----- Original Message -----
From: Joel Geier <jgeier at attglobal.net>
To: Obol <obol at lists.orst.edu>
Sent: Monday, January 21, 2002 9:38 AM
Subject: Re: Winter swallows and other passerines


> Hello OBOlers,
>
> This has been an interesting discussion, and I have not
> noticed anyone trying to claim that there is a single,
> simple answer to the puzzle.
>
> In the interest of avoiding oversimplification, I suggest we
> should disabuse ourselves of the notion that there is
> "nothing special" about this year's weather. The average
> daily low temperature data I cited yesterday show that this
> is one of the six mildest winters in the mid-Willamette
> Valley, out of the past 40 years. Beyond that, there has
> not been even a single day of sustained freezing weather --
> the lowest daily high temperature so far at Corvallis
> (Hyslop field station) was 39 deg F.
>
> In other words, this is clearly a notably (if not
> freakishly) mild winter. While I agree with Dave Irons and
> Phil Pickering that this may not be a primary cause for
> unusual birds to show up in our area, it certainly needs to
> be considered when pondering why unusual birds are being
> observed.
>
> As Irons points out, the reason why various passerine
> species have occurred in our area likely varies from species
> to species. Some like the Hermit Warblers are presumably
> lingering birds (or birds whose migration from a bit farther
> north in their breeding range was for some reason
> abbreviated). The eastern vagrants may have shown up here
> due to a flip of their "magnetic compasses." The Barn
> Swallows might be getting pushed north into our area on
> sustained southerly winds -- well, maybe. All of these are
> fine explanations (some more speculative than others) as to
> why these birds arrived or simply stayed here.
>
> Irons rightly points out that "year-to-year weather patterns
> probably affect nothing other than the survival rates" of
> semi-hardy passerines. I would add that weather patterns
> also affect detection rates: To be observed well into the
> winter months, the birds first need to survive. That's
> leaving aside the intangible effect of weather on observer
> activity.
>
> In the same vein, Steve Mlodinow points out, "When a Barn
> Swallow wanders a thousand miles north of its normal winter
> range, it is in a very tenuous position. It is hard to
> imagine that these birds are not doomed." Unless, of
> course, they find that unusually mild temperatures have kept
> an adequate supply of their usual forage on hand.
>
> Regarding the "pineapple express" theory for Barn Swallow
> occurrence, the absence of Violet-green Swallow reports and
> sparseness (within normal ranges) of Tree Swallow reports
> does raise some problems. As Mlodinow suggests, one might
> logically expect these more northerly wintering swallows to
> be pushed north by the same prevailing winds, and to arrive
> in numbers ahead of the more southerly wintering Barn
> Swallows.
>
> Let me throw out another word: DISPERSION. When a
> population of birds starts to move north (with or without
> the help of prevailing winds), a few birds generally arrive
> well in advance of the main horde, and then we see a steep
> ramp-up as the main population arrives. Think of the spring
> arrival of Violet-green Swallows -- for a few days we get
> scattered reports, and then suddenly, one day they seem to
> be everywhere.
>
> A similar phenomenon is seen in the study of how chemicals,
> colloids, bacteria etc. spread out as they migrate along
> with groundwater currents. In that field the process is
> called "dispersion" (Probability freaks may be entertained
> to know that the shape of the curve is the classical
> Gaussian error function). Mathematically it can be shown
> this phenomenon is the natural outcome of combining small,
> random movements of itty-bitty things (whether birds or
> bacteria) in a prevailing current (water flow, prevailing
> winds, or the "migratorial urge").
>
> I would argue that what we have seen so far does not look
> like anything like the "dispersion front" associated with a
> northward movement of a Barn Swallows population. Apart
> from two instances of small flocks (8 birds and 15 birds,
> each seen on one day only), all other sightings have been of
> 3 birds or fewer. If a sizeable population is being pushed
> north, at some point we should see more than the dispersed
> fringe, but it hasn't happened yet. Here in Oregon, the
> Yaquina Bay flock on 5 Jan was not followed by numerous
> flocks in the following week. Instead we went back to the
> pattern of a few individuals seen here or there, which had
> been the pattern through most of December.
>
> If tomorrow we start getting reports of Barn Swallows
> everywhere in western Oregon, that would add some credence
> to the northward-drift theory -- but we haven't seen that
> yet.
>
> I would also argue that a bit of a reality check is called
> for regarding the "sudden occurrence" of Barn Swallows.
> There have been a few unsupported statements made about
> this, e.g.: "After an apparent absence in late Fall and most
> of December these birds have suddenly appeared." This seems
> to be based more on sudden attention to Barn Swallow
> sightings since those notable flocks in Newport and Everett
> were reported, than on a full consideration of sightings in
> the early part of the winter.
>
> If you look at a nominally unbiased record of sightings
> since late fall, the grounds for such statements are rather
> scant. Here's the full set of actual & possible Barn
> Swallow reports from the "OBOL reporting area," since the
> tail end of fall migration:
>
> Date No. Place (observer)
> 29 Oct 1 Scravel Hill pond, Albany area (Marcia
> Cutler).
> 1 Nov 5 Ankeny NWR south of Salem (Roy Gerig)
> 2 Nov 8+ Ankeny NWR (Jeff Harding)
> 9 Nov 1 Barn near Ankeny (Stuart Sparkman)
> early Dec "several" Monmouth, OR (Maggie Meikle)
> 11 Dec 2 Smith & Bybee Lakes, Portland (David Bailey)
> 14 Dec 2 Smith & Bybee Lakes, Portland (Tom Ewert)
> 17 Dec 1(*) Fern Ridge Reservoir (fide Harry Nehls)
> 28 Dec 1 "regular" at Hansen NWR (Mike Patterson)
> 30 Dec 1 Hansen NWR (Mike Patterson, Russ Copenheim)
> 3 Jan 1 Near Shedd, mid-Willamette Valley
> 5 Jan 8 Yaquina Bay South Jetty (where did they go?)
> 6 Jan 3 Livermore Rd., Polk Co.
> 13 Jan 1+ Steigerwald NWR, WA (Pamela Johnston)
> 14 Jan 1 Monmouth, OR (Maggie Meikle)
> 14 Jan 1 Amazon Cr, Fern Ridge area (Dave Brown)
>
> (*) Reported as "Swallow sp.," may not have been Barn
> Swallow.
>
> The only gap in the above is from 10 Nov to "early" December
> (whenever that might have been). As I recall, that was a
> time of near-continuous rain in our area. Was the level of
> birder activity comparable to that in the CBC season, or in
> the couple of weeks of relatively clear weather since then?
> And even if birders were out, might not swallows have been
> hunkered down somewhere or just plain missed, considering
> the rain? Given that the numbers observed outside of that
> period have been scant (except the one flock), it seems
> probable that these few individuals could just have been
> missed during that period of rainy weather. The fact that
> the Newport flock was seen on only one day illustrates that
> swallows can indeed hide from us.
>
> This is not to say the "pineapple express" theory might not
> be at least part of the explanation for why a few Barn
> Swallows are being observed. However, I'd caution that the
> evidence of a dramatic change since the start of the year is
> exceedingly thin. Let's wait and see what this week brings
> .... which I guess means going outside for a look!
>
> Good birding,
> Joel
>
> --
> Joel Geier
> jgeier at attglobal.net
>