From south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] Wed Nov 5 16:24:58 2003 Received: from www.plaidworks.com (localhost [127.0.0.1]) by plaidworks.com (8.12.9/8.12.9) with ESMTP id hA60KO6T027609 for <[[email protected]]>; Wed, 5 Nov 2003 16:20:24 -0800 (PST) Received: from rtjones.nas.nasa.gov (rtjones.nas.nasa.gov [129.99.19.30]) by plaidworks.com (8.12.9/8.12.9) with ESMTP id hA60J8MU027559 for <[[email protected]]>; Wed, 5 Nov 2003 16:19:12 -0800 (PST) Received: from rtjones.nas.nasa.gov (localhost [127.0.0.1]) by rtjones.nas.nasa.gov (SGI-8.12.5/8.12.5/NAS-6n) with ESMTP id hA60ImLF095888 for <[[email protected]]>; Wed, 5 Nov 2003 16:18:48 -0800 (PST) Received: (from mrogers@localhost) by rtjones.nas.nasa.gov (SGI-8.12.5/8.12.5/Submit) id hA60ImcO096264 for [[email protected]]; Wed, 5 Nov 2003 16:18:48 -0800 (PST) Date: Wed, 5 Nov 2003 16:18:48 -0800 (PST) From: "Dr. Michael M. Rogers" <[[email protected]]> Message-Id: <[[email protected]]> To: [[email protected]] Subject: [SBB] sexing and ageing goldeneyes X-BeenThere: [[email protected]] X-Mailman-Version: 2.1.2+ Precedence: list List-Id: South Bay Birding List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] Errors-To: south-bay-birds-bounces+south-bay-birds-archive=[[email protected]] All, Yesterday I suggested that the female/immature BARROW'S GOLDENEYE at Shoreline Lake might be a male based on size (it seemed similar or even slightly larger in size than a nearby adult male Common Goldeneye and notably larger than the female/immature Commons). Madge and Burns list the following average weights in their "Waterfowl" book: Male Female Barrow's 1100g 800g Common 990g 710g suggesting that a bird of about 1000g is indeed more likely a male Barrow's than a female. However, all juvenile goldeneyes have dark bills and first-winter males do as well; apparently young males never go through a period when they have yellow in the bill. Yesterday's bird had an interesting mix of yellow and gray in the bill and thus must have been a large female. The underlying pattern was typical of an adult female Barrows Goldeneye, namely all yellow with a black nail, but there were patches of diffuse gray suffused around the base of the bill, including some fairly dark areas along the cutting edges of the mandibles and some gray splotches on the bill saddle between the nares. This could be easily explained if the bird was an immature female transitioning from juvenile (dark bill) to adult (yellow bill) plumage. Plumage features do not suggest this is the case, however. When the bird flapped it clearly showed dark tips to the greater secondary coverts, resulting in a dark bar dividing the white wing patch. Most field guides are vague about just how the white in a goldeneye wing varies with age, sex, and species, but the U.S. Fish and Wildlife Service website at http://www.npwrc.usgs.gov/resource/tools/duckplum/duckplum.htm is much more definitive. All immature goldeneyes (both Barrow's and Common) should have at most "a trace of black at the tips" of the greater secondary coverts, resulting in an apparently undivided white wing patch. This suggests that the bird was an adult female. Adult females also typically have some pale in the median coverts as well, although I did not see this when the bird flapped yesterday. The Shoreline Lake bird's very yellow iris (approaching the brightness of the adult male Common's eyes) as compared with the brown iris of a juvenile and its white collar also suggest an adult bird (or at least a bird well out of juvenal plumage). If the bird is an adult then the gray suffusion to the bill requires explanation. Eastern Barrow's Goldeneyes do not have complete yellow bills like our western birds, being more similar to female Common Goldeneyes in this respect. However, purported Barrow's Goldeneyes lacking all-yellow bills out west have been regarded with some suspicion and this bird did have the underlying pattern of a western bird. I suspect that the answer lies in a statement in "Waterfowl" regarding eclipse plumage females, namely "females bill duskier in summer". Given the relatively early date in the winter season, I suspect that the gray suffusion/spots in the bill are remnants of eclipse plumage and that this bird is indeed a large adult female BARROW'S GOLDENEYE, not an immature male as speculated yesterday. Mike Rogers _______________________________________________ Do not post admin requests to the list. They will be ignored. south-bay-birds mailing list ([[email protected]]) Help/Unsubscribe/Update your Subscription: http://www.plaidworks.com/mailman/options/south-bay-birds/south-bay-birds-archive%40plaidworks.com This email sent to [[email protected]]