A few comments on the previous comments:

At 04:28 PM 3/25/98 -0800, Jerry Tangren wrote:
>A few comments...

>>I'll quote from Rising and Beadle's book, 'A Guide to the Identification and
>>Natural History of the Sparrows of the United States and Canada' (and the
>>Oscar for the longest book title in the natural history category goes to...):
>...
>
>>...Cassiar Juncos breed east of the coastal ranges from central Yukon
>>southeast to north-central and eastern British Columbia and west-central
>>Alberta. These birds represent a stable population of apparent hybrids
>>(intergrades?--m) between Slate-colored and 'Oregon' juncos (J.h. oreganus),
>
>Your insertion is quite appropriate--what is a stable population of apparent
>hybrids? That statement makes no sense! Please emphasize that this is a
>population of birds intermediate along a continuum between eastern birds
>and western birds. They aren't hybrids. I'm not even sure they are
>intergrades?

According to "Ornithology" the text book by Frank B. Gill the concept of a
"Stable Hybrid Zone" is valid. He also refers to the concept of intergradation
and hybrid zone in the same sentence making no distinction between the two
in a reference to Audubon's and Myrtle Warblers ("Westward movement of Myrtle
Warbler genes and eastward movement of Audubon's Warbler genes has extended
the zone of intergradation - hybrid zone - far beyond the mountain passes where
hybridization takes place"). His concept of hybridization involves two distinct
populations not necessarily separate species. He goes on to talk about the
stable
hybrid zone of "red-shafted" and "yellow-shafted" Northern Flickers in the Great
Plains and the 2 theoretical models that have been proposed to explain
stable hybrid
zones. He also mentions that the Guilded Flicker may be of hybrid origin,
being the
result of two hybridizing populations that fused into a single taxon with
intermediate
characters. I think this supports Rising's description of Cassiar Juncos.

>
>In a gross simplification--what is occuring is a change in certain gene
>frequencies from here to there. We may perceive subspecies only because
>the environmental features that select for the frequencies in the population
>change at sufficiently high rates at the "subspecies" boundaries.
>
>And then perhaps only because the gene combinations produce birds that
>are readily separable from others with slightly different combinations.
>
>>and they are intermediate in coloration between the two.

What you're talking about is a geographic cline but clearly there is
not an even gradation across the "Dark-eyed Junco" range and it seems
reasonable to assume that the intermediate forms are a result
of secondary contact among previously isolated populations (I mean isolated
thousands of years ago). Since there is agreement as to
the characteristics of the common subspecies it seems reasonable to refer
to the intermediate forms as hybrids.

Just my $.02 on this interesting subject.

Charles.



^__^ ========================
(0 0) Charles E. Swift
| \/ | charless at umich.edu
\ / Moscow, Idaho
\/ I'm for the birds!
========================