The Dark Ghost

written by a guest blogger

Rob Miller, the author of this post, is a Raptor Biology student at Boise State University. His thesis is focused on the breeding Ecology of Northern Goshawks within a unique forest landscape in Southern Idaho. He is specifically investigating the diet of Northern Goshawks and how prey abundance influences nest occupancy and success. He maintains a blog, Rob's Idaho Perspective, and will be an occasional guest author here at Nemesis Bird.

As many birders can attest, species identification can be very challenging at times. The challenge arises from the similarity of many related and sometimes not so related species. Complicating this is the ephemeral nature of birds in general. The dominant feature which makes them so intriguing - the ability to fly - helps them maintain their distance from would be predators and admirers alike. A bird in nature seldom presents itself to a viewer in a fashion presented in most field guides. But, sometimes they do...

Rough-legged Hawk. Snake River Birds of Prey NCA.

Increasing the challenge of identification is the seasonal and regional differences in plumage within the same species. Plumage coloration is also related to the condition of the bird. In many songbirds for example, it has been shown that the coloration can be an honest signal of fitness. The greater the color richness and vibrancy of the adults within a species, the better the body condition, the stronger the immune system, and the higher the reproductive success. While raptors generally have more neutral coloration, there is evidence that the expression of certain pigments correlates with the strength of the immune system. Consider the expression of the color red in the tail of a Red-tailed Hawk or American Kestrel. This red color is believed to be directly correlated to immune system function. This connection is currently being investigated by another raptor biology student at Boise State University - Chris Porterfield.

American Kestrel interacting with Red-tailed Hawk. Idaho.

Bird identification can also be challenging when a species presents itself in different color-morphs. As a raptor biologist this is a central issue in my field. Most raptor species present different color morphs, sometimes as many as three or four. These color morphs are not to be considered sub-species unless individuals of a given color morph begin to assortatively mate - mate only with other individuals with that color morph - and they evolve away from the central population. Color morphs are controlled by genetic factors and thus their prevalence within a population is related to the reproductive success of the individuals carrying the morph. If the morph increases reproductive success then you would expect a greater percentage of individuals in the population to carry the morph or vice-versa. There is some evidence that color morphs have little influence on reproductive success, but the relative rarity of dark morphs leads me to believe otherwise. The morphs are produced by an increase or decrease in the gene expression of melanin - the dark pigment producing the color of most raptors. Dark-morphs are sometimes called melanistic birds. Dark-morph raptors are much more common in the old world than the new world and are more common in some species than others, but it is not understood why.

The story of the ghost

Last week I was helping another graduate student with his field work. Neil Paprocki's project involves winter surveys of raptors within the Morley Nelson Snake River Birds of Prey National Conservation Are a (NCA). Last Monday Neil and I, along with another biologist Liz Urban, were surveying for raptors via a road survey south of Boise. The term "road" is very generous. This 4-wheel-drive path, mostly buried in Russian Thistle tumbleweeds, traverses the western portion of the NCA. We observed Red-tailed Hawks, Rough-legged Hawks, Golden Eagles, American Kestrels, Prairie Falcons, and Northern Harriers. Toward the end of the survey we flushed a bird just ten feet from the edge of the "road". "What was that?!?"

First glimpse of the ghost!

It's a harrier! No! Is it? What? Confusion took over as our brains shuffled the various field marks and signs we use for identification. We kept coming back to Harrier, but then the issue - all Harriers have a wide robust white rump patch easily seen from almost every angle. However, this one did not... The bird flew a short distance before landing out of sight in the sage. We discussed if it was worth flushing this bird again for an identification. In general we try to minimize our disturbance. However, this bird was notable enough and our identification was not positive enough that we decided we were justified to get one more look.

'Normal' Northern Harrier with white rump patch. Roswell, Idaho.

Neil and I exited the truck and started hiking through the sagebrush and grass landscape that makes up the NCA. The bird flushed again. Dang! My camera wouldn't focus... Then I got the shots! In all manner of flight style, shape, behavior, and habitat, this was a harrier. However, there was no white tail band... Do dark-morph harriers exist? None of us had ever seen or heard of one. This is clearly an unusual bird.

dark-morph Northern Harrier.

Or is it?

Hmm.

Yes it is!

According to various reports (see citations below) only three dark morph Northern Harriers have ever been reported and only one has ever been photographed (Liguori 2009). Dark morphs have been well documented in Northern Harrier's closest relatives, a number of other harrier species in Europe.

This was a fantastic find and an exciting topic to explore. The photos are being shared with the top raptor experts in the country for their expert analysis. Neil returned to the area the next day but was unable to relocate the raptor.

Literature Cited:

Liguori, J. 2009. Distant Raptors. Birding 41:74-76.

Olson, C.V. and S.A.H. Osborn. 2000. First North American Record of a Melanistic Female Northern Harrier. Journal of Raptor Research 34:58-59.

Paprocki, N. 2012. Do Dark Northern Harriers Exist? Wild Lens Blog. Retrieved January 13, 2012 - http://www.wildlensinc.org/blog/