Monday, March 7, 2016

Aberrant Red-tailed Hawk

A Partially White Red-tailed Hawk in Yarmouth
A few images of an intriguing Red-tailed Hawk were posted to the Nova Scotia Bird Society Facebook page by Robert Surette. The hawk was photographed on March 4, 2016, soaring above the Town of Yarmouth and Robert was fortunate enough to capture both the underside (Fig. 1) and upperside (Fig. 2) of this magnificent bird.

Figure 1. Aberrant Red-tailed Hawk in the Town of Yarmouth, Yarmouth County, March 4, 2016. Notice the asymmetric pattern to the white flight feathers. Photo by Robert Surette.

With a quick glance, it almost looks like the bird is missing many secondaries and primaries (Fig. 1). If a bird were moulting flight feathers, the same feathers would be replaced at the same time on both wings, creating symmetry in the feather gaps. Upon closer inspection you see that there are no gaps, the feathers are actually present, but they appear entirely white. See my previous post named Ghost Grackle for some introductory information on colours and colour aberrations in birds.

Figure 2. Aberrant Red-tailed Hawk in the Town of Yarmouth, Yarmouth County, March 4, 2016. This view of the upperparts of this hawk shows that is it mostly white. The head and tail appear normally coloured, as do some of the flight feathers.  Photo by Robert Surette.

The aberrant feathers appear entirely white. There is no indication of any pale brownish or greyish feathers. Aberrant feathers are known to bleach quickly; this requires checking of feathers that are protected when the bird perched for an indication of what a feather looked like when it was new. The secondaries of a Red-tailed Hawk are covered when the hawk is perched, and these feathers also appear to be pure white from below (Fig. 1) and from above (Fig. 2).

This hawk is definitely not an albino. This requires that all feathers lack both melanin pigments, which would render a Red-tailed Hawk's plumage entirely white. We can also eliminate all other aberrant conditions except leucism and progressive greying due to the fact that the unusual feathers appear pure white and not brown or grey. These two remaining conditions are difficult to tell apart if all feathers are affected. Luckily, this is not the case here. When leucism affects only part of the bird's plumage, the pattern produced is typically symmetrical. As previously mentioned, the pattern is asymmetrical. The differences between the wings are annotated by arrows in Figure 3. (Grouw 2013)

Leucism almost always affects the extremities (outer primaries, head, feet, belly...), so a bird showing many white feathers on the back and few in the wing along with normally coloured outer primaries, head and belly is likely not a leucistic individual. Progressive graying is also documented to occur with some regularity in Red-tailed Hawks. (van Grouw, pers. comm.)

The left wing has only one white outer primary (P10) while the right wing has three (P7, P9 & P10). There are also clear differences between each wing in the secondaries, the right wing having a large patch of white feathers not present in the left wing.

Figure 3. Aberrant Red-tailed Hawk in the Town of Yarmouth, Yarmouth County, March 4, 2016. This view of the underparts of this hawk shows that the white feather pattern is not bilaterally symmetrical. We also see that the head, body and underwing coverts appear normally coloured.  Photo by Robert Surette.

This pattern of white feathers is characteristic of the colour abnormality called progressive greying, a condition that is more common than leucism. Progressive greying arises once a bird reaches a certain age and is specifically the progressive loss of pigment cells with age. Once the condition begins, the bird will gain an increasing number of white feathers after every moult until the entire plumage is white. (van Grouw 2013)

History of White Red-tailed Hawks in Nova Scotia
Red-tailed Hawks with white plumage have been documented fairly regularly in Nova Scotia. See the list below for NS Birds issues from Jan 1997 until today with mention of white Red-tailed Hawks. All but the latest issue include a link to the PDF on the Nova Scotia Bird Society website. Follow the link to read about the sightings.

NS Birds Vol 57 No 2 (Fig. 4)

Two long-staying birds were at Sheffield Mills from 1988 until 1996 and the Wolfville/Hortonville area from 2001 until 2010 (Fig. 5). Some other white Red-tailed Hawks have been spotted in Kingston, Kings Co. in 1998; Meiklefield, Pictou Co. in 1998; Pomquet, Antigonish Co. in 2007; West Hants in 2008 and the recent bird at Canning in November 2014 (Fig. 4).

Figure 4. A mostly white Red-tailed Hawk at Canning November 6, 2014. Without better photos, we can't be sure of the exact condition, but it is likely leucism or progressive greying - the affected feathers do look pure white. Photo by Mel Carlton.

Figure 5. White Red-tailed Hawk at Grand PrĂ© in November 2009. The feet and eyes of this bird appear normally coloured, so it isn't an albino. The affected plumage appears pure white, so again it is likely caused by leucism or progressive greying through a lack of melanin. Photo by Tuma Young.

The Christmas Bird Counts in The Valley have consistently produced a relatively high number of Red-tailed Hawks in comparison to the rest of the province. A higher number of birds would definitely give an observer a higher chance of spotting an aberrant Red-tailed Hawk.

In the process of researching this hawk, it has been clear that most of the birding community in North America hasn't jumped on van Grouw's aberrant colour mutation terminology. While both leucism and progressive greying produce plumage and skin that is all white or partly white through a lack of melanin, they do differ in that leucism's effects on an individual does not change over time while progressive greying's effects on the bird's pluamge spreads over time.

Grouw, Hein van. 2013. What colour is that bird? The causes and recognition of common colour aberrations in birds. British Birds 106: 17-29.

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