DO WOODPECKER DRUMS ENCODE INFORMATION FOR SPECIES RECOGNITION?1
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The Condor 103:143–150
q The Cooper Ornithological Society 2001
DO WOODPECKER DRUMS ENCODE INFORMATION FOR
SPECIES RECOGNITION?1
DANIELLE J. DODENHOFF2,3, ROBERT D. STARK2 AND ERIC V. JOHNSON
Biological Sciences Department, California Polytechnic State University, San Luis Obispo, CA 93407
Abstract. We investigated whether woodpecker drums are exchanged interspecifically
and attempted to ascertain the drum variables responsible for species recognition. Playback
experiments were conducted on four woodpecker species (Nuttall’s Picoides nuttallii, White-
headed P. albolarvatus, Hairy P. villosus, and Downy P. pubescens Woodpeckers) com-
paring behavioral responses to sympatric hetero- and conspecific drums. There was a sig-
nificant difference in behavioral responses to sympatric hetero- versus conspecific drums;
individuals responded less intensely to drums of heterospecifics when drum cadences were
dissimilar. Allopatric species drums with similar cadences were used to examine whether
cadence was a variable that encodes species identity. There was no significant difference
between responses to allopatric hetero- versus conspecific drums with a similar cadence,
indicating that the drum’s cadence encodes information for species recognition. Further
playbacks eliminated drum duration and spectral properties as important variables for species
recognition. Results suggest that if an individual drums at a similar cadence to a sympatric
heterospecific, then their signal may elicit a conspecific response.
Key words: drumming, interspecific communication, nonvocal communication, species
recognition, woodpeckers.
INTRODUCTION of individuals (Short 1982, Wilkins and Ritchi-
Acoustic communication in birds normally oc- son 1999). Given that many of these functions
curs by the use of songs and calls. However, correspond to bird song, some researchers have
woodpeckers lack the ability to generate songs concluded that drumming should have similar
similar to those used by passerines (Bracken- characteristics and have even postulated drum-
bury 1982). Along with a vocal repertoire of ming to be the evolutionary counterpart to pas-
calls, woodpeckers employ a long-distance non- serine song in woodpeckers (Pynnönen 1939,
vocal acoustical signal aptly referred to as drum- Lawrence 1967), although others disagree
ming. A woodpecker drum is a rapid, repetitive (Winkler and Short 1978).
series of strikes with the bill on a substrate, not Although there are several accounts of inter-
associated with foraging or cavity excavation specific reaction to drumming, references have
(Bent 1939, Pynnönen 1939, Short 1974). tended to consolidate the variety of responses as
Drumming is unusual in that a separate instru- equivalent to responses given to conspecific
ment in addition to the bird’s bill is required to drums (Lawrence 1967, Winkler and Short
produce the signal (Skutch 1985). 1978). The problem with comparing woodpeck-
Limited information is available concerning er responses to drums stems from the lack of
nonvocal acoustical signals in avian species information on the function of the multiple sig-
(Prum 1998). As an instrumental signal, drum- nals woodpeckers use to communicate. There
ming has been noted to be a form of long-dis- are three classes of calls (call notes, intimate
tance communication that may or may not elicit calls, and rattle calls) which may be given in
hetero- or conspecific responses (Crockett 1975, response to a drum, and it is unknown if these
Duncan 1990). Functions attributed to drum- calls are equivalent responses to drumming
ming include territorial establishment, mate at- alone.
traction, pair bond maintenance, and localization There is no consensus as to whether drums
encode species-specific information detectable
1 Received 2 March 2000. Accepted 27 September by receivers. Four hypotheses have been sug-
2000. gested. First, drums are species-specific and not
2 Present address: Department of Evolution, Ecology
and Organismal Biology, The Ohio State University,
normally exchanged interspecifically (Perrins
Columbus, OH 43210. and Middleton 1985, Welty and Baptista 1988).
3 E-mail: dodenhoff.1@postbox.acs.ohio-state.edu Second, drums are not distinctive because other
[143]144 DANIELLE J. DODENHOFF ET AL.
TABLE 1. Descriptive statistics of the drum variables (mean 6 SD) for the playback tapes.
