The VCE crew made its final 2015 foray to Mansfield's ridgeline on
Tuesday evening, hoping to intercept some passage migrants and hear the
Bicknell's Thrush (BITH) parting chorus. The trip was a success on both
counts. We left the valley's stifling heat and arrived at our study site
- the toll road uppermost parking lot - at ~5 pm, with cooler
temperatures and a surprisingly brisk west wind. We scurried about
setting mist nets and had 23 open by 6:30 pm. In typical mid-September
fashion, BITH put on a solid show at dusk, as 18-20 birds vocalized with
vigor over ~15-20 minutes. We even heard a brief snatch of song from one
individual. Among the 10 birds we captured before closing nets as
darkness fell, the most surprising by far was a male Wilson's Warbler,
the first we have ever encountered on Mansfield, even during the three
autumns we banded on the ridgeline from 1995-97. We also captured 3
BITH, one an adult male we had banded in mid-July, and two
young-of-the-year.
The season's reduced daylength afforded us an opportunity to sleep in,
and we reopened our nets at the leisurely hour of 5:30 am. Again, BITH
vocalized as they did back in July, though only calls were heard, from
the same (presumably) 18-20 birds. Why BITH undergo this annual
resurgence of calling just prior to their southward departure (most will
be gone by October 1) is anyone's guess. For a species that doesn't hold
true breeding territories, there would seem to be little imperative to
restake claim to any turf, in hopes of reoccupying it next spring. Both
male and female adult BITH return to Mansfield at relatively high rates,
so perhaps there is some social context/message we humans can't
decipher. Other songbirds are known to resume limited vocalizing
(usually song) in fall, but I'm not aware of any that do so to the same
extent as BITH. Hormones are likely involved, perhaps triggered by
daylength(?). Whatever the cause/reason, it's a delight to hear, when
all other species are virtually silent! Birds continued to call
sporadically throughout the morning, some of them quite insistently.
Overall, we captured 82 birds in our nets. In numerical abundance, they
included:
Yellow-rumped Warbler - 29 (all new)
Bicknell's Thrush - 15 (10 new birds [7 young, 3 adults] and 5
recaptures [4 from earlier in the summer, 1 banded as an adult on 12
Sept 2011 and not encountered since!])
Black-throated Blue Warbler - 13 (all immatures. This species was by far
the most abundant non-locally breeding species during our mid-1990s
autumn banding study. Of the 338 individuals we banded i those 3 years,
328 [97%] were immatures, and not a single one was ever recaptured on
site. We believe montane forests are an important habitat for
post-fledging dispersers, but the fact that birds appear to rapidly
depart is puzzling. If you can't bear not to learn more, check out our
2000 paper
<http://www.bioone.org/doi/abs/10.1676/0043-5643%282000%29112%5B0124:MSAPDA%5D2.0.CO%3B2>
in the Wilson Bulletin.)
Golden-crowned Kinglet - 5
White-throated Sparrow - 4 (including 3 recaptures of adults from
June-July)
Dark-eyed Junco - 4
Ruby-crowned Kinglet - 2
Blackpoll Warbler - 2
Red-breasted Nuthatch - 1
Brown Creeper - 1
Swainson's Thrush - 1
Ovenbird - 1
Nashville Warbler - 1
Magnolia Warbler - 1
Black-throated Green Warbler - 1
Wilson's Warbler - 1
We'll be back up there in early June of 2016 to launch our 25th
consecutive year of field work. Until then, safe journeys to all the
migrants!
Chris
--
Chris Rimmer
Vermont Center for Ecostudies
P.O. Box 420
Norwich, VT 05055
802-649-1431 ext. 1
www.vtecostudies.org
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