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Fall 2006
Notes and Highlights for
August
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Banding totals for the month of
August were about average with just over 1000 birds banded. The large
majority of the birds banded this month, however, represents individuals
from the local or nearby breeding population and few migrants, despite
the arbitrary definition of August as the start of our fall migration season
at Powdermill. For example, all of the 80 Gray Catbirds and 78 Cedar
Waxwings that contributed to this months total were locally hatched young
and parent birds. Of the 78 American Redstarts, few if any were caught
showing any subcutaneous fat deposits (fuel for migration). As we've
discussed before on our website, AMREs banded during the first 3/4 of the
month of August most likely are individuals (adults and young) from local
breeding populations. Our "truest" migration catch of AMREs occurs
between the end of August and into September (see fall
migration timing graphs). Ruby-throated Hummingbird (most of
these being migrants) topped the list with 138 banded and Hooded Warbler
(65) rounded out the top five with the birds above. Despite the large
proportion of local breeding birds in our August banding total (ca. 75%),
our totals for the following species that do not nest locally undoubtedly
represent migrant birds: Yellow-bellied Flycatcher (43 banded in
August), Chestnut-sided Warbler (41), Least Flycatcher (32), Magnolia Warbler
(32), Traill's Flycatcher (28), Northern Waterthrush (22), Canada Warbler
16, and Mourning Warbler (5).
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We thank the following volunteers
for their help with banding this month: Lauren
Schneider and Dean Thompson, Jessica Scopel, Pam Ferkett, Andrew McGann,
Kris Jerrell, and Bobbie Franks, and Mary Shidel.
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In August we caught a number of species that have been
conspicuous "misses" for us over the last few seasons (i.e., species with
a long-term fall average of >1). Three of these were caught on August
3rd.
(1) - This HY male Orchard Oriole, our tenth ever in
a fall season....(Juvenile Orchard Orioles are sexed primarily based on
wing length because the black chin of males does not develop until just
before the bird returns to its breeding grounds in the spring. While
less extensive than the juvenile
Tufted
Titmouse discussed in last month's update, this bird also had a conspicuously
aligned fault bar in the tail, a clear sign of a juvenile, which grows
its rectrices concurrently).
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(2) One of two Golden-winged Warblers banded thus
far this fall, and...
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(3) this HY male Cerulean Warbler. The bottom photo
is of an HY female banded a week later, on 11 August, for comparison of
the plumage dimorphism between immature males and females. The female
CERW certainly qualifies as one of the four or five quintessential "confusing
fall warblers."
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A very "good get" for the month was this HY Marsh Wren.
Given there are no confirmations of this species breeding within about
75 miles of Powdermill, the origin of this, and the angry, fuzzy MARW juvenile
banded in August 2001,
remains a mystery. It is unlikely that post-fledging dispersal occurs
to the extent that would bring these young wrens ca. 100 miles to Powdermill--a
more likely explanation is that a nearby breeding location in southwestern
PA has yet to be discovered.
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While we regularly catch a Savannah Sparrow (or two) each
year, we rarely see more, so every one we catch is a "good" bird for us.
This is because we lack any extensive grassland habitat in our banding
area. Unlike the juvenile Marsh Wren above, it is no mystery that
the bird pictured below probably fledged from a nest in the extensive open
hayfields and pastures less than a half mile west of the banding area.
It was actively undergoing its first prebasic molt with the greater coverts,
carpal covert and upper alula (A1), or alula covert, in sheath and flakes
of sheathing from pin feathers coming in all over its head and body.
This HY Great Crested Flycatcher banded on 25
August was another good (i.e., uncommon) fall catch for us.
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Adult birds already undergoing their "fall" molt during
the breeding season were main features of our summer highlights this year.
American Redstarts, in particular are regularly caught during summer months
actively molting. By August, the proportion of individuals having
completed their molt increases and in the case of adult (second year and
after second year) male redstarts, results in the renewal (in ASYs) or
acquisition (in SYs) of a striking black-and-orange plumage. Upon
completion of this molt, SY and ASY male AMREs can no longer be distinguished
from one another--thus, all AMREs in this plumage in fall are simply lumped
together as after hatching year (AHY) males.
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Two Black-and-white Warblers pictured below were caught
together on August 3. They provided a nice comparison of the subtle
sexual plumage dimorphism in immatures of this species in fall. The
principal difference is the wider, more defined, and darker black streaking
on the bright white sides and flanks of the male (top photo) compared to
the more indistict streaking against buffy white sides and flanks in the
female (bottom photo).
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Two Baltimore Orioles caught together provide an equally
nice comparison between the plumages of juvenile male and females of this
species.
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The male on the right in the photo above is generally
more brightly colored and juvenal males may acquire one or two black feathers
in the throat, but the color of the upper wing bars (yellowish orange in
males [top photo] and white in females [bottom photo]) is a more definitive
sexing criterion for birds of this age.
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The photo below not only is a perfect fit in this sequence
of HY male/female comparisons, but also addresses a common statement/question
we hear from visitors this time of year, "[why do] I have many more females
than males coming to my feeders now than earlier in the summer?"
In actuality, many of those "extra females" are immature males. Young
males even have white outer tail spots like females. While a few
HY males can acquire one to a few iridescent red throat feathers, the majority
of their "ruby-throat" molts in on their wintering grounds. As the
photo shows, however, young males (bottom bird) tend to have a much more
distinctly streaked (with green) throat, compared to females (top bird)
of any age.
The most notable difference between the sexes in
immatures, however, is size. Ruby-throated Hummingbirds exhibit reversed
(females larger than males) sexual size dimorphism, a topic thoroughly
discussed in Mulvihill, R. S., et. al. 1992. A possible relationship
between reversed sexual size dimorphism and reduced male survivorship in
the Ruby-throated Hummingbird. Condor 94: 480-489. The average
wing length of an HY male is 3.5 mm shorter than that of the female, and
males weigh 0.2 grams less on average. For banders, the appearance
of certain feathers on the wing also differentiates between sexes.
For example, the tertials on a male have a glossy, green iridescent sheen
while female tertials are usually a duller green with thin buff edges.
In addition, the leading edge (i.e., outer web) of primary number six in
males is much more narrow than that in females (see Leberman, R. C.
1972. Key to age and sex determination of Ruby-throated Hummingbirds
in autumn. Inland Bird Banding News 44:197-202).
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As mentioned above, female warblers, in general, sport
a more subdued plumage than males, with immature (HY) females being the
dullest across the entire age/sex plumage gradient. Below, we compare
an AHY female (left) and HY female (right) Mourning Warbler banded this
month. Note--the conspicuous whitish eye arcs in both age classes
of female MOWAs in fall are not as thick and prominent as they would be
in the corresponding age/sex classes of the similar MacGillivray's Warbler,
a western counterpart of the MOWA.
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Finally, while there was no mistaking this adult male
Blue-winged Warbler banded on August 25 for anything else,
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the bird in the photo below, banded on August 24, has
a couple more weeks and a little more molting to do to before it loses
its fuzzy juvenal feathers and begins to really look like a.....
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Black-throated Green Warbler!
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Last Updated on 09/07/06
By Adrienne J. Leppold