Cadence Duration Interstrike
Playback signal (strikes sec21) (sec.) No. strikes interval (sec.)
Nuttall’s 20.8 6 0.4 1.03 6 0.1 21.5 6 0.7 0.050 6 0.001
‘‘Slow’’ Nuttall’sa 17.1 6 0.5 0.78 6 0.2 13.1 6 3.2 0.064 6 0.001
Downy 15.8 6 0.4 0.79 6 0.4 12.5 6 0.7 0.066 6 0.002
Hairy 26.6 6 0.5 0.83 6 0.3 22.0 6 7.1 0.039 6 0.001
Northern Flicker 19.3 6 1.2 0.99 6 0.4 19.3 6 7.8 0.055 6 0.005
White-headed 20.1 6 0.5 1.64 6 0.2 33.0 6 2.0 0.048 6 0.001
Three-toed 15.3 6 0.6 0.78 6 0.1 12.0 6 1.0 0.070 6 0.007
Black-backed 15.6 6 0.3 1.83 6 0.1 28.5 6 0.7 0.066 6 0.002
Ladder-backed 31.1 6 1.2 1.13 6 0.1 35.0 6 1.2 0.033 6 0.001
Pulse tone 400 Hz 15.9 1.3 20 0.066
Nuttall’s altered to Hairy 26.3 1.0 26 0.041
Hairy altered to Nuttall’s 18.8 1.0 19 0.053
a Drums . 2 SD below species average.
signals, perhaps rattle calls, are used for species other species were used as playback stimuli:
identification (Short 1974, 1982, Winkler and Black-backed (P. arcticus), Three-toed (P. tri-
Short 1978). Third, drums are fairly species-spe- dactylus), and Ladder-backed Woodpeckers (P.
cific even to the human ear, but woodpeckers scalaris), and the Northern Flicker (Colaptes au-
may react indiscriminately (Winkler et al. 1995). ratus). Determinations of sympatry were based
Fourth, drums are sympatrically, but not allo- on each species’ breeding range within sampled
patrically, species-specific, with the drum ca- areas. White-headed Woodpeckers could be con-
dence (in strikes sec21) as the predominant var- sidered marginally sympatric with Nuttall’s and
iable for species identification (Stark et al. Downy Woodpeckers, but only during nonbreed-
1998). ing months (Beedy and Granholm 1985, Gaines
Two unresolved questions remain in the afore- 1988).
mentioned hypotheses. First, whether responses The following procedure was used for all
to hetero- versus conspecific drums are different, playback trials: Subject birds were observed for
and whether the variable(s) used by woodpeck- a minimum of 5 min before each trial. Only in-
ers for species recognition are contained within dividuals that were foraging, preening, or cavity-
the drum. We designed three experiments to re- excavating, and within acoustical range were
solve these questions. Experiment 1 established used. Playback experiments were conducted in
a method of scoring responses to drums, and ex- a random, balanced, pairwise design (conspecif-
periment 2 compared behavioral responses to ic:heterospecific), with a minimum of 30 min
playbacks of sympatric hetero- versus conspe- between stimuli (based on log survivorship
cific drums. Experiment 3 compared behavioral curves calculated for the maximum natural du-
responses to playbacks of allopatric heterospe- ration a bird remained responsive during drum-
cific and computer-generated drums versus con- ming sequences, Dodenhoff 1996). Birds unre-
specific drums, to test possible variables used for sponsive to both stimuli were excluded from the
species recognition. analysis.
Drum variables for playback tapes were based
METHODS on those reported by Stark et al. (1998), and in-
GENERAL PLAYBACK DESIGN cluded duration (sec), number of strikes per
Populations in four regions of California, in- drum, cadence, and interstrike interval in one
cluding the Los Padres, Sequoia, Sierra, and drum (interstrike interval, in seconds; Table 1).
