Tony, Thank you for your detailed and thought provoking post. We agree that without genetic work on suspect birds, we, as field observers, really have no idea what the parentage of odd ABDU's are. When describing the birders' tendency to call ABDU x MALL, you say, "there was no way to determine that the ABDU that they saw with white trailing edges were actually hybrids." Touché. Most of the time, there is no way to determine if they were not actually hybrids. The "swamping" of the ABDU gene pool by MALL is a serious issue (for ABDU's). Even a bird described as having a white trailing edge to the secondaries in 1834 is suspicious, since presumably hybridization was occurring that long ago, but perhaps not at such a rate as we see today. When exactly did the swamping start? More importantly, why did it start? What was the situation that allowed the ABDU to deviate from MALL or MALL ancestor, and then to recombine in modern times?
The history of this issue is full of uncertainty and non-consensus, and Kirby et al. (http://www.npwrc.usgs.gov/resource/birds/diplume/index.htm) makes a valiant effort to work things out using wing morphology. Based on their identification key, "the proportion of American Black Duck × Mallard hybrids to the American Black Duck parental population (the ratio: hybrids/[hybrids + American Black Ducks]) may therefore be closer to 0.132 than 0.056, the historically reported value." So, hybrids are more prevalent in wild populations than previously reported. And according to their key, it's the presence/absence of white anterior to the speculum, and the number of dark feathers on the underwing, which determines a hybrid. And they also make the point that some animals just can't be classified. It's unfortunate that they didn't present measurements of the thickness of white areas on the wing, which I think would be useful. So, I may be at fault for being too liberal with calling ABDU x MALL, but this is a conscious decision based on the fact that I don't believe we know what is really happening when looking through the bins. I agree now that a bird with a thin trailing edge on the secondaries is acceptable for pure ABDU: http://www.billhubick.com/photos/birds/american_black_duck.php (note "typical" hybrid at bottom of page) http://ibc.lynxeds.com/photo/american-black-duck-anas-rubripes/male-preening-showing-speculum And what I really like to see: http://sdakotabirds.com/species/photos/american_black_duck.jpg Birds that I am very suspicious of birds that look like this: http://photography-on-the.net/forum/showthread.php?t=803208 It's a thin line between "pure" and "not pure." Can the human, with the tools at hand (optics->eyeballs->brain), really make the call with confidence? I've learned a lot from this, and next time I study an American Black Duck with white on the trailing edge of the speculum, I'll make sure I take an even closer look at the anterior edge of the speculum and the amount of dark feathering on the underwing, as well as the other typical ID criteria like tail color, bill color, and curling of upper tail coverts. Like with so many other identification conundrums, physical evidence would be more useful than just a human's description of what they saw. Christian Nunes -- You received this message because you are subscribed to the Google Groups "Colorado Birds" group. To post to this group, send email to cobirds@googlegroups.com. To unsubscribe from this group, send email to cobirds+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/cobirds?hl=en.
