Short Communications The Wilson Journal of Ornithology 119(2):259–262, 2007
Similarities Between Campephilus Woodpecker Double Raps and Mechanical Sounds Produced by Duck Flocks Clark D. Jones,1,4 Jeff R. Troy,2 and Lars Y. Pomara3 ABSTRACT.—The search for the Ivory-billed Woodpecker (Campephilus principalis) in eastern Arkansas and northwestern Florida has yielded double raps recorded by autonomous recording units (ARUs) in White River National Wildlife Refuge and along the Choctawhatchee River, respectively. These double raps have been presented as suggestive evidence for the presence of the species in those regions. We present data comparing double raps produced by wing collisions from an aerial Gadwall (Anas strepera) flock to double raps documented by ARUs. Close similarities in amplitude ratios, peak-to-peak times between raps, and auditory quality between ARU recordings and wing collisions from a Gadwall flock illustrate the ability of flying ducks to produce sounds easily mistaken for the double raps of Campephilus woodpeckers. All ARU double raps suggesting the presence of an Ivorybilled Woodpecker should be reconsidered in light of the phenomenon of duck wingtip collisions, especially those recorded during winter months when duck flocks are common across flooded bottomlands of the southeastern United States. Received 25 January 2007. Accepted 28 February 2007.
Evidence for survival of Ivory-billed Woodpeckers (Campephilus principalis) in Arkansas and Florida includes recordings of ‘‘double raps’’ (DRs) from autonomous recording units (ARUs) in White River National Wildlife Refuge (WRNWR) and along the Choctawhatchee River, which sound much like the double raps of some Campephilus woodpeckers (Charif et al. 2005, Hill et al. 2006). Although no unequivocal recording of the double rap of an Ivory-billed Woodpecker (IBWO) exists, written descriptions indicate it was similar to those of other members of the same genus (Tanner 1942). We believe many 1 Tall Timbers Research Station, 13093 Henry Beadel Dr., Tallahassee, FL 32312, USA. 2 4009 Ermine Trail, Temple, TX 76504, USA. 3 Department of Geography and the Environment, University of Texas, 1 University Station A3100, Austin, TX 78712, USA. 4 Corresponding author; e-mail:
[email protected]
ARU DRs from Arkansas and Florida were likely produced by aerial duck flocks when flock members flying in close proximity hit wingtips. We observed three sets of DRs produced by flying Gadwall (Anas strepera) flocks at WRNWR on 21 December 2005. Prior to this observation, we were unaware of the ‘‘duck wingtip collision’’ (DWC) phenomenon, and we subsequently discussed our finding with members of the 2005–06 Cornell Laboratory of Ornithology search crew. We then observed and recorded an identical event from a Gadwall flock at Eagle Lake, Texas on 2 January 2006. In both cases, these sounds were associated visually with duck flocks. No visual sightings were associated with ARU recordings. CDJ worked as a member of the IBWO search team from December 2004 to April 2005. During this time, to his knowledge, field crews were not informed of the DWC phenomenon. Here we compare DWCs from our recording with ARU DRs from 24 January and 5 February 2005 (http://www. birds.cornell.edu/ivory/multimedia/sounds). METHODS We recorded a series of DWCs from a Gadwall flock using a Sony TCM5000EV cassette recorder with a Sennheiser MKH416T shotgun microphone at Eagle Lake, Texas (96⬚ 19⬘ 43⬙ W, 29⬚ 35⬘ 15⬙ N) on 2 January 2006. We measured times between rap peaks and peak-to-peak amplitude ratios for two Eagle Lake DWCs, and three ARU DRs from 24 January and 5 February 2005 (Table 1). In addition we made similar measurements for DR recordings of four Campephilus species: Pale-billed Woodpecker (C. guatemalensis), Powerful Woodpecker (C. pollens), Robust Woodpecker (C. robustus) (audio S3 in Charif et al. 2005), and Red-necked Woodpecker (C. rubricollis) recorded by LYP in Peru in November 2005 (Table 1). We produced a lowvolume recording from the C. rubricollis re-
259
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007
260
TABLE 1. Peak-to-peak times and amplitude ratios for five double rap recordings (two DWCs and three ARU DRs), with ranges for four Campephilus woodpeckers; Pale-billed (C. guatemalensis), Powerful (C. pollens), Robust (C. robustus), and Red-necked Woodpecker (C. rubricollis). Recording
Peak-to-peak time (ms)
Amplitude ratio
Eagle Lake A Eagle Lake B ARU January ARU February A ARU February B Campephilus
65 108 102 84 75 57–82
1.78 1.36 2.76 0.79 3.72 0.73–1.00
cording for equitable comparison with the distant Eagle Lake recording (Fig. 1). We calculated peak-to-peak amplitude ratios by dividing the peak amplitude of the second rap by that of the first, for each DR. Times between raps were calculated by subtracting the time of peak amplitude occurrence of the first rap from that of the second. All audio spectrogram measurements were made using program Raven (Charif et al. 2004).