Inyo National Forests, were sampled during the Tapes were generated by editing together field
1993–1996 breeding seasons. Four species were recordings and commercially available drums
studied using playbacks: Nuttall’s (Picoides nut- (Cornell Laboratory of Ornithology 1992a,
tallii), Hairy (P. villosus), Downy (P. pubes- 1992b), computer generated tones, and altered
cens), and White-headed Woodpeckers (P. al- natural drums reset to specific cadences. Play-
bolarvatus). In addition, the drums from four back tapes were standardized for decibel level.DO WOODPECKER DRUMS ENCODE INFORMATION FOR SPECIES RECOGNITION? 145
The results from the analysis of behavioral servable changes from one behavior to another,
observations were used to construct a scoring were noted for each individual during a drum-
method for comparing responses to playback ming sequence (adapted from Halloran and Be-
signals for experiments 2 and 3. Although there koff 1995). Behaviors that significantly preceded
are differences between calls and displays of or followed drumming were determined by se-
woodpeckers, stereotypical behaviors are ob- quence analysis using a first-order Markov
served within the family Picidae (Winkler et al. chain.
1995). Therefore, generalizations from obser- Independence between consecutive behaviors
vations of Nuttall’s Woodpecker correlated with was tested using categorical modeling proce-
behaviors observed for other subject species. We dures (SAS Institute 1990), which used maxi-
used these results to rank the behavioral data mum likelihood values to estimate the G-test sta-
appropriately concerning responsiveness to tistic and its associated x2 statistic (Sokal and
drum playbacks. Rohlf 1981). Data collection excluded the pos-
Behaviors observed more often during drum-
sibility of behaviors following themselves.
ming sequences from experiment 1 were used to
Therefore, the ‘‘expected’’ values where the pre-
create a categorical scoring method similar to
ceding and following behavior were the same
Emlen (1972) for experiments 2 and 3. Behav-
were considered ‘‘logical zeros,’’ and the ex-
iors included alarm calls, approach, drumming,
flights over the speaker, display, attack/supplant, pected values were calculated using a log-linear
and frozen antipredation posture. These behav- model (Fagen and Young 1978). Significant dif-
iors composed the intensity scale. The intensity ferences were detected when the transformed
scale ranked the behavioral level achieved dur- frequency zYz . 1.08.
ing each playback and ranged from 0–7 (low– To investigate which behaviors occurred more
high). Given the subjective nature of categorical often during drumming sequences versus other
scoring, we also employed a hybrid scale, which activities, a comparison of the frequency of each
subdivided the intensity scale into differing lev- behavior was conducted using a x2 analysis for
els of response (using frequency data), weighing contingency tables. Twelve different Nuttall’s
each of the seven categories equally. Hybrid Woodpeckers were observed for 30–60 min
scores ranged from 0–14 (low–high). Behavioral each during the months of March and April,
responses were compared using two-tailed Wil- 1993, during non-drumming sequences. Behav-
coxon signed-ranks tests. For the test to be able iors observed were classified categorically and
to detect significance at the a 5 0.05 level, n compared to the behaviors observed during the
must be at least 6. If n 5 6, then differences had drumming sequences.
to be of like sign (Sokal and Rohlf 1981).
EXPERIMENT 2: SYMPATRIC SPECIES
EXPERIMENT 1: BEHAVIORAL SEQUENCE PLAYBACKS
ANALYSIS
Behavioral responses of the four subject species
Since there were no previous measurements of
to sympatric hetero- and conspecific drums were
drum responses for these species, we conducted
compared. Trials with Nuttall’s Woodpeckers in-
a behavioral sequence analysis to categorize be-
cluded drums from three sympatric species
havioral responses to drumming. Observations
of Nuttall’s Woodpeckers during natural and in- (Downy and Hairy Woodpeckers, and Northern
duced drumming sequences were made in the Flickers), and trials with Downy Woodpeckers
Los Padres National Forest, San Luis Obispo included drums from three sympatric species
County, California during the 1993–1995 breed- (Nuttall’s and Hairy Woodpeckers, and Northern
ing seasons. Behaviors were identified according Flickers). Northern Flicker drums used for play-
to previous descriptions (Short 1971, Winkler back were recorded from individuals sympatric
and Short 1978) and categorized for analysis. with Nuttall’s Woodpeckers. Hairy Woodpecker
Behaviors for this analysis included foraging, trials were conducted using the drums from two
flying or changing locations, intimate calls, sympatric species (Nuttall’s and Downy Wood-
preening, rattle calls, look around (with bill peckers), whereas White-headed Woodpecker
wave), call notes, agonistic attacks, and copu- trials consisted of drums from a single sympatric
lations. Behavioral transitions, defined as ob- species (Hairy Woodpecker).146 DANIELLE J. DODENHOFF ET AL.
