PREDATORY ATTACK BY A WESTERN TERRESTRIAL GARTER SNAKE ON A NESTLING DARK-EYED JUNCO.
At some localities, birds appear relatively frequently in the diet of Western Terrestrial Garter Snakes. On Mitlenatch Island, British Columbia, Campbell (1969) reported that this garter snake species readily preyed on nestling Glaucous-winged Gulls (Lams glaucescens) and the eggs and nestlings of Northwestern Crows (Corvus caurinus). James and others (1983) reported recovering 15 nestling White-crowned Sparrows (Zonothrichia leucophrys) from 9 of 39 Western Terrestrial Garter Snakes they examined near San Francisco, California. In general, however, birds and their eggs appear to be a minor dietary component for this garter snake species (Fitch 1941; Cunningham 1955; Anderson 1977).
On 7 July 2018 at 11:48 MDT, while walking in the Rattlesnake Creek drainage near the north end of Missoula, Missoula County, Montana, my attention was drawn to a pair of Dark-eyed Juncos (Junco hyemalis) about 20 m distant emitting a series of "tik" calls (Hostetter 1961; Nolan and others 2002) as they fluttered and perched low to the ground; they were accompanied by a pair of Chipping Sparrows (Spizella passerina), which also were vocalizing and acting agitated, but the sparrows did not fly to the ground with the juncos. The location (46.92226[degrees]N, 113.96020[degrees]W; 1090 m elevation) was under an open canopy of Douglas-fir (Pseudotsuga menziesii) and Ponderosa Pine (Pinus ponderosa) with very little shrub understory, about 40 m from a water-filled irrigation ditch and <100 m from Rattlesnake Creek and associated riparian habitat. When I approached the site where the juncos were directing their attention I found a nest on the ground, containing a junco nestling grasped around the head by a Western Terrestrial Garter Snake approximately 45 cm SVL in size (Fig. 1A). The nestling was still alive, and struggled weakly, but never escaped the snake. The snake switched positions in the nest once during the next 3 min as it continued to grasp the nestling in its mouth. I never saw the snake attempt to coil around the nestling, a behavior sometimes used by this garter snake species to hold prey (Gregory and others 1980). The snake released the now-dead nestling only when I reached toward the nest to move some grass that partly obstructed my view. The snake then slowly crawled away, leaving the dead nestling on the rim of the nest.
The dead nestling was mostly feathered and about 9-d-old, based on the stubby tail feathers and the flight feathers still mostly sheathed (wing chord = 42 mm); the only visible wound on the dead nestling was a patch of torn skin on the neck near the base of the mandible (Fig. 1B). No other junco nestlings were present in or near the nest, and I did not notice any irregularities along the snake's body that would suggest it had already consumed other nestlings. Other nestlings that may have been present when the snake appeared probably abandoned the nest, something juncos of this age may do when disturbed by a potential predator (Hostetter 1961; Nolan and others 2002). The dead nestling was still present in the nest when I checked 2.5 h later.
Dark-eyed Juncos are rarely noted as prey for garter snakes. Hostetter (1961) reported 3 cases of predation by the Common Garter Snake (T. sirtalis) for 72 junco nests in Virginia; additional instances for Virginia and Ontario are noted by Nolan and others (2002), but without including the number of cases or additional details. For juncos nesting within the range of the Western Terrestrial Garter Snake, the incidence of nest predation by this snake species also appears to be low (Phelps 1968; Smith and Andersen 1982; Nolan and others 2002; Robertson 2009). I found only 2 prior accounts of predation by Western Terrestrial Garter Snakes on Dark-eyed Juncos. Fitch (1941) listed Junco among 4 birds found in 70 stomachs of garter snake specimens collected across the range of this species outside of California, but provided no additional details regarding age of the bird or collection location of the snake in which it was found. Thatcher (1968) found a garter snake with a live fledgling junco a few days out of the nest at 2896 m elevation in Colorado; the elevation of the incident eliminates all but Western Terrestrial Garter Snake as the species observed.
Although predation of Dark-eyed Juncos by garter snakes is rarely reported, juncos are exposed throughout much of their breeding range to 1 or more garter snake species (Nolan and others 2002; Stebbins 2003). In western North America, juncos nest on or near the ground in montane regions from valley bottoms to at least upper tree line (>3000 m elevation), often near water and in more mesic habitats, such as Aspen (Populus tremuloides) stands and spruce (Picea spp.) woodlands (Phelps 1968; Thatcher 1968; Nolan and others 2002). Similar habitats are also favored by Western Terrestrial Garter Snakes, which range into mountains near streams and wetlands over a similar elevation gradient (Hendricks 1996; Hammerson 1999; Werner and others 2004). With more intensive study of both species it may be shown that predation by Western Terrestrial Garter Snakes on Dark-eyed Juncos occurs with greater frequency than currently appears to be the case.
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Montana Bird Advocacy, 909 Locust Street, Missoula, MT 59802, USA (PH); paul.hendricks@mso. umt.edu. Submitted 12 July 2018, accepted 15 October 2018. Corresponding Editor: Robert Hoffman.
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|Title Annotation:||GENERAL NOTES|
|Publication:||Northwestern Naturalist: A Journal of Vertebrate Biology|
|Date:||Mar 22, 2019|
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