RESULTS Two DWCs from the Eagle Lake recording were of sufficient quality for analysis (http:// www.talltimbers.org/research/duckraps.htm). Additional DWCs were audible in the recording, but measurements were inconclusive due to background noise and distance from the flock. We analyzed only the second ARU DR from the 24 January 2005 recording and the second and third from the 5 February 2005 recording, as the other raps were too faint for conclusive measurement. Peak-to-peak rap times and amplitude ratios of ARU DRs from 24 January and 5 February 2005 were similar to those of DWCs recorded at Eagle Lake, Texas. Peak-to-peak rap times for ARU DRs and DWCs were either similar to or slower than examples from Campephilus species (Table 1). Amplitude ratios for two ARU DRs and both DWCs were greater than those measured for these Campephilus species (Table 1). The peak-to-peak time and amplitude ratio ranges shown (Table 1) for these extant Campephilus species do not represent full ranges of possible values, but are intended
FIG. 1. Audio spectrograms of (A) duck wing collisions from Eagle Lake, Texas, (B) ARU recording 24 January 2005, (C) ARU recording 5 February 2005, and (D) low volume C. rubricollis double rap to simulate distance and quality of Eagle Lake recording. Color value is proportional to power in decibels (e.g., black represents lowest power, white represents highest power). Amplitude (not shown) of second rap is greater than amplitude of first rap in panels A, B, and C. Amplitude of first rap is greater than second in panel D.
SHORT COMMUNICATIONS
to represent ranges within which typical Campephilus DRs would be expected to fall. These comparisons illustrate the ease with which DWCs could be mistaken for Campephilus DRs. DISCUSSION Two ARU DRs from Arkansas and both DWCs consisted of a second rap greater in amplitude than the first, strengthening the possibility that these and other such ARU raps were produced by ducks and, more importantly, decreasing the likelihood the sounds were produced by Ivory-billed Woodpeckers. While Tanner (1942) described the typical DR of C. principalis as a louder first rap followed by a softer second rap, other Campephilus species are known to occasionally produce DRs with a second rap louder than the first. However, these are considered atypical; such DRs occurred in only 18% of a sample of 119 DRs recorded from seven Campephilus species (Fitzpatrick et al. 2006). Only 45% of 99 DRs recorded from Choctawhatchee River fit the description of Tanner (Hill et al. 2006). Therefore, DWCs provide a more plausible explanation for the source of ARU DRs than do multiple recordings of atypical Campephilus DRs. The second DR from the February ARU recording appears to fit Tanner’s description. However, all DRs from this recording show a particularly strong likelihood of being DWCs, as they are accompanied by audible wing sounds produced by flying ducks and a Gadwall vocalization. This suggests that DWCs may even account for some ARU DRs in which the first rap is greater in amplitude than the second; considerable variability in the amplitude and spacing of the two raps in such random events as duck wingtip collisions is to be expected. Our suspicion that many ARU DRs were produced by ducks is further reinforced by the similarity in auditory quality between the ARU and Eagle Lake recordings, in contrast to the acoustic quality of other Campephilus recordings (audio S3, audio S4 in Charif et al. 2005). Campephilus drums on a wooden substrate typically consist of a crisp tone followed by brief reverberation, whereas our DWCs and publicly available ARU DRs consist of dull tones lacking reverberation.
