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Note on the Woodcock Rocking Display
Bernd Heinrich

Northeastern Naturalist, Volume 23, Issue 1 (2016): N4–N7

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2016 Northeastern Naturalist Notes Vol. 23, No. 1 N4 B. Heinrich Note on the Woodcock Rocking Display Bernd Heinrich* Abstract - Scolopax minor (American Woodcock) is well-known for its crepuscular foraging, nuptial sky dance, and concealment by both cryptic coloration and immobility. It occasionally engages in a highly conspicuous rocking motion that has been interpreted by some as a strategy for worm-hunting. However, the literature on this behavior, combined with current observations and theory, can instead more plausibly be interpreted as an advertisement of unprofitabi lity. Scolopax minor Gmelin (American Woodcock, hereafter Woodcock) is a woodland bird of eastern North America (Bent 1927, McAuley et al. 2013, Mendal and Aldous 1948, Sheldon 1967) that commonly feeds on earthworms. It is best known for the male's aerial display (Longcore et al. 1996). While walking, Woodcocks sometimes shift their body up and down and forward and back with each step, while maintaining their head steady. This rocking behavior, which is also reported for Scolopax rusticola L. (Eurasian Woodcock; De Forges 1975, Severinghaus 1978), has been described as a means of foraging for earthworms, although various alternative hypotheses have been proposed for it as well. The worm-hunting hypothesis for the rocking walk apparently originated from Bent (1927), who mentioned that “it [the Woodcock] is said to beat the soft ground with its feet or wings, which is supposed to suggest the effect of pattering rain and draw the worms to the surface.” His only support for this idea is reference to C.J. Maynard, who had in 1896 observed a captive Woodcock which “stamped with one foot, giving several sharp, quick blows, after which he would bend its head near the ground and then listen” before catching a worm. Subsequently Christy (1931:14) observed Woodcocks in Pittsburgh, PA, in August on lawns, gardens, and parks and concluded that “most of the bird’s movements were very deliberate” involving “peculiar pressing movements of the feet—and I gained the distinct impression that the angleworms were located by the sense of feeling through the feet.” But Pettingill (1936:269) conjectured instead that the behavior is a “nervous action resulting from fear and suspicion”, and Worth (1976:374) proposed that it may mimic either moving leaves or act to reduce detection by “minimizing shadows”. Marshall (1982) observed these rocking movements of 2 Woodcocks in early spring in suburban St. Paul, MN. Because his observations were, like Christy’s (1931), on lawns under overcast sky on apparently wind-still days, he excluded Worth’s (1976) idea of the birds’ mimicry of either moving leaves or shadows. Marshall’s (1982) observations were made in early spring when “the frost line varied from 5 to 20 cm in depth.” Furthermore, on his second observation after 15–20 cm of fresh snow had fallen, the Woodcock were “very conspicuous as they rocked, whether on snow, lawn, or leaf litter.” It was therefore unlikely that the Woodcock were either stamping their feet to cause worms to come up out of the ground, or to hide. Unable to find a plausible explanation, and despite his strong evidence against it, Marshall (1982) nevertheless concluded, “I would suggest that a minute movement of earthworms close to the surface in response to the slight changes in pressure from the rocking bird’s foot, allows the Woodcock to detect them by sight or perhaps by an infinitesimal soun d.” Bent (1927) included observations of Dr. Robert Cushman, who described a Woodcock leaving her nest and spreading her tail “to show full advantage [of] the double row of *PO Box 153, Weld, ME 04285; bheinrich153@gmail.com. Manuscript Editor: Glen Mittelhauser Notes of the Northeastern Naturalist, Issue 23/1, 2016 N5 2016 Northeastern Naturalist Notes Vol. 23, No. 1 B. Heinrich glistening white spots at the ends of the double row of retrices and under coverts. Next, flashing this striking banner slowly, she would move off among the trees in the attitude of a strutting cock, stopping when we refused to follow, and then tripping ahead a few steps, all the while bleating softly. The effect was astounding: the ordinary low visibility of the Woodcock against the forest floor no longer held, for the spotted fan became a most conspicuous and arresting mark.” In certain situations, some animals make themselves highly conspicuous to predators, as a defense. Alcock (2013) summarizes these deliberately conspicuous behaviors, such as the stotting of antelope (Pitcher 1979), the waving of the tail by an Anolis lizard when it had been discovered by a potential predator (Leal 1999), and the white tail flashing of fleeing Odocoileus virginianus (Zimmermann) (White-tailed deer), as “advertisements of unprofitability” that act to reduce the likelihood of attack by a potential predator. This hypothesis for the function of a potentially conspicuous behavior gives relevance to the visually conspicuous “rocking” behavior of Woodcock, given the circumstances of when it occurs and an anecdote of my own. In the spring of 2011, I observed the rocking display of Woodcock near Weld, ME, at the edge of a