Iberian Imperial Eagle: Age classes and plumages

El Iberian imperial eagle stands, without a doubt, as one of the winged jewels endemic to our country. An emblem of biodiversity conservation, it is one of the distinctive species that we find in our peninsular territory, and, today, fortunately, it is quite common to see it in our skies if we pay a little attention. We have already talked about his story.

If we look closely at its livery, we will discover that the Iberian imperial eagle is one of the raptors with the most variety in terms of the appearance of its plumage from birth until it reaches adult status, and it is relatively easy to distinguish, or at least, to locate in which age groups the specimen in question that we are observing may be in, without entering into much more complex areas such as the molting of flight feathers. In this publication, we will try to summarize, in a basic and didactic way, the different ages of this species based on their plumage, in order to go one step further and enrich our field trips and our observations of, as Félix Rodríguez de la Fuente would say, this very beautiful and heraldic species.

We must bear in mind that four age groups can be reliably identified in this species by the appearance of the plumage and the molt (juvenile, second plumage, third plumage and adult). A key factor is the time of year in which we are carrying out the observation, a clear example occurs with juvenile specimens, seeing a bird born that same year in September (a few months ago) is not the same as seeing the same individual at the beginning of the following year, and although it is still not a year old, we can no longer speak of a juvenile, but of a later phase, as we will see later. It should also be clear that, fortunately, everything in nature is subject to variability both at the population and individual scales, which often differs from the theory presented.

Broadly speaking and roughly speaking, to get a quick idea, these are the four “phases” that we have to consider:

Juvenile plumage Straw (1st/2nd) Damero (light/dark) Subadult/adult Age 2-7 months8 months — 2.5 years 2.5 — 4 years 4.5 — 5.5 years

1ST AGE CLASS: JUVENILE

It is the first plumage of all, from the time the chickens leave the nest until they are approximately 7-8 months old. Perhaps it can cause quite a bit of confusion and overlap with the second plumage referred to in ornithological jargon as straw. In this first phase, that characteristic reddish-orange brown color stands out. (1). In flight, it is very visible that the wing cover feathers have a clear terminal end, giving a very obvious appearance of a line or wing stripe (2). Something very similar happens in the tail, the lines at its terminal end lighten and give a less colored line appearance (3). Finally, also very visible in the field, at the end of the flight feathers, you can see a much whiter area, like a winged “window” (4), which will be lost over time, but which is very evident especially in juveniles.

2ND AGE CLASS: STRAW

In this case, we have to think of a bird with a longer plumage than that of the juvenile one, more worn and discolored feathers (4), much more heterogeneity caused by these new pens that stand out over the older ones. All the characteristic features of juveniles begin to disappear. We compare it with a specimen in flight, and then with this same specimen posed. We started with that winged window (1), so evident in juveniles, that it fades more and more until it disappears depending, as mentioned before, on the variability and degree of advance of the plumage, as well as the end of the lines on the tail (2). Some specimens, (although this is subject to a lot of variability), may have some darker striation in the area of the mouth, below the throat (3).

Finally, in this phase of development, more typical features of the plumage that precedes it can begin to be observed, showing darker areas on the head and throat (5), and in the region of the scapular feathers (shoulders), where you can already see some white feathers (6), which so characterize individuals of a higher age group.

3RD AGE CLASS: CHECKERS

Continuing on, we reached the third age group, individuals who range between 2 and 4 years old, which may be the most complex phase, because individual variability and the degree of progress of molting, are factors that, to be as precise as possible, must be taken into account, and that divide the specimens into light or dark checkerboards depending on the evolution that the plumage has undergone.

The covering feathers on the wings and body are brown - dull yellowish and brownish - blackish, giving a “stoned” appearance (1), like a checkerboard (hence the origin of its name). The throat and head take on very dark tones, already typical of the definitive plumage (2). In some specimens, the characteristic white feathers on the “shoulders” are already beginning to appear (3), in this case they are very obvious, still not as clean a target as in adults.

4TH AGE CLASS: SUB-ADULTS AND ADULTS

Subadult

Finally, we reached the last two age classes, which begin around 5 years of age and older, a much more homogeneous, distinctive and familiar plumage, giving a feeling of very dark specimens, but highlighting the contrast of those white feathers like shoulders so characteristic. In this case, we start with a subadult individual, who, even if posed, shows us many traits characteristic of this age group.

As just pointed out, the plumage is very dark, leaving some older lighter checkerboard feathers retained, mainly by the lower parts of the body (1). The white of the scapular feathers is becoming more vivid and more extensive (2), and also the golden-yellowish color of the pileus and the nape of the neck stands out a lot, which contrasts with the rest of the body (3). In flight, just by seeing the tone of the plumage, we could be sure that it is an adult or subadult specimen, and, therefore, determine that we have in front of us a bird of at least 4.5 - 5 years of age.

Adult

Adults acquire their definitive plumage starting at 5 and a half or 6 years old, and this in turn is very variable. Very dark appearance, with the exception of the head and a large part of the neck, which obtain a brown-yellowish color (1) even more extensive and intense than subadults. Very white shoulder border (2), a typical and unmistakable trait in flight. Grayish, barred tail, with a characteristic black terminal stripe (3).

In short, and making a short final summary, the Iberian imperial eagle goes through several different age classes, increasingly darkening its plumage, and perhaps the central interval being the most conflicting and complex when trying to determine what stage of its life it is in. But what is clear to us, and we must appreciate, is that, at present, we are very fortunate to be able to enjoy this majestic species in our fields with relative ease and patience.

BIBLIOGRAPHY

• Forsman, D. (2016). In-flight identification: birds of prey from Europe, North Africa and the Middle East.
• Gonzalez, M.L. (2001). Iberian imperial eagle. From imperial emblem to symbol of nature.
• Gonzalez, M.L. (2011). The Iberian imperial eagle. The re-emergence of a threatened species.