Ok, this is getting to be a deep thread, possibly for a very limited OBOL
audience .... BUT this is a very good explanation by Dave and worth a slow
read with a hot cup of Java.
Understanding a bit about the occasionally subjective science of taxonomy
and specie and subspecie assignment helps and it does increase the interest
and challenge for those so inclined.
Take it or leave it for your own great birding!
Roger Freeman
Silverton OR
On Fri, Mar 8, 2019 at 11:58 PM David Irons <llsdirons@xxxxxxx> wrote:
Steve Harder posed this question to me this morning: *For those of us
less versed in this discussion, could you please define the criteria used
to determine when a group of individuals from a known species is or can be
construed as a subspecies? Does a certain amount of geographic isolation
matter? *
In the context of this discussion, here's how I would answer Steve's
question.
In the simplest terms a "subspecies" is a subset or sub-population of a
species (in this case Song Sparrow) that through reproductive isolation has
developed genetic and morphologic characteristics that differ from the rest
of that species' populations/sub-populations. The morphologic differences
may or may not be readily evident in the field to the average birder.
Some period of geographic separation or habitat fragmentation ("island
effect" if you will) typically facilitates the isolation. As an example,
"Ice Ages," or glacial maximums are believed to have precipitated divergent
evolution within many species, as sub-populations were separated from other
members of their species by continental ice sheets for long periods.
Subspecies are typically only marginally different genetically from other
populations within their species and when pairs of subspecies come into
contact on breeding grounds they may interbreed somewhat freely. Good
examples of this are Slate-colored and Oregon Dark-eyed Juncos and
Yellow-shafted and Red-shafted Northern Flickers. These two pairs of
distinctive subspecies interbreed frequently resulting in swarms of
recognizable intergrades. With Song Sparrows subspecific differences in
appearance are for the most part subtle, thus intergrades are unlikely to
be as identifiable as the intergrade juncos and flickers.
With most species, interbreeding that produces hybrid offspring is
somewhat rare, even in situations where two species within the same genus
share a broad zone of contact on their breeding grounds. Of course there
are notable exceptions to this rule. In our own region Western and
Glaucous-winged Gulls, Townsend's and Hermit Warblers and Red-breasted and
Red-naped Sapsuckers hybridize routinely. These species pairs act more like
subspecies pairs on the breeding grounds and in fact not that long ago
Red-breasted, Red-naped, and Yellow-bellied Sapsuckers were considered
conspecific. I find it interesting that Townsend's and Hermit Warblers
interbreed routinely in their contact zone, but neither species seems to
hybridize with "Audubon's" Yellow-rumped Warbler, which occurs in abundance
across the breeding range of both Townsend's and Hermits.
It can be difficult to wrap our heads around the idea of a subspecies.
Genetically speaking, the line between where subspecies and species begin
and end is a murky and subjective. It's vital to remember that 'species'
and 'subspecies' are human constructs that satisfy our inherent need to
sort, categorize, define relationships and put names to things based on
their differences and similarities. How the birds sort themselves on the
breeding grounds doesn't always align with our human notions about their
relationships and the differences we humans see in their appearances may or
may not be integral to this process.
The various Song Sparrow subspecies being discussed represent
sub-populations of birds that are generally found breeding and or residing
permanently within a certain range that does not include Western Oregon. We
call them subspecies because each of these sub-populations shares some
common set of morphological characteristics that were at some point
described and quantified to the satisfaction of the ornithological
community. Some Song Sparrow forms are migratory while many others are not.
The relative similarity in appearance of many of these subspecies makes
trying to determine where and how much they move around a considerable
challenge. What we do know is that the Sauvie Island bird is clearly
different in appearance from the Song Sparrow types found in Western Oregon
year around. In appearance it resembles some eastern birds, at least one
subspecies that is widespread in California and perhaps the population that
breeds in the Northern Rockies and Great Basin. Hence our thirst for trying
to understand where it may have come from.
It makes my head hurt to think about some of this stuff. I hope that I
haven't sent too many of you looking for the Excedrin after reading this.
Dave Irons
Beaverton, OR
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