hill-side clearing by a stand of Acer rubrum L. (Red Maple) and A. sacharum Marshall (Sugar Maple). The clearing had been used as a Woodcock spring display area for at least the last 30 years. The Woodcock were at the lower end of a slope in a level area between the maple trees and several Viburnum dentatum L. (Arrowwood Viburnum) and Ilex verticillata (L.) A. Gray (Winterberry) bushes. At the time of the observations, the leaves and grass in that area were conspicuously marked by dozens of white blotches of bird scat, hence indicating previous presence of the birds. On 25 April 2011, I flushed 1 Woodcock from the area and a second started to walk away from me, taking slow small steps, and, after each step, rocking its body back and forth in the typical and often-described rocking walk display. I remained to watch this bird for about a half hour. Whenever I followed it in a slow walk, it continued its rocking motions as it walked keeping about 6–10 m ahead of me. It stopped when I stopped, and resumed its rocking walk display when I again drew closer to it. I returned to the same water-soaked, unfrozen, fecal-marked area in both the early and late afternoon of 9 May 2011, and a Woodcock was there each time. It again did not flush, and I again followed the bird. As before, it rocked up and down in place, or backward and forward during its walking in the typical rocking behavior. It occasionally accentuated this display by also flaring its tail, prominently flashing white feathers up and to the sides (described as the “tail flare” alarm by Morgenweck and Marshall [1984]) and as previously described by Bent (1927). After 10 minutes, I withdrew ~100 m to climb 10 m into a Red Maple tree to hide behind its trunk and watch the bird with binoculars. During my 40 minutes of watching from a distance, the Woodcock ceased its display, and did not vocalize. Since the fecal markings indicated that Woodcock had spent considerable time feeding at this location, potentially on worms in or on the open, wet ground, I searched and discovered several earthworms under the wet leaves on top of the water-saturated soil. The worms were not moving and did not respond to my handling them. The hypothesis (Christy 1931, Marshall 1982) that the bird(s) I was watching were, by their rocking display, attempting to activate worms to come to the surface at near 0 °C seemed suspect. The Woodcock’s rocking-walk behavior was noteworthy because it seems to violate the usual cryptic behavior of individuals of this species when approached by humans or dogs, when the birds usually flatten themselves to the ground which they often resemble and where they often remain until flushed into flight after being approached closely. Throughout October 2015 (when Woodcock were on migration), I attempted, with the help of 2 2016 Northeastern Naturalist Notes Vol. 23, No. 1 N6 B. Heinrich assistants, to find Woodcock and again observe this rocking display by searching forests within 300 m of my 25 April observations. We flushed Woodcock on 15 occasions, but were never able to detect one before flushing it. Even when we were about 2 m from birds, they always flushed before we saw them, flying about 50–80 m before either settling back onto the ground or departing out of sight. However, we flushed 2 of the birds 3 times in succession and the openness of terrain allowed us to see where they landed. When we approached them the second time they flew up when we came to within 4–5 m from them, and the third time we approached the same individuals they flushed at a distance of 8–10 m. Although knowing where these transients had landed, we were still unable to see them before they flushed. Apparently these birds’ lack of the rocking walk display was associated with their being in their normal habitat where they could escape detection by hiding, but they were willing to leave any area within it. The only 2 fecal spots found indicate that the site was only a temporary one for these birds. The Woodcock rocking-walk observations by Christy (1931) and Marshall (1982), and those by Cushman quoted by Bent (1927) had been made under conditions that contradict the worm-hunting hypothesis. Furthermore, the rocking up and down and back-and-forth movements could arguably be judged as either gentle or forceful, but not the kind designed to cause earth vibrations. Furthermore, the above-quoted rocking-walk display from Bent (1927) closely approximates the “tail flare” alarm signal (Morgenweck and Marshall 1984), which the rocking walk of the bird I was following at times exhibited. It was likely an accentuation of the rocking display, just as the flushing of Woodcock from increasingly greater distance after greater pursuit was likely a sign of greater alarm. It is especially noteworthy that the Woodcock exhibited the rocking display precisely when the potential threat came near, and stopped abruptly on my withdrawal to watch from a distance through binoculars. Given those behaviors and sequences of events, the worm-hunting hypothesis is contradicted; no animal proceeds to forage the moment it senses it is being stalked. I suggest that the Woodcock rocking-walk display is a response to what it perceives as a mild potential threat situation that is not severe enough to initiate predator-avoidance tactics to disrupt it into flight or cryptic hiding. The Woodcock’s rocking-walk display