February 2000 Mystery Birds

Thanks to everybody for some very thoughtful insights. The discussion was terrific.

I belive these birds are a Common Nighthawk and a Mallard. The nighthawk illustrates an illusion in which the white patch appears to be out near the tip of the wing because the wing narrows, leaving relatively less dark wing area outside the white than inside. As pointed out by David Fix and others, if you measure the wing from the bend to the tip, Common Nighthawk has the white intersect the leading edge about half way. On Lesser Nighthawk the white intersects it about two thirds of the way out. Sometimes the white bends as it approaches the trailing edge of the wing, giving the impression that the white is closer to the tip than to the bend of the wing. The suggestion about the difference in wing formula is also valid but young Common Nighthawks in which the outer primaries are not fully grown may show a wing formula similar to Lesser Nighthawk. I am aware of at least one case where a nighthawk specimen from Colorado was incorrectly identified as a Lesser because it supposedly showed the Lesser's wing formula. Caution is advised whenever an out-of-range nighthawk is seen.

I agree that the duck does not appear to be a hybrid. I received some private comments that I will share. Al Jaramillo studied this bird at the Palo Alto Duck Pond and wrote:

I have an unfair advantage in that I have seen the real duck and have observations that others do not have on the thing. This bird has been hanging around with a normal male Mallard, and behaviourally it behaves like a female and the two like a pair. All one needs to see is copulation to confirm this, but my sense is that this is a male-like female!!!!! I do not know if this is known in the literature or not. The bill certainly is a female type bill, and up close you can see that the breast while brown is actually streaked as it would be on a female. The flanks look like female feathers, and these have not moulted out as the season has progressed. I will look for it again in the coming week, and see if it is still behaving as a female, and then maybe I will be more convinced that this is what the thing is.

In another message Al added:

Given that there have now been several people that have suggested either an "intersex" or male-like female being a possibility I feel better about having seen the behaviour of the pair at the pond. I bet it is a female! I tried to find it the other day again, but had no success. The information from the folks that have banded these birds is of great interest as duck banders can easily accertain the sex of the bird by extruding the cloca. I have done this myself while Mallard banding, and it really is quite obvious.

I thought the genetics discussion was fascinating and certainly such genetic oddballs cannot be ruled out. For excellent photographs of a real Gadwall x Mallard hybrid ("Brewer's Duck") check out the photo gallery.

On 8 March 2000, I received the following important additional comments from Roland van der Vliet co-editor of Dutch Birding and member of Dutch rarities committee (CDNA). It describes a similar female Mallard which acquired male-like plumage characters:

I looked up the reference regarding the male-plumaged female Mallard in Dutch Birding. This is:

Dutch Birding 14 (4): 131-134, 1992: Uitwendige geslachtsverandering bij vrouwtje Wilde Eend, by J N J Post and E J O Kompanje. It is in Dutch, but with an English summary. It also includes four plates, depicting the bird in its fourth year (typical female plumage) and after it died, in its 14th year. Then it had an almost male-type plumage, including the 'curl' in the tail, and typical male head- and neck markings. Also, it had a male-type bill coloring.

I give you the summary of the article:

"External change in sexual characters in female Mallard.

On 12 August 1977, four deserted Mallard Anas platyrhynchos ducklings were found and subsequently hand-raised [and ringed; RvdV]; they fully matured and lived for many years as garden pets in Goes, Zeeland. After c 10 years, one of the females gradually started to show male plumage and bare part characters. Within the group (now consisting of seven birds), however, the female still behaved as a females and was also treated like a female by the other ducks (which usually is not the case). The bird died on 3 September 1991. Dissection revealed that the left ovary and part of the oviduct were cysteous, possibly as a result of age. The right ovary, however, was still in its normal embrynic state. Birders are regularly confronted in the field with ducks that show incompletely developed adult male plumage characters. Although these are often moulting or juvenile birds, there is a possibility that they are females showing male plumage and bare part characters due to an ovarian abnormality."

References include Swennen et al 1989 which describe a similar case for a 18-year old Common Eider (Swennen, C, Duiven, P & Wintermans, G J M 1989. Abnormal plumage in possibly senile female Eiders Somateria mollissima. Wildfowl 40: 127-130).

The original photos are below. To view public comments or join the fray by adding your opinion, click here. For the March mysteries, I am trying a new message board. Let me know what you think of it. The idea is that each comment will be in its own file for faster downloading. Have fun!

Photos © Summer Wilson and Michael Rogers.
All rights reserved.

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