“Winter” specimens of Swainson’s Thrush in eastern North America


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ANSP193361 — C. u. swainsoni, collected by J.H. Weber on 3/3/1904. Photo: Matthew R. Halley

I came across a specimen of Swainson’s Thrush (Catharus ustulatus swainsoni) in the ornithology collection of the Academy of Natural Sciences (ANSP193361) that was collected by J. H. Weber in Fort Lee, New Jersey on March 3, 1904! This sparked my curiosity because this taxon, considered a subspecies by most authorities but a species by the Brazilians (Piacentini et al. 2015), spends the winter in South America and does not typically arrive in North America until April. This set me on a brief search for other North American “winter” specimens of C. u. swainsoni. For the purpose of my investigation, I was hesitant to rely on sight records, because it is easy to misidentify these thrushes in the field (trust me), and eastern Hermit Thrushes C. guttatus faxoni are normal winter residents in these regions. For these reasons, I wanted hard evidence, and a search of major specimen collections did the trick.

I looked for specimens collected during November through March. A query on VertNet.org returned 800 specimen records from that time period, of which 24 were collected in North America east of the Mississippi River. Of those, 9 were erroneous or dubious (e.g., several records had the date 1/1, the default of some databases when a date field is left blank). After separating the wheat from the chaff, I managed to uncover 15 (apparently) legitimate “winter” records, which I plotted alongside an eBird chart compiled from three “Bird Conservation Regions” that cover a large portion of the ‘transient zone’, i.e., in between the breeding and wintering grounds of C. u. swainsoni. Interestingly, the specimen that started my search (ANSP193361) was actually one of three taken by J. H. Weber in Fort Lee, New Jersey—20% of the “winter” C. u. swainsoni specimens taken in eastern North America were found at the same place by the same guy!

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The orange curves in the figure show the annual cycle (in total # of birds detected) from the perspective of birders in the ‘transient zone’. There is a sharp decline in autumn (far right of the graph), and detections typically cease by the end of October. The birds are then absent for the next five months (they are “on holiday” in South America). Then, the spring migration brings a rapid increase in detections in early- and mid-April, as Swainson’s Thrushes migrate northward to their breeding grounds.

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C. u. swainsoni in the ‘transient zone’. Photo: Matthew R. Halley.

Three of the “winter” specimens were collected in early November, and can easily be attributed to late departures, and not overwintering birds (MCZ338962, UMMZ234592, UWBM86964). To my knowledge, there have been no specimens collected during December, and only one legitimate record from January, a specimen collected in Brown Co., Wisconsin (FMNH351309); and that specimen was collected in late January (the 21st). Two specimens from mid- and late-February were collected in Frederick Co., Maryland (UWBM 38236), and New Haven Co., Connecticut (YPM ORN 002630), respectively. The remaining 9 specimens were taken in March, one month before the expected arrival of the species in North America, and so can be explained as early arrivals. Indeed, given the distribution of these “winter” specimens, clustered near the tails of the eBird distributions, there is very little to suggest that any of them were actually over-wintering individuals. Rather, they seem to be aberrant individuals with early or late departure or arrival dates, outliers way out on the tail ends of the normal distribution, but still a part of that distribution. However much I might like to think one of these Swainson’s Thrush specimens was an over-wintering bird, the null hypothesis is that they are just super late/early migrants (i.e., a migrant with wacky timing seems more plausible than a bird that ceased to migrate altogether). At present, the weight of the evidence is not enough to reject the null.

So no—sorry to get you excited, but at the present moment there is no evidence of a Swainson’s Thrush wintering in North America. Many thanks to the folks at ebird.org and VertNet.org, and to the many collectors, preparators, and curators of the collections.

[UPDATE, 12/21/2016: I recently came across an article in Cassinia (62:65) in which Keith Russell reported a Swainson’s Thrush at Cape May Courthouse, NJ, on December 24, 1985! Identification was corroborated by Bob Ridgley. This indeed is a good candidate for an individual *actually* overwintering in this region, but I do not know if it was seen in subsequent weeks.]

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Categories: Climate Change, Natural History, OrnithologyTags: , , ,

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