Date: 6/21/19 5:20 pm
From: Wayne Hoffman <whoffman...>
Subject: [obol] Re: So You Think You have Seen Allen's Hummer in OR?
Hi - 

Mike wrote:

"I looking forward to ... the argument from the cheap seats on why Rufous/Allen's should not be lumped but Western complex flycatchers should..."

It's simple.  Many birders think they can tell the hummers apart but are not at all confident about the flycatchers... and if you cannot tell them apart you cannot count them both, so go ahead and lump them!

More seriously, as Dave alluded, "species" are human constructs.  We use them to help us understand the diversity of nature.  In a large majority of cases with birds, we draw species boundaries around aggregations that are pretty natural, meaning all the birds inside are very closely related to each other, and distinctly less-related to all the birds outside.  Its the minority of cases where boundaries are not clear that cause all the controversy.

Hybridization is one of the kinds of unclear boundaries.  The essences of the Biological Species Concept (BSC) - the main approach used by the American Ornithological Societies - is that members of a species (1) interbreed freely, (2) without consequence and (3) are well-connected genetically.  

On the first part, if there is SOME hybridization, that does not automatically make the participants one species.  The Mallard X Wood Duck hybrid I photographed at Crystal Springs about 10 years ago does not mean Mallards and Wood Ducks are the same species.  Occasional hybrids are best treated as mistakes rather than evidence of conspecificity.  If these ducks were one species we would expect them to choose mates pretty randomly (interbreed freely), and obviously they do not.  Gadwalls and Mallards hybridize more often that Mallards and Wood Ducks but again, not "freely."  Rose-breasted and Black-headed grosbeaks hybridize much less freely than Red-shafted and Yellow-shafted flickers.

The second part of that statement about the BSC - "without consequence" means that mixed pairs are similarly likely to produce viable, fertile offspring as non-mixed pairs.  A good example, mentioned in a recent post, is California Scrub Jay vs Woodhouse's Scrub Jay.  They do indeed hybridize in a limited area of overlap in southern Nevada.  Recent research there has shown that they interbreed pretty frequently (mixed pairs are common)  BUT intermediate adult offspring are rare enough to indicate that they are less viable, or anyway somehow disadvantaged, i.e. selected against. The recent split would not have happened without this final result.  I found, and Douglas Bell two decades later, that while Western Gulls and Glaucous-winged Gulls hybridize a lot, it is a bit short of "freely" or at least of random choice.  Bell also found indications of selection against the hybrids.  

The third part "well-connected genetically" deals with some unusual situations where hybridization appears to be pretty free, but the hybrids are confined to a narrow band connecting the two parental populations.  The clearest example of this in North America is with Tufted Titmice and Black-crested Titmice.  They hybridize, apparently freely, in a narrow band in Texas, but outside that narrow band the birds do not show evidence of the "alien" genes spreading out into the "pure" populations.  A better-known example in Europe is the Hooded Crow and Carrion Crow.  Both species are widespread and hybridize extensively in a narrow band snaking across several nations.  This third part also covers some cases where hybridization occurs in a small area of overlap but apparently not in all overlap areas.  Two local examples are Sooty and Dusky grouse hybridizing in eastern Washington, and Oak and Pinyon Titmouse hybridizing in the Klamath Basin.

We also tend to think of current conditions as representative of a longer history, when the incidence of hybridization may be episodic.  For example, when one species expands its range into the range of another, there may be quite a bit of hybridization on the leading edge of the expansion, but much less "behind the front" where both forms are more common.  The best documented example i know of is a major range expansion of Syrian Woodpeckers into Europe in the 20th century, hybridizing with Great Spoted Woodpeckers at the leading edge of the expansion.  The high frequency of Baltimore X Bullock's oriole hybridization followed by a big drop in frequency of hybridization in the 20th century coincided with a major increase, followed by a big decrease, in available oriole habitat on the high plains.  The rapid and ongoing decrease in sea ice in arctic Canada undoubtedly is affecting the opportunities for overlap, and hybridization, of Kumlein's and Thayer's gulls.

So to decide that Rufous and Allen's hummingbirds should be treated as one rather than two species (under the BSC) the research should indicate that 

(1) mate choice is pretty random where they overlap.  They are promiscuous without male parental care, so genetic (i.e. paternity) testing would be the way to do this.

(2)  Interbreeding is without negative consequences. Again this would be a statistical analysis of genetic data.

(3) Allen's Hummer characteristics are spreading north from the obvious hybrid zone, and Rufous Hummer characteristics are spreading south.

I should note that  there are multiple other species concepts championed by taxonomists - the BSC is just the one used most by North American ornithologists.  Birders should understand however that most of the other species concepts would lead to recognizing many more species and many more examples of populations that hybridize regularly being recognized as separate species.

If i have not totally bored you to tears, let me recommend a reference that describes the current understanding of the BSC as used by the AOS;

Johnson, Ned K.,  J.V. Remsen, and Carla Cicero.  1999.  Resolution of the debate over species concepts in ornithology: a new comprehensive biologic species concept.

Quite a bit of hubris in the title (and in the paper) but it does lay out the approach that is being used'

Wayne


Available online:

www.researchgate.net/publication/281444708_Resolution_of_the_debate_over_species_concepts_in_ornithology_a_new_comprehensive_biologic_species_concept

.   



On 6/21/2019 10:47:40 AM, Mike Patterson <celata...> wrote:
> There is another simple solution.......they are one species.

With a hybrid zone as extensive as the one Tim describes and no
consistent behavioral barriers for positive assortment. There would
have to be some REALLY COMPELLING DNA evidence to support keeping
them separate.

I looking forward to reading the scholarly articles on this and the
argument from the cheap seats on why Rufous/Allen's should not be lumped
but Western complex flycatchers should...

--
Mike Patterson
Astoria, OR
Bald Eagles - a gateway bird
http://www.surfbirds.com/community-blogs/northcoastdiaries/?p=3709
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