©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

©elfruler 2018

Acquiring accurate measurements of a bird is a tricky business, and it turns out that there are relatively few peer-reviewed publications that report measurements for Bald Eagles.  In the pages that follow I present tabulations of the credible numbers of which I am aware.  I begin with some general information on measurements and measuring.  General References are given at this link.

What are the sources of measurements?

The most reliable measurements are taken from a live, wild, healthy eagle, but capturing one is difficult and rarely attempted.  Some researchers have used captive eagles, recently deceased eagles, and museum specimens for measurements, but these present some issues that must be considered:

• Permanently disabled eagles in captivity may have different measurements from those of their cousins in the wild because of richer diets, less exercise, irregular feather molt, abnormal beak length and depth (due to inconsistent feaking), etc.
• A rescued eagle in distress is usually dehydrated and emaciated, so its weight is likely to be lower than normal. After being treated in rehab for days or weeks during recovery, its diet may be richer and more abundant than it would be had the eagle continued living in the wild, which coupled with a reduced level of activity may result in a higher weight than normal at its release.
• A recently deceased eagle can yield accurate measurements unless the eagle succumbed to injury or starvation, or the body has decomposed or been disfigured by a predator.
• Measurements of carcasses that have ended up as prepared skin specimens in museums or other collections can be useful for some features, such as talons, bill depth, wing length, and tail length. But a specimen is subject to drying and shrinkage which can affect some measurements such as total length, bill width, tarsus width, wing chord, wingspan, and individual feather length.  And since most bones are absent from prepared skins, weight and most skeletal measurements are impossible.  Specimens also can yield erroneous sex classifications, especially with younger birds.  (Bortolotti 1984c)

How do researchers measure?

• Scales, calipers, and tape measures are standard tools.
  • Digital scales give more precise readings than analog scales. All scales should be well calibrated.
  • Methods for positioning the bird on scales and for a linear measurement can be surprisingly challenging and require study of best practices as well as training and practice.
• Measurements are best reported in universally used metric units (grams, millimeters, etc.), although some earlier studies and a few recent ones use the “traditional” American system of pounds, ounces, feet, and inches.
  • For ease of comparison, here I use metric units including conversions where necessary.
• For features that incorporate a curve or arc, such as the folded wing, talons, beak, and feathers, the measurement is of the chord, or a straight line between two end points, thus disregarding the curve. Measuring the curve itself requires a calculation based on the shape of the curve and may lead to only an approximate measurement that can make comparisons difficult.  Feathers and wings are not rigid and can be flattened to eliminate the curve, but this can introduce inconsistencies or inaccuracies and is generally avoided by researchers.
• Reliable statistical analysis requires a sampling of a large enough number of birds to provide minimum, maximum, and average measurements. The notation “N=x,” where x is the number of birds examined, conveys the size of the sampling.  In general the average is the mean.  Some researchers calculate a standard deviation (SD).

What features do researchers measure?

P. Baldwin, H. C. Oberholser, and L.G. Worley, Measurements of Birds (1931) established a comprehensive and standardized system of obtaining linear measurements of the various parts of birds, which continues to guide researchers. It includes detailed discussion of procedures and challenges and the tools needed to obtain accurate and consistent measurements, along with drawings showing what to measure, how to position the bird, and where to place the tools.

• Subsequent studies of raptors have offered nuances and modifications necessary for measuring these species.
• The authors of some studies do not always make clear what exactly what they measured or what procedures they followed, or their procedures may not conform to standard practices. For example, some authors do not indicate whether they used live birds or specimens, they may omit important information on the geographical origins, age, or sex of the birds, or they may use unorthodox techniques without explanation.

The following measurements of Bald Eagles have been reported in published literature.  (Figures from Baldwin et al. 1931 are used under the license agreement with Creative Commons.  Figures from Bortolotti 1984a are used with permission of The Journal of Field Ornithology.  Figures from Bortolotti 1984c are used with permission of The Wildlife Society, Bethesda, MD.)

• Weight, which would seem to be a straightforward measurement to obtain simply by putting the bird on a scale.  But several factors can skew the reading, and researchers do not always indicate whether they have addressed them:
  • A malnourished or otherwise unhealthy bird.
  • Contents of the stomach and crop, which can vary widely from hour to hour or day to day (according to Gerrard & Bortolotti 1988, a Bald Eagle’s crop can hold about 15% of its body weight). A desirable reading would be a “net” weight with empty stomach and crop.
  • Time of year the bird is weighed, as for instance for females in the lead up to egg-laying when they can gain weight and afterwards when they can lose weight, or around migration or dispersal season which could begin with weight gain and end with weight loss.
  • Recently deceased birds can give an accurate weight, but specimens will not.
• Length of the bird from the tip of the beak to the tip of the tail.  This measurement is taken with the bird lying flat on its back, its head and tail stretched as flat against the table surface as possible.  (Baldwin et al. 1931, Fig. 1)
  • This is not the “height” of the bird, which on many internet sites seems to mean a measurement from the crown of the head to either the tip of the tail or the bottom of the feet while the bird is perched or standing. This “height” usually is stated as being about 3 feet for a Bald Eagle, although the method of measuring this is rarely specified.
  • This measurement can be difficult to obtain with a high degree of accuracy because of variability in stretching the neck.  Skin specimens will not give an accurate measurement.
  • There are few reported measurements of a Bald Eagle’s length in the ornithological literature.
• Depth of the beak, from the maxilla at the front (anterior) edge of the cere to the opposite point on the lower mandible.  (Bortolotti 1984c, Fig. 1c)
  • The exact place at which this measurement is taken can result in significant variation in the measurement and thus can be unreliable for comparison purposes.
    • Specimens can provide accurate readings unless the carcass was dried with the beak open.