may act as a signal in a situation of a perceived potential audience or a predator, indicating that it is aware and can explode off the ground and escape if the predator seems likely to attack. The display saves the bird the energy and bother of flying off and possibly being chased. The rocking walk display is likely to occur during foraging, because foraging is centered at a food-rich place that the bird may be reluctant to leave. In most instances, available video clips of the behavior (displayed on the Internet by interested citizens) were made of Woodcock in potentially threatening ecological settings or during a discrete threat. The Woodcock were either out of their usually preferred woodland habitat (Bent 1927, McAuley et al. 2013) or near humans and were thus potentially “nervous from fear and suspicion” as Pettingill (1936) had originally supposed. That the bird’s behavior while walking is variable, including a teetering motion, is undisputed, but the reason(s) for this behavior is still uncertain. To test the display hypothesis that I propose would require observing the bird unseen (perhaps by remote video camera), both undisturbed in its preferred habitat and then under the influence of a mild (but not strong) potential threat in the presence of a resource of significant va lue to the bird. Conversely, to secure the alternative worm-hunting hypothesis and distinguish the incidental from the causal would require proving all of the presumptions that would make the behavior work: proof of buried worms, confirmation of their moving in response to a Woodcock’s rocking walk, proof of a Woodcock’s ability to sense these movements when N7 2016 Northeastern Naturalist Notes Vol. 23, No. 1 B. Heinrich they occur, the Woodcock’s responding to these movements by probing there into the ground, and lastly then also locating the worms. Earthworms can indeed be caused to come up out of the earth by vibrations. In Florida's Apalachia National Forest, people have for generations harvested a large endemic earthworm (Diplocardia mississippiensis Smith) by driving a stick into the ground and vibrating it and the soil mechanically by power tools, historically using a model T Ford, and now chainsaws (Catania 2008). These vibrations may mimic falling rain or digging by moles and elicit the worms’ escape response (Catania 2008). However, the walking by Woodcocks are unlikely to cause such vibrations. Since no strong evidence for the rocking walk as a worm-hunting strategy exists, it seems premature to call the worm-hunting hypothesis an established “fact”. Another hypothesis which may illuminate the function of the Woodcock’s fascinating behavior is at least consistent with available facts. As per the discussion by Alcock (2013) of the conspicuous signals given by prey to potential predators, this behavior is in accord with the theory of advertisement of unprofitability, to prevent said predators from pursuing the alerted prey. Acknowledgments. I thank Craig Neff and Pamelia Markwood for showing me a video clip of a Woodcock; John Alcock, Glen Mittelhauser, and an anonymous reviewer for helpful comments and suggestions on a draft of the manuscript; and Lynn Jennings and Joshua Brown for Woodcock flushing. Literature Cited Alcock, J. 2013. Animal Behavior: An Evolutionary Approach, Tenth Edition. Sinauer Associates, Inc. Sunderland, MA. 522 pp. Bent, A.C. 1927. Life histories of familiar North American birds: American Woodcock, Scalopax minor. Smithsonian Institution. United States National Museum Bulletin 142(Part1):61–78. Available online at http://birdsbybent.netfirms.com/ch61-70/woodcock.html. Catania, K.C. 2008. Worm grunting, fiddling, and charming: Humans unknowingly mimic a predator to harvest bait. PloS ONE 3(10):e3472. doi:10.r371/journal.pone.0003472. Christy, B.H. 1931.Woodcocks in a dry season. Cardinal 111:13–14. Des Forges, G. 1975. Behavior of an incubating woodcock. British Birds 68:421–428. Leal, M. 1999. Honest signaling during prey–predator interactions in the lizard Anolis cristatellus. Animal Behaviour 58:521–526. Longcore, J.R., D.G. McAuley, G.F. Sepik, and G.W. Pendleton, 1996. Survival of breeding male American Woodcock. Canadian Journal of Zoology 74:2046–2054. Marshall, W.M. 1982. Does the Woodcock bob or rock—and why? Auk 99:791–792. McAuley, D., D.M. Keppie, and R.M. Whiting Jr. 2013. American Woodcock (Scolopax minor). No. 100, In A. Poole (Ed.). The Birds of North America. Cornell Lab of Ornithology, Ithaca, NY. Available online at http://bna.birds.cornell.edu/bna/species/100. Mendall, H.I., and C.M. Aldous. 1943. The ecology and management of the American Woodcock. Maine Cooperative Wildlife Research Unit. University of Maine, Orono, ME. 201 pp. Morgenweck, R.O., and W.H. Marshall. 1984. Observations on postures and movements of nonbreeding American Woodcock. Wilson Bulletin 96:720–723. Pettingill, O.S., Jr. 1936. The American Woodcock (Philohela minor (Gmelin). Memoirs of the .Boston Society of Natural History 9:169–391. Pitcher, T. 1979. He who hesitates lives: Is stotting anti-ambush behavior? American Naturalist 113:453–456. Severinghaus, S.R. 1978. Diurnal behavior of an Eurasian Woodcock (Scolopax rusticola). Auk 95:748. Sheldon, W.G. 1967. The Book of the American Woodcock. University of Massachusetts Press, Amherst, MA. Worth, C.B. 1976. Body-bobbing woodcocks. Auk 93:374–375.