©elfruler 2018

See MEASURING AN EAGLE for details on procedures and challenges of acquiring measurements and descriptions and figures of the features measured.   General References are given at this link, while References specific to each table below are given at the end of each table.

The charts below give measurements of adult and subadult Bald Eagles as reported in peer-reviewed publications.  I have omitted measurements that are questionable or not standard.  (If a reader knows of reports that I do not include here, please contact me with details.)

These numbers provide some context for consideration of several factors relating to size in Bald Eagles:

Age

The age of a Bald Eagle during its first five years affects several measurements.

• Beak and talons increase in size.
  • A Bald Eagle’s beak and talons are not fully grown at fledge but increase slowly in size over approximately its first 3 years. This is probably caused by a gradual buildup of the keratin layer over the underlying bones (which do appear to be fully grown at fledge) (Bortolotti 1984d).
• Feathers decrease in length.
  • A juvenile, a fledgling eagle in its 1^st year, has longer flight feathers (wing and tail) than it will ever have again.
  • With each successive molt of a subadult from its 2^nd year through its 5^th, the new flight feathers are a few millimeters shorter.
  • After reaching maturity, feather lengths of adults remain steady. But feathers wear down over time:  An individual Bald Eagle primary or secondary feather molts only every 3-4 years so it becomes progressively shorter over that period.  Also, a new flight feather can take 40-50 days to grow to its full length, so a measurement before it has finished growing will be misleading.
• Weight.
  • Primarily because of the decreasing feather lengths, an eagle’s weight decreases slightly over its 1^st 5 years.
• Researchers cannot always be certain of the age of a particular bird, and some offer vague or imprecise descriptions of age, such as “second winter,” “immature,” or “subadult.”
• Only measurements of the same eagle from one year to the next would yield meaningful comparisons, and this is possible only with a captive bird or with nestlings that are visited more than once before they fledge. Few such measurements exist.

Sexual dimorphism

• Females are larger than males in general, although exceptions can exist. The numbers tabulated here indicate that the difference can be from 13-23%, although some publications and internet sites claim as much as 25-30%.
• Adult females are larger than subadult females.
• Adult males are not significantly larger than younger males. (Bortolotti 1984c)

Geographical location and “Bergmann’s rule”

• It has been stated often that the size of Bald Eagles increases from south to north, and the so-called “Bergmann’s rule” is cited as an explanation for this phenomenon. The numbers in my tables do not necessarily confirm this “rule” for Bald Eagles, as discussed below.
• “Bergmann’s rule” has roots in an 1847 article by Carl Bergmann, entitled “On the relationship of the warmth economy of animals to their size” (trans. Salewski and Watt 2016).
  • Bergmann described a “law” pertaining to warm-blooded animals (birds and mammals):
    • Since larger animals have a smaller ratio of body surface area to body volume, they expend less effort than smaller animals to maintain a constant internal body temperature. (The surface area is important for the dissipation of heat from the body to compensate for excessive environmental heat, while the volume is pertinent to heat production to warm the body in excessive cold.)
  • From this “law,” Bergmann hypothesized that larger animals need a cooler climate than smaller animals. Since in general environmental temperature is lower at higher latitudes (further north), it follows that larger animals would favor northern environments and smaller animals would favor southern environments, a concept that biologists refer to as a latitudinal size cline, a gradation of size from larger to smaller, in this case from north to south.
  • Bergmann tested the size cline hypothesis by comparing the relative sizes of species within a genus, only once mentioning the sizes of individuals within a single species (the White-tailed Eagle, see below). Using wingspan (not body volume, or weight) to compare size, he examined 310 species across 86 genera and concluded that the hypothesis of a latitudinal size cline is true in most (but not all) cases.
  • To address the exceptions, Bergmann noted that other factors besides latitude might be in play:
    • Altitude (mountainous habitats generally are cooler than lower elevations).
    • The reliability of wingspan as an indicator of size (e.g. the Merlin has a smaller wingspan than the European Hobby but can be of comparable weight and remains in northern climates through the winter).
    • Migratory habits (which may affect wingspan).
    • Quality of plumage.
  • Bergmann included Bald Eagles (Haliaeetus leucocephalus) in a sea-eagle taxon with White-tailed Eagles (Haliaeetus albicilla), Short-toed Eagles, and Ospreys, although he acknowledged that in his day there was disagreement about whether they all belong in the same genus (taxonomists today agree that they do not).
    • He noted that among White-tailed Eagles, which conform to the hypothesis in general, some smaller individuals may be found in the north and some larger ones in the south.
  • Bergmann himself never articulated a “rule” about a relationship between the size of an animal and its geographical location. Later researchers have formulated the “rule” in different ways, and there remain disagreements about its underlying assumptions, its application, and even its validity.
  • A latitudinal size cline does not apply to all species and genera of birds. Meiri and Dayan 2003 surveyed studies of 94 species of birds that provide reliable data on size and locale and concluded that “over 72% of birds…follow Bergmann’s rule.”
    • Among raptors found in North America, that includes Turkey Vultures, Peregrine Falcons, Sharp-Shinned Hawks, and Ospreys.
    • But several North American raptors do not follow “Bergmann’s rule”: Cooper’s Hawks (Whaley and White 1994) , Northern Goshawks (Whaley and White 1994), Red-tailed Hawks (Fitzpatrick and Dunk 1999), and Merlins (Temple 1972).
    • Meiri and Dayan did not include Bald Eagles in their survey because not enough studies were available that provide “data that were statistically tested for geographic variation.”
  • My tables here do not provide such data for Bald Eagles either. The sampling in the published literature is too small, variable, and arbitrary to either confirm or refute “Bergmann’s rule” in the case of Bald Eagles.
  • In fact, the numbers I have tabulated suggest that, as with White-tailed Eagles, which are close genetic relatives of Bald Eagles, some smaller individual Bald Eagles may be found in the north and some larger ones in the south. The ranges of weight and wingspan measurements in my tables illustrate some exceptions to “Bergmann’s rule” (ranges are given in parentheses and italics below the averages):
    • The highest weight among females was recorded in Illinois (6577g) and the lowest in Saskatechwan (4540g) – higher in the south, lower in the north.
    • The highest weight among males was found in Alaska (5625g) and the lowest also in Alaska (3633g) – both high and low in the north.
    • The longest wingspan among females was recorded in Alaska (2333.5mm) and the shortest in Illinois (2035mm) – longer in the north, shorter in the south.
    • But the longest wingspan among males was found in Alaska (2171.7mm) and the shortest in Saskatchewan (2027mm) – both long and short in the north.
  • In conclusion, until more systematic studies of Bald Eagles are done with large samplings of measurements across a full range of geographic locations, we cannot be certain which of the following is true:
    • Bald Eagles are more similar to Turkey Vultures, Ospreys, Peregrine Falcons, and Sharp-Shinned Hawks in always or almost always conforming to “Bergmann’s rule,” or,
    • Bald Eagles are more similar to their sister White-tailed Eagles in following the “rule” generally, but having many individual exceptions. Our limited data suggests that this is a more accurate statement.

ADULT BALD EAGLE MEASUREMENTS TABLE

SUBADULT AND JUVENILE BALD EAGLE MEASUREMENTS TABLE

REFERENCES