanges between a breeding pair inhabiting a nest furnished with a webcam and microphone located in Decorah, Iowa and managed by the Raptor Resource Project (www.raptorresource.org).
In a previous study centered on captive bald eagles at the University of Minnesota Raptor Center, five call categories, including so-called grunts, screams, squeals, chirps and cackles, were identified. The primary goal of this study was to extend the investigation into the field to begin efforts to characterize and compare the acoustic properties of calls produced in captivity and in the wild.
Predictably, many of the acoustic features of calls produced in captivity and in the wild are generously shared. However, preliminary findings suggest that at least a subset of calls exchanged by breeding pairs may take on a hybrid character, exhibiting blended variations of the chirps, squeals and screams characterized previously in captive birds. Calls analyzed here were taken from a variety of settings that include mating, exchanges associated with feeding at the nest, vocal reaction to intruders near the nest, and short distance call exchanges that appear to function as hailing signals.
The source of raw materials used to relate the behavior of the interacting pair to their vocal exchanges can be appreciated by observing the following audiovisual recording examples.

 

VIDEO 1
In this video, the female of the pair, an eagle known affectionately as Mom, is not so patiently awaiting the arrival of her partner, known by the less endearing name DM2. As DM2 arrives at the nest with a meal, Mom produces a call sounding a lot like the call of a sea gull; a call with the characteristics of a lower frequency version of the scream observed in captive eagles.

VIDEO 2
Here, Mom appears to be calling out to DM2 for a break from nesting. DM2 arrived shortly after the footage shown here and Mom takes off for higher ground. The call appears to be a commonly produced, seemingly multipurpose utterance closely resembling a spectrally complex version of a call observed in captive eagles known as the chirp.

VIDEO 3
In this sequence, Mom appears to summon DM2 in response to what appears to be an intruder, possibly another bald eagle, in the airspace surrounding their nest. Again, a complex variation of the chirp observed in captive eagles appears to serve as a territorial marker.
The take-home message of preliminary findings reported here is that the acoustic structure of at least a subset of calls produced by free-ranging bald eagles appears to be more nuanced and complex than those representing their captive counterparts. Elements typically representative of three primary call types in captive birds, namely chirps, screams and squeals, intermix in calls produced by free-ranging eagles, creating a vocal repertoire with subtle, but potentially meaningful structural variation. If differences reported here remain stable across a larger sample size, these findings will serve to underline the relative importance of our work in the field.

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