EXPERIMENT 3: ALLOPATRIC AND quences to its occurrence during non-drumming
COMPUTER GENERATED PLAYBACKS sequences. The behaviors ‘‘preening’’ and ‘‘rat-
Behavioral responses of three subject species to tle call’’ were observed more often during drum-
allopatric hetero- and conspecific drums were ming sequences (x29 5 183, P , 0.01). Rattle
compared. Trials with Nuttall’s Woodpecker in- calls have been noted to elicit drums, and are
cluded four allopatric species drums, one with a considered similar in function to drumming
similar cadence (White-headed Woodpecker) (Short 1971, Winkler and Short 1978). Although
and three with different cadences (Ladder- ‘‘look around with bill wave’’ did not always
backed, Black-backed, and Three-toed Wood- precede or follow drumming, it was observed
peckers). Trials with White-headed Woodpeck- more often during drumming sequences. Forag-
ers included playbacks with Nuttall’s Wood- ing was observed during both sequences more
peckers, whereas trials with Downy Woodpeck- than expected. Previously, call notes have been
ers included three allopatric species: two with considered a conspecific response to drumming
similar cadences (Black-backed and Three-toed (Short 1982, Kilham 1983). However, our anal-
Woodpecker) and one with a dissimilar cadence ysis indicated call notes were observed frequent-
(White-headed Woodpecker). Although Black- ly in non-drumming rather than drumming se-
backed and Three-toed Woodpeckers’ drums are quences. Furthermore, these call notes preceded
similar in cadence to Downy Woodpeckers foraging and were more often observed during
(Short 1982), they differ in the spacing pattern non-drumming activities. Thus, call notes with-
of individual strikes; Black-backed and Three-
out supporting behaviors were not considered a
toed Woodpeckers increase cadence towards the
response to drumming.
end of their drum, whereas Downy Woodpeckers
decrease (Stark et al. 1998). Also, the drum of EXPERIMENTS 2 AND 3: RESPONSE TO
Black-backed Woodpeckers is significantly lon- PLAYBACKS
ger than Downy Woodpecker drums (Stark et al.
1998). Individuals demonstrated a significant difference
Because behavioral responsiveness to allopat- in the level of response between sympatric het-
ric drums may be observed, computer-generated ero- versus conspecific drums, except between
drums were used to test whether drum cadence Nuttall’s Woodpecker and the Northern Flicker
was the primary variable responsible for species (Table 2). Furthermore, both Downy and Nut-
recognition (as predicted by Stark et al. 1998). tall’s Woodpeckers were similar in their respons-
Two types of trials were conducted using com- es to the Nuttall’s playback tape generated from
puter-generated drums to test spectral properties. an individual greater than 2 standard deviations
First, a 400-Hz tone was repeated to simulate a below the mean cadence for Nuttall’s Wood-
drumming Downy Woodpecker. Second, a Nut- peckers (‘‘slow’’ Nuttall’s, Stark 1996).
tall’s Woodpecker strike was set at a cadence Responses to conspecific drums were not sig-
similar to a Hairy Woodpecker drum, and recip- nificantly different from responses to allopatric
rocally, a Hairy Woodpecker strike was set at a species, provided the drum cadences of the two
cadence similar to a Nuttall’s Woodpecker drum. species were comparable. Conversely, responses
to allopatric species with dissimilar cadences
RESULTS were of significantly lower intensity. The results
EXPERIMENT 1: BEHAVIORAL SEQUENCE for the hybrid scale were similar to those for the
ANALYSIS intensity scale (Table 2), except for a marginal
Nuttall’s Woodpeckers (n 5 68 birds, with 574 difference in the Hairy versus Ladder-backed
transitions) were observed for the behavioral se- Woodpecker comparison; there was a significant
quence analysis. The sample size was large difference for the intensity scale but a nonsig-
enough for the x2 distribution to approximate the nificant difference for the hybrid scale (Table 2).
