Foraging strategy of a California kingsnake in searching for fledglings of the least bell's vireo.
While checking marked nests of vireos at ca. 1000 h on 4 June 2007, I observed a nest ca. 1.5 m above the ground that had been partially torn, but two fledglings were nearby, perched in a shrub ca. 1 m away. The nest previously had contained three banded nestlings, and these two fledglings were now 11 days old, a little young to be out of the nest. These fledglings were being actively fed by the adults. One fledgling was standing on a low branch and appeared more developed, while the younger one was sitting on a low branch, flapping its wings and calling. As the parents were actively feeding both, I left the area with the intent to return later. About 1 h later, I returned and heard the adults loudly mobbing in the vicinity of the nest. As I approached, I saw the younger fledgling frantically flapping and crawling away in response to the alarm calls of the parents. I then saw a 1-m California kingsnake swallowing the more-advanced fledgling feet first. After ca. 2 min of actively mobbing the snake, one adult landed in front of the fleeing fledgling, and used food to lure it farther away. After the fledgling was ca. 15 m away, the mobbing ceased and the two adults left. The snake finished consuming the first fledgling and began moving away from the nest. After traveling ca. 6 m, it turned around and returned to the nest. It then proceeded to move out and back from the nest site over the next 15 min, each time on a new compass direction, traveling ca. [less than or equal to]6-10 m before returning. Once, it briefly climbed into low branches of the nest tree. The snake rapidly tongue-flicked the substrate as it traveled. About 15 min after consuming the first fledgling, the kingsnake found the original roosting branch of the surviving fledgling. It paused there for ca. 10 s, rapidly tongue flicking along the branch. It then proceeded quickly toward the fledgling that had moved ca. 15 m away and was now being fed by its parents. Beause the fledgling could not fly, it had left a long scent trail as it flapped and crawled to its new perch in a low shrub. The kingsnake traveled this distance in ca. 2 min, and killed and consumed the last fledgling. A few weeks later, the pair of vireos was observed feeding the third fledgling, which apparently had survived the initial attack and moved out of the area.
Bell's vireos characteristically make an open cup nest suspended from a forked branch within 1-2 m of the ground, making their nests potentially vulnerable to a variety of snakes. Other nest-raiding snakes observed in the area included coachwhip (Masticophis flagellum), red-sided garter snake (Thamnophis sirtalis infernalis), and gopher snake (Pituorihis catenifer). Peterson et al. (2004) documented a gopher snake consuming four eggs in a nest of least Bell's vireo within the same river system as the observation described here. Pemberton and Carriger (1916) reported predation by a California kingsnake of the nest of a least Bell's vireo in Colton, San Bernardino Co., California. California kingsnakes were also the most frequently documented predator of nests of rufous-crowned sparrows (Aimophila ruficeps) at upland sites in central San Diego County (Morrison and Bolger, 2002).
While California kingsnakes have been reported to feed on birds and bird eggs, this systematic searching behavior has not been described previously. It is unclear how the kingsnake found the nest originally, or whether visual or chemical cues encouraged the continued searching after the initial fledgling was consumed. Likely both visual and chemical cues were used. Mullin and Cooper (1998) described how the visual stimuli of a bird passing overhead improved the hunting efficiency of gray rat snakes (Elaphe obsoleta spiloides) in locating bird nests. Eichholz and Koenig (1992) showed that both visual and chemical cues were used by gopher snakes in finding nests of western bluebirds (Sialia mexicana) in artificial nestboxes. Goodman and Goodman (1976) postulated that California mountain kingsnakes (L. zonata) are aided in finding nests by intensive mobbing by adult birds. The initial proximity of the fledglings to each other (within 1 m) may have allowed the kingsnake to visually observe the second fledgling flee as it consumed the first.
Submitted 28 August 2007. Accepted 29 November 2008.
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PETERSON, B. L., B. E. KUS, AND D. H. DEUTSCHMAN. 2004. Determining nest predators of the least Bell's vireo through point counts, tracking stations, and video photography. Journal of Field Ornithology 75: 89-95.
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KEVIN B. CLARK
Clark Biological Services, 7558 Northrup Drive, San Diego, CA 92126x5115, email: e-mail:
Associate Editor was Michael S. Husah.
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|Author:||Clark, Kevin B.|
|Date:||Sep 1, 2009|
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