[obol] Re: Bell's Sparrow -- A Potential Addition to the Oregon List???
- From: David Irons <llsdirons@xxxxxxx>
- To: OBOL Oregon Birders Online <obol@xxxxxxxxxxxxx>
- Date: Sun, 4 Aug 2013 15:44:32 +0000
Wayne wrote:
Your comments about post-breeding dispersal of juveniles is partly what I had
in mind. If there is more info on distinguishing juvenile Bell’s from juvenile
Sagebrush Sparrow than the illustrations in Sibley, could you pass that on?
Adult Sagebrush Sparrows have narrow white edgings on their tails and Bell’s do
not (see Sibley’s inflight illustrations). Do you know whether juvs. also
differ in this?
In addition to post-breeding dispersal of juveniles, mid-late summer sometimes
is a period of northward or northwestward dispersal of dryland adults of quite
a few species. So a Rogue Valley adult Bell’s Sparrow in August could be a
possibility too. Back in the 1970s I knew some graduate students who were
studying bird populations on the Hanford Reserve in SE Washington. They
reported influxes, presumably from the south, of a variety of sage birds that
they associated with “heat low” weather conditions. Their study plots would
see big increases in numbers of things like Sage Thrashers, Gray Flycatchers,
Ash-throated Flycatchers, during the periods that daytime temperatures were
going above 100F. I think this phenomenon does not get widely recognized
because we do not bird as much in open country in those weather c onditions.
I think that Sibley's illustrations of the juv. Bell's and Sagebrush Sparrow
capture the differences pretty well. His juv. Sagebrush Sparrow seems to be
spot on based on my experiences with and the photos that I've taken of this age
class. I've not seen a Bell's Sparrow of any age (no armchair lifer for me), so
what I've been able to learn about telling them apart has come from the study
of many photos and illustrations. As mentioned in my post from last night, this
past week a juv. Bell's Sparrow (presumed based on location) was found in
Humboldt County. Rob Fowler circulated photos of that bird. It is decidedly
more gray headed and seems to have heavier streaking than a juvenile Sagebrush
Sparrow. Sibley's illustration of juvenile Bell's seems perhaps a bit too dark
overall and the streaking doesn't look quite right, but it looks very much like
the Humboldt bird. Juvenile passerines change quickly (preformative molt starts
very soon after they leave the nest), hence trying to capture a useful
representation of the typical juvenile has to present a challenge for
illustrators. I don't know about the white outer tail feathers, but I suspect
that juv. Bell's are like their parents in lacking white in the tail.
Post-breeding movements of dryland nesting birds are, as Wayne indicates,
poorly understood. Some species move upslope (warblers, vireos, flycatchers),
some move across the Cascade divide to the wetter west slopes (most notably
sapsuckers), and others move north. Herons and egrets are well-known for
dispersing north after the breeding season. The need to molt is likely a
driving force in most of these movements, as molting is energy and fuel (food)
intensive. Birds have to find a source of abundant food while they molt and
fuel up for migration. This explains the massive build-up of birds at Lake
Abert every year. Most of the species that amass at Lake Abert do not breed
there. Wilson's Phalaropes (mostly juveniles), which gather by the hundreds of
thousands, molt out of juvenile plumage July-August before undertaking their
first southbound migration. The brine fly hatch occurs just when the phalaropes
and other species need food the most, which creates the spectacle that
increasing numbers of birders are enjoying.
Dave IronsPortland, OR
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