G distribution. Results of the categorical model Hairy Woodpeckers responded to Ladder-backed
implied a strong dependency between behaviors Woodpecker drums, but less intensely than to a
(x229 5 190, P , 0.01). Twelve Nuttall’s Wood- conspecific signal (although the hybrid scale
peckers were used to analyze behavioral re- score approached significance). The results from
sponses to drums by comparing each behavior’s playbacks with computer-generated signals in-
frequency of occurrence during drumming se- dicated no significant difference between the re-DO WOODPECKER DRUMS ENCODE INFORMATION FOR SPECIES RECOGNITION? 147
TABLE 2. Wilcoxon signed-ranks test results for comparisons of responses to sympatric, allopatric, and com-
puter-generated drum signals versus the conspecific signal. Nonsignificant results indicate no significant differ-
ences in responses to conspecific and heterospecific signals. Listed are P-values (n birds). Int 5 Intensity scale;
Hyb 5 Hybrid scale; see Methods for a description of the two scales.
Species
Playback signal Scale Nuttall’s Downy Hairy White-headed
Nuttall’s Int 0.01 (8) 0.04 (9) 0.58 (10)
Hyb 0.01 (8) 0.02 (9) 0.61 (10)
‘‘Slow’’ Nuttall’sa Int 0.23 (9) 1.00 (9)
Hyb 0.17 (9) 0.83 (9)
Downy Int 0.01 (29) 0.01 (11)
Hyb 0.01 (29) 0.01 (11)
Hairy Int 0.01 (20) 0.01 (11) 0.02 (7)
Hyb 0.01 (20) 0.01 (11) 0.02 (7)
Northern Flicker Int 0.16 (18) 0.04 (6)
Hyb 0.29 (18) 0.04 (6)
White-headed Int 0.40 (19) 0.02 (8)
Hyb 0.08 (19) 0.02 (8)
Three-toed Int 0.03 (11) 0.18 (13)
Hyb 0.01 (11) 0.23 (13)
Black-backed Int 0.03 (13) 1.00 (12)
Hyb 0.01 (13) 0.55 (12)
Ladder-backed Int 0.04 (6) 0.04 (13)
Hyb 0.04 (6) 0.06 (13)
Pulse tone 400 Hz Int 0.06 (7)
Hyb 0.06 (7)
Nuttall’s altered to Int 1.00 (7)
Hairy Hyb 0.89 (7)
Hairy altered to Nut- Int 0.28 (8)
tall’s Hyb 0.25 (8)
a Drums more than 2 SD below species average.
sponses of woodpeckers to these signals versus These results differ from Duncan (1990), be-
conspecific drums. cause Duncan’s comparisons did not include re-
ciprocal playbacks with species sharing similar
DISCUSSION cadences.
Although most researchers agree drums have Our results between species that share similar
species-typical characteristics (Short 1982, Kil- cadences indicated interspecific responses may
ham 1983, Winkler et al. 1995), it was unknown occur between sympatric species. The responses
whether observed interspecific responses to by Nuttall’s Woodpeckers to the drums of North-
drums were predictable. Previous studies includ- ern Flickers were similar to responses elicited
ed multiple responses as equivalent indications by a conspecific drum. Thus, interspecific re-
of species recognition (Short 1982, Winkler et sponses may occur between these sympatric spe-
al. 1995), which implied ambiguity within the cies. Individual drums of both species often have
drum, but did not indicate the absence of dis- a similar drum cadence, and were predicted to
crimination. Individuals may respond to a het- have reciprocal heterospecific ambiguity in their
erospecific drum, but those responses can differ drum (Stark et al. 1998). Despite this prediction,
from responses to conspecific drums, which we did not observe interspecific responses to
could indicate discrimination between stimuli. natural (i.e., non-playback) drums between in-
Results from the playbacks of the ‘‘slow’’ Nut- dividuals of these species over the course of this
tall’s stimulus to Nuttall’s and Downy Wood- study. We attributed the lack of interspecific re-
peckers indicated that interspecific behavioral sponse to observations of restricted drumming
responses to drums can occur naturally between by Northern Flickers. Northern Flickers within
sympatric species, if the drum cadences overlap. the study areas drummed for a few weeks during148 DANIELLE J. DODENHOFF ET AL.
