A Bald eagle adapts to a handicap

16 Jan

A handicapped Bald eagle on the Bosque del Apache NWR has been reported by a few individuals the last three days. A visiting photographer*  shared with me his magnificent photographs and observations capturing how this bird has adapted to daily life. How the injury occurred and when is unknown to us. But the animal appears to have adapted quite well to flying, landing, perching and even obtaining its own food.

Our one-legged Bald eagle. Photo courtesy of Mike Endres.

How does this handicap affect an eagle’s ability to perform its life functions? As this individual demonstrates, missing half a leg and one taloned foot probably does not significantly impair its ability to fly, perch and eat. However, depending on its sex, could it affect its ability to reproduce? That is a good question, especially for a female. Although no obvious impairments might directly affect courtship and nesting, we won’t know conclusively unless the bird is observed during breeding, nesting and fledgling time.

All eagles are sexually monomorphic, which means both sexes look alike. Thus determining the gender of an individual Bald eagle is difficult. Although the female may sometimes be slightly larger than the male, this difference is often subtle and only determined if a pair are together, such as at a nest. The only other differentiating characteristics, measurements of the bill depth and length of the rear talon, can only be discerned with the birds ‘in hand’.

How can we know if the bird is a male or female? We won’t unless it is seen with its mate, if it is paired, to compare size. The only other recourse is using molecular biology. For this, biologists rely on blood samples, which is an invasive and stressful experience for animals, or collecting feathers, which is the preferred non-invasive approach.

Feathers are processed and analyzed by techniques that are sometimes referred to as ‘molecular sexing’. DNA is extracted from feather segments using a common laboratory kit. Small aliquots are then prepared and run through a polymerase chain reaction (PCR) with molecular primers that ‘bind’ to a specific portion of a gene that is associated with either the male or the female. The ‘bound’ segment(s) of DNA is then amplified many times. Aliquots of the amplified PCR products are then digested into smaller segments and run on an electrophoretic gel. The resultant banding patterns, which indicate the sizes of  all the products, are then matched with what is expected with the male or the female gene segment.

The entire process may take from 3-5 hours in a well-equipped lab. With many sexually monomorphic birds, this is often the only way to determine their gender, but it is non-invasive with little stress (if any) for the birds. When we banded American pelican juveniles (pre-fledge) last summer, the final step (after attaching two leg bands and weighing) was plucking a feather and putting it in a plastic bag with a corresponding code. The sex of each of the seventy-five birds was molecularly determined back in the lab at a later time.

What might be the prognosis for a normal life for this bird? Perhaps we may be bold enough to predict it will survive and live normally. Bald eagles tend to have nearly equal contribution to mated life. During courtship, they fly and lock talons together. Paired bald eagles share duties in nest building, incubating eggs, and providing food for the hatchlings. Consequently, the bird’s life as a mated pair may not be jeopardized. In fact, compared with other raptor species, Bald eagles share nesting duties between the sexes more than many other birds of prey.

However, as the number of hatchlings increase, the female’s role of providing food increases because the male tends to range further for food. Thus, if this bird is a female and her ability to catch and provide food for herself and a large brood is compromised, the probability of higher chick mortality may increase.

Another factor is defending the nest and caught food. The loss of one taloned foot might be decisive in a battle with another bird, especially if it is a large challenger such as a Great Horned owl. But as we have seen demonstrated by this bird here at the Refuge, it seems to have adapted well thus far.

Let’s hope his or her future is bright and fruitful.

The eagle has landed.  Photo courtesy of Mike Endres.

* I gratefully acknowledge and thank Mike Endres of Little Wing Photography  for sharing his photographs and observations of this eagle on the Refuge. I hope readers will visit his website (follow link above), and view his other excellent photographs. Thank you again, Mike. I enjoyed your visit and our chat.

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