Before going into the main theme about corvids, I'll provide a little
background. Today (May 17), here in the village area of Norwich, there
was some suggestion of possible arrival of new migrants overnight last
night. Specifically, this morning there was a singing Magnolia Warbler
here in hemlocks bordering the backyard, and along Elm Street near the
Jones Circle intersection there were a singing Tennessee Warbler and a
singing Scarlet Tanager. None of these three species had been detected
yesterday in their respective localities of this morning.
To see whether there might be additional migrants around, early this
afternoon I decided I'd check the Norwich nature area even though the
time of day was not especially favorable for finding migrants. I
entered the nature area through the trailhead and walked the short
distance through the woods into the meadow to the south. As I was
crossing the meadow toward the orchard, I noticed a couple of American
Crows repeatedly swooping downward over the top of a live white pine
near the southeastern border of the meadow. Through binoculars I could
see towards the top of that tall pine a fairly large nest upon which
there was a darkly colored bird larger than a crow but partially
hidden by the branches of the pine. The bird on the nest had a corvid-
shaped bill, and, during a number of minutes of watching, I could see
bill movements indicating feeding. Eventually the large bird flew out
of the nest about 1:45 PM while carrying a conspicuous, irregularly
shaped, dark object in its bill and while closely pursued by a crow
which was obviously considerably smaller in size. The shape of the
tail confirmed that the larger bird was a Common Raven. Although I
could not determine through the binoculars exactly what was being
eaten or carried away, indications were that nestling crows, rather
than eggs, were the probable food.
While the raven was on the crow nest, at least two crows were
occasionally swooping above the raven, and at least three other crows
were flying around in the vicinity. Crows were calling loudly, but I
heard no raven vocalizations.
This was the first time I've seen predation on the American Crow by
the Common Raven. However, such observations might be expected because
here in the Upper Valley region crows are seen pursuing ravens in much
the same way that crows pursue Red-tailed Hawks. Such "mobbing"
behavior indicates that the bird being "mobbed" is a predator for the
species doing the pursuing.
Other dates on which I have seen a raven being pursued by one or more
crows in the Upper Valley include the following: May 24, 1997; June
20, 1997; May 19, 1998; February 11, 1999; March 16, 1999; April 21,
1999; April 11, 2001; May 14, 2001; October 1, 2001; May 14, 2004;
November 22, 2004; May 14, 2005; December 6, 2005; February 24, 2006;
May 5, 2007. I've not seen pursuit of ravens by crows in July,
August, or September. The dates on which pursuits have been seen show
that mobbing of ravens by crows is not limited to spring and early
summer when crows would be nesting.
Relations between crows and ravens should not be described as
neighborly!
Incidentally, I did not find any obviously new migrants in the nature
area this afternoon, but there earlier today, as reported on UVB,
Chris Rimmer found a Wilson's Warbler, which might fit in the category
of new arrivals as none of that species had been reported from that
locality on the previous day.
George
George Clark
Norwich, VT
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