Identifying Bald Versus Golden Eagle Bones: A Primer for Wildlife Biologists and Law Enforcement Officers

1. What they did

Bald eagles and golden eagles are North America’s largest birds of prey apart from the California condor, and both are protected by federal law in the United States. Their remains turn up in bird mortality surveys at wind farms and power lines, and in wildlife crime investigations. With feathers, the two species can almost always be told apart. But sometimes only bones are left after decomposition and exposure, and eagle bones are also found in crafted items with no feathers attached. Until this paper there was no guide to telling the two species’ bones apart that non-specialists could use.

The author, of the U.S. Fish and Wildlife Service National Fish and Wildlife Forensics Laboratory (NFWFL), worked as follows:

• Comparing bones. He examined all the major bones of bald and golden eagles in the laboratory’s reference collection (10 to 24 individuals of each species, depending on the bone).
• Choosing useful bones. Five bones (wishbone or furcula, scapula, radius, carpometacarpus and femur) were dropped because they look too alike. Eight bones showed reliable shape differences: skull, sternum, pelvis, coracoid, humerus, ulna, tibiotarsus and tarsometatarsus.
• Making a photo guide. He photographed an example of each bone from both species and marked the features that tell them apart.
• Testing the guide. Six volunteers, ranging from no biology training to a PhD in mammalogy but none with experience of eagle bones, were given mixed, unmarked bones of both species and asked to identify them using only the annotated photographs. Features they found hard to judge were dropped.
• Measuring size. He measured the total length of the eight bones in both eagles and six other large North American raptors, and compared them with published lengths for large non-raptor birds.

2. What they found

Size

• Size alone almost always separates eagle bones from those of other North American raptors.
• But bald and golden eagle bones overlap widely in size, made worse by differences between the sexes and between regions, so the two eagles should not be told apart by measurements.
• Several large non-raptor birds (swans, Canada goose, wild turkey, pelican, great blue heron, cranes) have limb bones large enough to be confused with eagle bones. Skulls, breastbones and pelves of these birds look clearly different from an eagle’s. If only limb bones of an unknown large bird are present, the author advises not to attempt identification without a reference collection.

Shape features that tell the two eagles apart

• Skull, side view: bald eagles have a solid plate of bone between the eyes; golden eagles have an obvious “window” in it. Bald eagle beaks are more massive and often yellow, golden eagle beaks always dark, but beaks should not be relied on alone (young bald eagles have dark beaks, and beaks found in the field are often damaged).
• Skull, back view: the bony projections behind the eyes are slender and pointed in bald eagles and broad plates in golden eagles.
• Sternum (breastbone): long with parallel sides in bald eagles; compact and broadening towards the rear in golden eagles, often with small holes at the rear corners, which are never seen in bald eagles.
• Pelvis: the hip crest is a smooth curve in bald eagles and angled and flared in golden eagles.
• Coracoid: golden eagles have a raised “mushroom cap” of bone near the shoulder end; bald eagles have a smooth transition.
• Humerus (upper wing bone): the hollow in the head of the bone is large and round in bald eagles, smaller and more pointed in golden eagles.
• Ulna (lower wing bone): the ridge at the wrist end is rounded and almost C shaped in bald eagles, elongated and D shaped in golden eagles. This is the hardest bone to identify.
• Tibiotarsus (middle leg bone): a long slender curve on the side crest in bald eagles, broader in golden eagles.
• Tarsometatarsus (lower leg bone): golden eagles have an obvious deep notch at the top of the bone; bald eagles do not. This bone alone can give a reliable identification.

The volunteer tests

• Errors ranged from 0% (skull, side view) to 7% (ulna, 9 wrong out of 132).
• Each volunteer was 95 to 100% correct across all bones combined.

3. What they concluded

• If an intact skull or the lower leg bones (tarsometatarsi) are present, identifying the species should be straightforward and unequivocal.
• Even without them, untrained volunteers using the photographs identified the pelvis, sternum, coracoid, humerus and tibiotarsus correctly 95% of the time, and the ulna 93% of the time.
• In the field, remains usually include several bones, and identification should be based on all the informative bones together. This should allow the two eagles to be told apart in almost every case.
• The ulna should not be used alone when other bones are available. It is included because ulnas are the bones most often used to make Native American style bone flutes, so they are often seen in wildlife crime cases.
• The author encourages wildlife biologists and enforcement officers to carry printouts of the figures as a field reference.

4. How tracking was or was not used

No tracking in the field. The paper is about one kind of sign: identifying the species from bones left after death. The guide assumes enough remains are present to show they are an eagle (for example, part of a skull or a foot with large talons).

5. Tracker or observer certification

The identification guide was tested by six volunteers, named in the acknowledgments, with varying backgrounds in biology and no previous experience with eagle bones. They had no certification; their accuracy with the guide was measured in blind tests. Two of the volunteers also measured the bones.

6. Other technologies, skills or innovations

• Annotated photographs of each bone, designed for use in the field and at places without a reference collection.
• Digital calipers for bone lengths.
• Supplementary table of bone measurements for large raptors in the NFWFL collection.

7. Implications for conservation (from the paper)

• Eagle deaths at wind energy facilities are a growing concern, and these facilities are spreading into areas used by golden eagles. Reliable species identification is essential for the mortality surveys run at wind farms and along power lines.
• Golden eagle populations in the United States are stable or slowly declining, and most recorded deaths are caused by people (shooting, electrocution and lead poisoning).
• The guide supports wildlife law enforcement, where eagle remains and crafted items are examined as evidence.