North American Birds

VOLUME 69 NO2 2016

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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Page 27 of 139

N O R T H A M E R I C A N B I R D S 202 T H E C H A N G I N G S E A S O N S : C A R B O N C O P Y records of Cassin's Finches (none elsewhere in the Midwest or East) and zero records of Western Purples Finches out of range. Unlike, for the finches, migration of Var- ied Thrush is fairly well described in the lit- erature (see George 2000). Twenty years ago, Jeff Wells and colleagues (1996) described population cycles of the species, showing peaks every few (2-5) years in survey data, with abundance in a given winter correlated to abundance in the breeding season before it. They suggested that local abundance of acorns, on which this species is known to feed in winter, could strongly influence local movements and abundance. Like the finch- es, Varied Thrushes sometimes stray far out of range, most frequently to the Midwest (es- pecially western Great Lakes) and Northeast but sparingly farther south as well. These irruptions have been discussed often in this column in the past (Brinkley 2009, 2013), with tentative suggestions that the northerly/interior subspecies meruloides, in which females are paler gray above, are involved in many or most records, though distinguishing among the subspecies is a dodgy affair, and some authors suggest that the 2-4 named subspecies are not valid or at best weakly differentiated. As Peder Svingen (1995) noted, the trajectory of the move- ment into and across Minnesota and Wis- consin appears to be from the northwest to- ward the southeast, supporting the idea that meruloides rather than nominate naevia is involved. Moreover, meruloides appears to be and also numerous records across central and southern California, including in many areas where rarely recorded in such num- bers. Indeed, a quick glance at eBird maps for the winter (December through February) period for each winter shows a multiple- orders-of-magnitude increase from 2013- 2014 to 2014-2015, even when one makes adjustments for birds seen by multiple ob- servers. To see these observations displayed in the eBird maps is just stunning. Farther north, in central California, Brian Sullivan birded Point Pinos in Monterey County for 90 minutes on 17 November 2014, logging a conservative 750 Varied Thrushes over- head among thousands of American Rob- a longer-distance migrant than naevia, and passerine vagrants far out of range do tend to be long-distance migrants; the species has reached Bermuda, every Canadian province, and all U.S. states except Mississippi and Ha- waii. Importantly, George (2000) notes that the irruptive movements as far as the Mid- west and East are positively correlated with flights of the species into southern Califor- nia but not with general population trends, "so it is likely that factor(s) responsible for the cyclic changes in abundance within nor- mal range differ(s) from those that cause irruptions." A California correspondent of Arthur Cleveland Bent (1949), W. A. Kent, speculated about the cause of these faculta- tive flights: "The winter migration of the Var- ied Thrush south into Los Angeles County is a food problem rather than [one] of climate. Should there be an acorn shortage in Ven- tura County, north of us, varied thrushes will be here in numbers [...] in the oak groves." And that correlation appears to apply to fall/winter 2014-2015, which saw more than three times the va- grants in the East than the previous winter— Figure 6. These Band-tailed Pigeons were among at least 1250 birds seen moving northward high over Monterey Bay from Point Pinos, Monterey County, California 17 November 2014. Photograph by Brian L. Sullivan. Figure 7. Among the first ever recorded on the floor of the Central Valley in Fresno County, these Band-tailed Pigeons at Kerman 7 January 2015 were part of a major fall/ winter invasion into portions of northern California. Photograph by Gary Woods.

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