the breeding season, usually when Nuttall’s species identity. However, these results cannot
Woodpeckers were unresponsive to drums. This exclude the possibility that either interstrike in-
may indicate a shift between species in the tim- terval or duration encodes species identity for
ing of drumming during the breeding season; Downy Woodpeckers that are sympatric with
species with similar drums could reduce ambi- Black-backed or Three-toed Woodpeckers.
guity through temporal separation and maintain Computer-generated drums tested whether
species distinctiveness (Stacier et al. 1996). spectral properties were important in species
Allopatric playbacks followed the same pat- identification. Results from trials with Nuttall’s,
tern as observed in sympatric playbacks: drums Hairy, and Downy Woodpeckers indicated that
of species with similar cadences elicited behav- spectral properties of a drum were not charac-
ioral responses equivalent to conspecific signals, teristics responsible for species recognition. In-
whereas those with dissimilar cadences did not. stead, the spectral properties generated in drum-
This indicates that drum cadence is one variable ming have been noted to depend on the drum-
that encodes species identity. Playback trials ming substrate selected (Eberhardt 1997). Fur-
with Downy Woodpeckers tested whether the thermore, differential signal attenuation of
variables ‘‘interstrike interval’’ and ‘‘duration’’ frequencies through an environment may make
were used to encode species identity. Downy the spectral properties of a drum a poor variable
Woodpeckers responded similarly to conspecific, for species-specificity and signaler identification
Black-backed and Three-toed Woodpecker (Wiley and Richards 1982). However, these
drums, which indicated that drum duration and spectral properties may be important in other as-
interstrike interval were not used by Downy pects of communication, including ranging
Woodpeckers to differentiate between these spe- (Morton 1986, Naugib 1995, 1998).
cies. This is similar to the finding that, for many In summary, our results support the conclu-
passerines, signal duration is usually not a spe- sion that the drum’s cadence encodes species
cies-typical variable (Becker 1982). However,
identity of the signaler, and that receivers per-
because heterospecific drums used as playbacks
ceive these differences within the acoustical en-
were no shorter than Downy Woodpecker
vironment. Individual woodpeckers did not react
drums, our test does not eliminate the possibility
indiscriminately to drums. Responses to heter-
of a minimum duration required for species rec-
ospecific drums with dissimilar cadences were
ognition. Shorter song durations may elicit less
less intense and did not include vocalizations or
intense responses, and minimum durations are
behaviors observed during playbacks with con-
required for species recognition for passerines
specific drums. Sympatric woodpecker drums
(Becker 1982).
Our results also eliminated the interstrike in- are not usually exchanged interspecifically, but
terval differences in drumming as a character- interspecific responses may result if individuals
istic responsible for species recognition in from different species have an overlapping drum
Downy Woodpeckers. Kaiser (1990) suggested cadence. However, due to the amount of overlap
that the spacing pattern within unmodulated between species, both sympatric and allopatric,
drums was related to the woodpecker’s foraging we conclude drums are not species specific. The
method. In ‘‘wood-pecking’’ species the rhythm lack of signal specificity does not prevent the
accelerates, whereas ‘‘gleaners’’ slow down to- encoding of species identity. For example, dif-
ward the end or the rhythm and amplitude of ferential timing of drumming between species
their drums remain uniform (Winkler et al. within a breeding season could result in a lack
1995). Although Kaiser’s hypothesis was based of selection pressure on drums to significantly
on small sample sizes, it suggested that the spac- diverge. This may allow a few sympatric species
ing pattern within a drum may be controlled by to utilize similar cadences to encode species
the musculature used for foraging. Thus, this identity without resulting in ambiguity between
variable would have a greater likelihood of over- species. Nonetheless, given the similarities be-
lapping with a sympatric species with a similar tween allopatric species, it is unknown whether
foraging strategy. Given the high degree of over- the trend of signal divergence observed in this
lap between the foraging strategies of many study is applicable across broad geographic
sympatric woodpeckers, the interstrike interval ranges or under differing acoustical environ-
of a drum may not be a reliable indicator of ments. Clearly, considerable research still needsDO WOODPECKER DRUMS ENCODE INFORMATION FOR SPECIES RECOGNITION? 149
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