Originally posted @ http://hawkwatch.org/blog
This post is an interesting subject to explore. I was curious about an adult male Northern Harrier Peter Pyle published in Birding Jan/Feb 2014. He suggests that a male-plumaged specimen is actually an adult female in this caption: "...Although the specimen was labeled a male, it had a wing chord of 362 millimeters, indicating a small female; the label included no data on gonads, suggesting that it may have been (mis?)sexed based solely on plumage." He states in the article that he had studied this specimen before but did not mention he thought it to be a female at that time. Must be an interesting bird, right? Looking at this particular specimen (image #1), one question arises: does it make more sense to sex this bird as a male based on the fact that it has typical adult male plumage, or is it better to assume it's a female with male plumage based on a single measurement that has an overlap zone between male and female? Let's explore this a bit more and see what we can find out.
Because there has never been a known female Harrier showing adult male plumage banded in North America, housed in a museum, or reported in historical data or literature regarding nesting birds in North America, Brian Sullivan and I were very curious to explore Pyle's theory. So Brian went to the Museum of Vertebrate Zoology at Berkeley to review the specimen. We can say for sure that the bird appears it was labeled correctly as a male all along. This is good thing. Now, new birders won't be confused and wonder whether every adult male Harrier could be a female...it's hard enough to identify all the raptors out when you first start birding, so I'm glad it seems we were able to eliminate one potentially confusing ID issue! There is considerable size variation in both sexes of Harrier, and overlap occurs. Brian measured the wing chord of the male in question multiple times and the largest measurement he recorded was 352mm, which is towards the larger end for a male. Truth be told, it is more difficult to measure a "frozen" study skin than a live bird, and that could be the reason for Pyle's higher measurement. Brian had to have one of the museum staff assist him in order to measure the specimen and take a photograph of the measurement (image #2). I reviewed the literature Pyle cites in his article and found an adult male Harrier with a wing chord of 362mm in one study (JFO. K. Bildstein, F. Hamerstrom). I also reviewed measurement data on nearly 1,000 Harriers throughout North America (576 Harriers in the HawkWatch International database) and found overlap in wing chord between the sexes, and several additional males with wing chords of 362mm. In Pyle's 2008 reference alone, the wing chord range for male Harrier is listed as 322 - 360mm, female is listed as 362 - 400mm, a mere 2mm separation.
The specimen tag had some very relevant information. The weight of the bird the day it was shot was 382.5gms, easily within the male range. Brian's tarsus width measurement was on the small end for a male (right leg = 5.58, left leg = 5.84), but the legs of specimens can be slightly slimmer than live birds. A smaller looking male Harrier in the same tray as the specimen in question had a larger tarsus width: right leg = 6.35, left leg = 6.35.
Hawks From Every Angle (Princeton, 2005) states "some females show grayish remiges." However, they are not known to have remiges as gray as adult males, and Lack solid black wing tips, a black trailing edge to the wings, and white underparts. This specimen has distinct black wing tips typical of adult males. Pyle didn't show the wing tips in his article, so here is an image of the bird for clarification (see image #1) and a nice addition to the story. And, here's an example of a typical adult female that has grayish remiges (image #3), check out the banded wing tips. Interestingly, the eye is still brownish-yellow on this adult female suggesting it is only a few years old.
Pyle also says "...it is possible that adult males show some sort of polymorphism as older birds..." meaning males can be "browner" on the back or face as they age, but then he finishes by saying "...but I can't think of another example among species with delayed plumage maturation (gulls, albatrosses, boobies, etc.) in which plumage does not progress through early cycles toward a universally definitive plumage state, let alone showing sexual dimorphism as well." Several issues arise from the second part of this quote. Pyle previously stated: "browner adult males were "2nd-years" or "sub-adults." He also stated: "older females can look like males." As well as stating previously: "adult Harriers reach a definitive plumage." So, if older females (specimen Pyle refers to) look identical to what Pyle (2008, 2014) refers to as sub-adult males, how are we to know which is which in the future? And, what % of "browner males" are actually older females? If the classic brown adult female plumage is not now the definitive plumage, is the definitive plumage of every female ultimately a male plumage? Can grayer males change to browner plumage or even female plumage as they age? All of this would mean the only way to sex Harriers at all is by measurements (of which there is some overlap), unless of course it is mated pair. But, these questions are irrelevant for the time being. I credit Pyle for agreeing with Brian and I in saying "...it is possible that adult males show some sort of polymorphism as older birds..."
An interesting point to note: One might figure a bird old enough to change out of its definitive plumage might show 2 generations of feathers (as many adult females do), but this specimen only shows one, which unfortunately means there is no way to age the bird past after-hatch-year. The eyes are not present on skins of course, so we can't determine by eye color if it were a younger (brown eye) or older (yellow eye) adult female if it were a female anyway. For new birders or researchers, beware that study skins may look overstuffed, elongated (as the discussed specimen), or plumper, etc. than normal depending on who prepared it. I noticed this since I was taught taxidermy in 1988 and prepared skins for several years after. Check out these male Harriers (image #4) and male Merlins (image #5) (sexed by measurements, gonads, and plumage) that vary considerably in size due to preparation bias.
Having said all this, I am still open to the possibility of the bird being a female, since I don’t know everything there is to know about raptors (not even close), and for anyone to say that have “seen it all” would be just plain silly. I would just like more evidence to point that way, and for now I don’t think it does, but may very well one day…and that would be just fine with me!