• Width of the beak, the widest point where the cere meets the feathers.  (Baldwin et al. 1931, Fig. 13)
  • The exact point at which this measurement is taken can cause significant variation in the reading and thus is not always reliable for comparison purposes.
  • Specimens provide less reliable readings.
• Length of the exposed culmen of the beak, the chord of the beak from the tip of the culmen (the curve on the top of the upper mandible) to the front (anterior) edge of the cere, thus the length of the culmen without cere.   (See above, Bortolotti 1984c, Fig. 1a)
  • Specimens can provide reliable readings.
  • Some researchers measure the culmen with cere, i.e. to the back (posterior) edge of the cere where the feathers begin, although they do not always say so; these are not included here.
• Length of the beak from the gape, a straight line (chord) from the tip of the upper mandible to the gape or corner of the mouth.
  • Specimens provide less reliable readings. (See above, Bortolotti 1984c Fig. 1b).
• Length of the foot, or foot pad, the bottom of the foot from the end of the middle toe to the end of the hallux toe, with both toes fully extended and not including the talons.  (Bortolotti 1984a, Fig. 1)
  • Taking this measurement on a live bird can be challenging unless the bird is anesthetized and thus relaxed.  Specimens usually will not yield accurate results unless the toes were extended during preparation before the skin dries out.
• Tarsus or tarsometatarsus, the “foot” or lowest part of the leg, from which two measurements can be made:
  • Length, from the intratarsal joint where the ankle meets the heel to just above the base of the middle toe below the lowest scute (scale) on the leg. This measurement is a long diagonal from the intratarsal joint at the back of the leg to the base of the toe at the front of the foot.  Reports of this measurement can vary, perhaps because of imprecision of identifying the end points.  (Baldwin et al. 1931, Fig. 136)
  • Width, which Baldwin et al. 1931 prescribe should be taken at the midpoint of the tarsus. The tarsus is wider when measured from front to back and narrower when measured from one side to the other; generally the front-to-back diameter of the tarsus is measured (Baldwin et al. 1931, Fig. 137).   But the few reported measurements for Bald Eagles are taken just above the toes at the narrowest point of the tarsus, and calculations are of an average of two measurements taken front-to-back and side-to-side, yielding mean width rather than diameter.  Either way, width is a highly variable measurement in reports, and specimens do not always yield reliable readings (Bortolotti 1984a and Bortolotti 1984c).
• Middle toe (the 3^rd and longest digit), measured from above, from its base at the metatarsal joint outward to the end of the toe, not including the talon.  (Baldwin et al. 1931, Fig. 139)
  • The toe must be extended as straight as possible.
• Length of the hallux talon, the chord from the talon’s tip to the point on top where it emerges from the skin of the toe.  (See above, Bortolotti 1984c, Fig. 1d)
  • Specimens can provide reliable readings.
• Length of a feather, the chord of the shaft from where it enters the skin to its tip, without flattening the shaft. (Baldwin et al. 1931, Fig. 114)
  • For Bald Eagles this is usually done only on flight feathers.
  • See these photos of Bald Eagle primaries and secondaries from the U.S.F.W.S. Feather Atlas.
• Tail, the longest point from the bases of the feathers at the skin to the tips, measured with the tail closed, the ruler placed between the two innermost, or central rectrix, feathers.  (Baldwin et al. 1931, Fig. 120)
  • Specimens can give reliable readings of this measurement.
• Wing chord, the length of the closed, unflattened wing from the wrist joint to the feather tips.  (Baldwin et al. 1931, Fig. 101)
  • Some reported Bald Eagle measurements appear to have been taken with flattened wings, hence are longer than measurements of the chord; these are not included here.
  • Specimens may not give reliable measurements.
• Wingspan, between the tips of the outstretched wings with feathers, measured with the bird lying flat on its back.  (Baldwin et al. 1931, Fig. 98)
  • Attempts to measure while the bird is standing or perched are less reliable and consistent.
  • Care must be taken not to injure the wings by over-stretching or flattening them.
  • Specimens will not give accurate readings.
  • There are surprisingly few Bald Eagle wingspan measurements reported in the literature.

MEASURING ADULT, SUBADULT, AND JUVENILE BALD EAGLES

ADULT BALD EAGLE MEASUREMENTS TABLE

SUBADULT AND JUVENILE BALD EAGLE MEASUREMENTS TABLE

REFERENCES