By: Bryan Watts
3/30/24 

I carried two pairs of welders’ gloves and Stan Wiemeyer carried a wide-mouthed hand net as we walked down a long line of flight pens to the sound of cackling eagles.  The complex felt like cell block 9 at Riker’s but was orderly, clean and magnificently built.  It was the bald eagle captive breeding facility at Patuxent Wildlife Research Center in Laurel, Maryland.  It was late 1982 and Stan was collecting blood samples for an experiment evaluating methods for storing eagle blood.  The eagle restoration program was in full swing.  The program was a hopeful experiment intended to assess options for restoring the species.  We stopped when we reached the door of pen number 7 and looked in to see a wooden ramp leading up to a nest platform.  Standing on the ramp was Pewee, an adult male eagle from south Florida.  Pewee had lost the outer end of one wing and after rehab had been recruited into the breeding program.  He needed the ramp to reach the nest platform.  We entered the pen and Pewee immediately jumped down to the ground.  Despite his wing injury, Pewee was an experienced bobber and weaver.  We were out of breath by the time we were able to secure him into the net.  I had handled many eagles but was entirely unprepared for the feel of Pewee in the hand.  At only 5.5 pounds, he was a red-tailed hawk in eagle’s clothing.  I was stunned by his small size and delicate structure. 

The next day, Stan and I would go to the bird yard to work with Bomber.  The bird yard was a collection of flight pens that housed screech owls, kestrels, night herons and other assorted species including a small number of eagles.  Bomber was an Alaskan male that had found his way to Patuxent after being picked up under a nest before he was old enough to fly.  He had been in captivity for more than 20 years.  He was a favorite of staff and the public and amazingly was completely at ease around people.  At the sight of anyone approaching he would run across his pen cackling to greet them at the door.  Mostly, he enjoyed food and it did not matter what was on the menu for the day.  He would eagerly stand next to you and eat whatever you brought. I had worked with Bomber for several months.  Despite his cheery personality, Bomber did not like to be handled.  He had been the subject of several physiological experiments during his younger years and had had enough.  Just the sight of a net as we approached sent him back to the corner of his pen flipping on his back and extending his talons.  Bomber knew every trick in the net avoidance book and it was never certain who would gain the upper hand.  After several take downs and escapes we had him in hand.  There was nothing delicate about Bomber.  He felt indestructible and heavy like a rodeo bull and weighed 16 pounds.   

Over the course of two days, Stan and I had crossed the continent working with ten eagles from throughout their geographic range and feeling firsthand how their body size and proportions shifted.  The experience left an impression on me and a sense of wonderment about the places and conditions that had shaped such variation in size and shape.  Body size is an important morphological characteristic that has been correlated with a broad array of life history traits from metabolism to home range size.  Although researchers have focused on bald eagle body size around the extremes of their range, this provides only an empty shape or outline.  The details of how body size varies throughout the range and what factors may drive this variation has yet to be filled in.  For example, although the Chesapeake Bay supports one of the most prominent eagle breeding populations throughout the range, there has been no treatment of body size for this population.   

In December of 2003, Catherine Markham and I traveled to the Smithsonian to measure eagle specimens known to be resident within the Chesapeake Bay to evaluate body size and patterns of sexual size dimorphism.  We worked with 27 specimens that included samples of males and females and we obtained mass data from the Bald Eagle Necropsy Program.  We measured a series of morphological characters including tarsus length, tail length, wing chord, eight primary, bill length, bill depth and hallux length.  A paper describing this work entitled “Assessment of body size and sexual size dimorphism in Haliaeetus leucocephalus (Bald Eagle) of the Chesapeake Bay” was recently published in The Raven.   

Two themes emerged from the measurements including that body size for Chesapeake Bay eagles was approximately a midpoint between the northern and southern extremes along the Atlantic Coast and that males and females exhibited a high degree of reversed sexual size dimorphism.  Clinal variation in body size with latitude is a well-known pattern for many vertebrates, with northern populations containing larger individuals compared to southern populations.  In bald eagles this difference in size was the basis for the early separation of northern and southern populations into distinct subspecies.  The fact that Chesapeake Bay birds approximate a midpoint between southern and northern extremes supports the notion of clinal variation with latitude. 

Reversed sexual size dimorphism where females are significantly larger than males is common in predatory birds.  Chesapeake Bay females were significantly larger than males in all characters measured except for bill length.  Dimorphism was particularly pronounced in body mass (30% larger) and tarsus length (17% larger).  We used a dimorphism index to compare divergence to other populations.  Compared to northern populations, eagles in the Chesapeake were more divergent in body mass but less divergent in hard structures such as culmen and hallux length.  Divergence patterns for feathers appear to be similar across geographic areas.   

The bald eagle is a well-studied species.  However, many details of their most basic traits such as body size have yet to be filled in across their range.  Understanding geographic variation in body size and dimorphism may help us to better understand some of the underlying selective pressures that have shaped these traits.