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On 2013-01-04 3:31 PM, Alan Kaplan wrote:
A wonderful mob (many observers) appeared at Tilden Nature Area/Jewel
Lake for our first of the year First Friday Birdwalk sponsored by
Golden Gate Audubon Society: 37 people signed in, another 4 or so came
later. Thank you to all for getting us off on the right foot. Our
theme today was (Dr.) Terry Root's Atlas of Wintering North American
Birds: an analysis of Christmas Bird Count data.
Dr. Root's conclusion was that many birds are limited in their
northern overwintering distribution by the average minimum January
temperature, and the limit of their ability to generate more energy
than 2.45 times their Basal Metabolism Rate. After some back-and-forth
in the literature in the late 1980s and early 1990s, her conclusion is
accepted widely today.
Dr. Root's data were the 1962/63 to 1971/72 CBC counts. A 2007 paper
reviewed the 1975-2004 CBC data for 254 species and found northward
("poleward") shifts of 1.5 km/year (see La Sorte and Thompson, 2007,
Poleward shifts in winter ranges of North American birds. Ecology 88:
Christmas Bird Counts are fairly robust (accurate) over a wide range
of efforts and abilities of "citizen-scientists" like us, with
exceptions for irruptive species (like Red Crossbills), nomadic ones
(Bohemian Waxwings), aggregating species (Red-winged Blackbirds) and
rare ones (Peregrine Falcon). As the skill and knowledge base of
observers improved, corrections have been made (Semipalmated
Sandpipers don't overwinter in North America--a very few may be in
Florida-- but were once widely reported on CBCs). Nocturnal counts of
owls probably reflect the zeal of counters and not the true
distribution of birds.
More about birds in winter on our next First Friday walk (February 1,
2013, 8:30 am, Tilden Nature Area parking lot).
Meanwhile, here is today's wonderful list of 34 species:
Bufflehead (amazing pre-nuptial displays by males: head-bobbing,
nod-swimming, bridling; and females: sidling up to males and inciting.)
Virginia Rail (it ran through the group at the edge of Jewel Lake!
Bird O' the Day, for sure !)
Band-tailed Pigeon (120 plus of them)
Belted Kingfisher (she flew around a lot but didn't vocalize much)
Varied Thrush (for a Thrush Hat Trick !)
Best of Boids!