261
Campephilus species typically produce a series of DRs from a single perch that are delivered at fairly regular intervals over periods of many minutes. However, all available ARU DR recordings from WRNWR (http://www. birds.cornell.edu/ivory/multimedia/sounds) have been short events consisting of only a few DRs. Of those recordings, one contains only a single DR event. The remaining two each contain only three DRs recorded in very short time periods, the longer of the two lasting approximately 42 sec. From the Choctawhatchee River, 69 of 99 recorded DRs were singletons (Hill et al. 2006), and the remaining 30 are present in recordings containing between two and nine DRs. A recording containing repeated double raps spaced fairly evenly over a period of many minutes, rather than brief events lasting only a short time, would constitute more plausible acoustic evidence for the presence of an Ivory-billed Woodpecker than any current putative recordings that are publicly available. The occurrence of a distant DR followed by a closer, louder DR in the WRNWR ARU recordings has suggested to some that they were call-and-response interactions between at least two Ivory-billed Woodpeckers (Charif et al. 2005). An extremely similar event occurs in our Eagle Lake recording. However, these DWCs were produced by a Gadwall flock flying away from the recording unit, with the second DWC more distant than the first. DRs present in the January ARU recording were likely produced by a duck flock flying toward the ARU, with the first DR more distant than the second. A treatment of the DWC phenomenon, incorporating our observations but without additional confirmed recordings, was given in the Cornell Laboratory of Ornithology’s 2005–06 final report (Rohrbaugh et al. 2006), where the number of plausible ARU DRs was concurrently reduced from hundreds to 10. The temporal distribution of those DRs is highest during the winter months and declines in abundance in the spring months. This pattern accords well with a migratory winter duck population, but the reduced use of ARUs in the spring months could easily account for the apparent correlation (see Table 6, Rohrbaugh et al. 2006). A more detailed account of the specific dates of recordings would help
262
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007
elucidate this potential relationship and the ARU DRs accumulated in Florida should be given a similar treatment. Additional ARU efforts, if undertaken, should be extended to summer months when migratory ducks are largely absent from the southeastern United States, but resident woodpeckers would be expected to remain territorially active. Charif et al. (2005) and Rohrbaugh et al. (2006) acknowledged that DRs recorded by ARUs do not confirm the presence of Ivorybilled Woodpeckers in Arkansas. However, until this time no other North American bird (or other sound source) was known to consistently produce Campephilus-like DRs, making Ivory-billed Woodpeckers seem the most likely candidates. Before any ARU DR is considered to be even suggestive of the presence of an Ivory-billed Woodpecker in a region where duck flocks are common, it should be conclusively shown the sound was not produced by duck wingtip collisions. Considering WRNWR hosts a large wintering duck population, wingtip collisions produced by flying ducks provide an alternative explanation for the source of ARU DRs and must be considered as the search for the Ivory-billed Woodpecker continues. Any DRs delivered in series and recorded during the summer, when wintering duck flocks are absent from the southeastern United States, would merit especially careful analysis. ACKNOWLEDGMENTS We thank R. H. Barth, J. A. Cox and R. A. Page for assistance at various stages of this work. We also thank
J. A. Jackson, B. M. Whitney, and an anonymous reviewer for many helpful suggestions.
LITERATURE CITED CHARIF, R. A., C. W. CLARK, AND K. M. FRISTRUP. 2004. Raven 1.2 user’s manual. Cornell Laboratory of Ornithology, Ithaca, New York, USA. CHARIF, R. A., K. A. CORTOPASSI, H. K. FIGUEROA, J. W. FITZPATRICK, K. M. FRISTRUP, M. LAMMERTINK, M. D. LUNEAU, JR., M. E. POWERS, AND K. V. ROSENBERG. 2005. Notes and double knocks from Arkansas. Science 309:1489. FITZPATRICK, J. W., M. LAMMERTINK, M. D. LUNEAU, JR., T. W. GALLAGHER, B. R. HARRISON, G. M. SPARLING, K. V. ROSENBERG, R. W. ROHRBAUGH, E. C. H. SWARTHOUT, P. H. WREGE, S. B. SWARTHOUT, M. S. DANTZKER, R. A. CHARIF, T. R. BARKSDALE, J. V. REMSEN, JR., S. D. SIMON, AND D. ZOLLNER. 2006. Clarifications about current research on the status of Ivory-billed Woodpecker (Campephilus principalis) in Arkansas. Auk 123: 587–593. HILL, G. E., D. J. MENNILL, B. W. ROLEK, T. L. HICKS, AND K. A. SWISTON. 2006. Evidence suggesting that Ivory-billed Woodpeckers (Campephilus principalis) exist in Florida. Avian Conservation and Ecology 1(3):2. http://www.ace-eco.org/vol1/ iss3/art2/. ROHRBAUGH, R. W., K. V. ROSENBERG, M. LAMMERTINK, E. C. H. SWARTHOUT, R. A. CHARIF, S. BARKER, AND M. POWERS. 2006. Summary and conclusions of the 2005–06 Ivory-billed Woodpecker search in Arkansas. USDI, Fish and Wildlife Service. Grant Agreement #401816G060. Final Report. http://www.birds.cornell.edu/ivory/ current0607/finalreport06/documentview. TANNER, J. T. 1942. The Ivory-billed Woodpecker. Research Report. Number 1. National Audubon Society, New York, USA.
