With respect to swallows flying higher in the fall, here is just a thought
along these lines with no local evidence to back it up, but an interesting
possibility to consider. In 2016, an article in Science reported on some
studies done over 10 years in the UK concerning migrating insects. They found
that over southern parts of the UK in the fall, there were over 3.5 trillion
insects (one author said it was the equivalent mass of 20,000 reindeer), many
flying very high in a migratory pattern. Little is known about most insect
migration but if similar patterns are found here, this could be some of the
explanation why the swallows are flying high in the fall. I don’t know of any
studies of our local insects that would tell us which, if any, are flying high
in vast numbers in late summer and fall. In the spring and during the breeding
season there is an abundance of insects flying near or above the ground and
water surfaces. Populations of many of these insects diminish as fall arrives.
At least, to observers on the ground this seems to be the case. But from the
ground, none of us can see what is happening with regard to insects at high
elevations. For swallows, perhaps there are better foraging opportunities up
high, but we will never know until someone can do studies about this. In truth,
however, we really know almost nothing about what Tree Swallows eat after the
breeding season. During the breeding season there is a heavy dependence on
flies and other diptera emerging and flying low as well as other insects. But
Tree Swallows at this time also eat things that provide high levels of calcium.
This includes an odd assortment of such things as aquatic snails, egg shells,
crawfish parts, freshwater clam shells, fish bones etc. Presumably these help
with calcium used in the production of egg shells and rapidly growing
nestlings. We know some of the kinds of insects brought to the nest but it is
uncertain what the adults are eating for themselves during breeding times. Do
they eat anything different than they bring to their young? In the east, Tree
Swallows arriving early may encounter cold snaps that make flying insects
unavailable. At such times they may gorge themselves on bayberries. These
fruits have a waxy coating to help protect the berry and its developing seeds
over the winter. This wax cannot be digested by most birds and so these fruits
are mostly left alone. But Tree Swallows have the ability to digest wax. Thus,
these berries become a food source when insects are not sufficiently present.
(Yellow-rumped Warblers also have this capability and eat some of our winter
berries.) Violet-green Swallows do not have this ability and they feed only on
flying insects. They certainly fly low to obtain low-flying or newly emerging
insects but Violet-green Swallows also often forage on flying insects that are
higher than most swallows feed during the breeding season. Some interesting
questions yet to be resolved.
Dan Gleason
Owner, Wild Birds Unlimited of Eugene
Ornithology Instructor, retired, University of Oregon
dan-gleason@xxxxxxxxxxx
On Sep 15, 2021, at 8:11 PM, Thomas Gilg <tom@xxxxxxxxxxxx> wrote:
Outside of Brownsville, I find that the spring swallows fly low enough to ID
well, but come fall they are “way the heck up there” and hard to ID. I don’t
know if this is a common phenomenon with swallows, and thus if some fall
swallows escape our notice.
Up on the Santiam section of the PCT a few weeks ago, the VGs were “way the
heck up there”. A few days prior, White-throated Swift were reported a few
miles south of (hum).
--tg
From: obol-bounce@xxxxxxxxxxxxx <obol-bounce@xxxxxxxxxxxxx> On Behalf Of Tim
Rodenkirk
Sent: Wednesday, September 15, 2021 5:26 PM
To: noah.strycker@xxxxxxxxx
Cc: OBOL Oregon Birders Online <obol@xxxxxxxxxxxxx>
Subject: [obol] Re: Watch those white-bellied swallows...
I have noticed the same phenomenon that Noah mentioned. I think all the eBird
filters should be really low after the end of August and that it should be
flagged after mid-Sep. Just had this discussion with Russ Namitz recently in
fact. I would set eBird filters to zero from mid-Sep until the beginning of
Dec. In Dec any white- bellied Sparrow is likely a Tree and VGs are very
rare. Right now VGs are very common and Trees rare.
This reminds me of the Western Sandpiper “problem” in winter. There are
legitimate flocks of say 20 or 30 in Coos Bay in winter but in general they
are pretty rare in most of Oregon and are normally just found in the ones and
twos with Leasties and Dunlin. A great find on most CBCs in fact. However,
for the uninitiated, those huge flocks of Westerns in the fall are an easy
call on a distant flock of Leasts (or Dunlin) without doing visual ID in the
winter. As fall goes on Leasts start predominating the small peep flocks and
Westerns numbers drop off so that by CBC time they are long gone. Same with
Tree Swallows which become less and less common as it gets late in August and
are a good find in early Sep before they completely disappear by mid-month.
A real red flag are flocks of hundreds of Westerns in the winter or hundreds
of Tree Swallows in Sep and October.
Anyhow, count yourself lucky to find a Tree Swallow now (in Oregon).
Merry migration!
Tim Rodenkirk
Coos Bay
On Wed, Sep 15, 2021 at 4:31 PM Noah Strycker <noah.strycker@xxxxxxxxx> wrote:
Hi all,
For the past few fall seasons, I've been interested in the departure date of
Tree Swallows from Oregon, and have looked carefully at the white-bellied
swallows I see.
Going through my own records, I've evidently only ever seen one (!)
individual Tree Swallow in Oregon during the month of September, and none in
October. It seems like Tree Swallows pretty much disappear after August, at
least from the places I visit.
On eBird, there are only four definite Tree Swallow photos from Oregon in
September, and just one in October (compared to many Violet-greens), each
seemingly from observers who were aware that they were documenting stragglers.
But there are lots of sight reports of Tree Swallows in Oregon, on eBird and
elsewhere, often of double-digit numbers and usually without details,
throughout September and October. These are probably almost all misidentified
Violet-greens. It's easy to see white-bellied swallows flying in places where
they were common earlier in the summer, and put down some Tree Swallows
without thinking about it, especially with dusky-cheeked young Violet-greens
in the mix.
So, look carefully from now on: Do you really see a Tree Swallow? If so, get
a photo! Doubtless there are occasional late ones hanging around, but Tree
Swallows seem to be much rarer than many birders realize after mid-September,
and are certainly worth a second look. (Winter records starting in December,
of course, are another story...)
Just something to watch for as fall is upon us ;)
Good birding,
Noah
