DOI: 10.5061/dryad.sf7m0cg3d
title: Data from: Acorn woodpeckers vocally discriminate current and former group members from non-group members

description: In species with long-term social relationships, the ability to recognize
 individuals after extended separation, and the ability to discriminate
 between former social affiliates that have died and those that have left
 the group but may return, are likely to be beneficial. Few studies,
 however, have investigated whether animals can make these discriminations.
 We presented acorn woodpeckers (Melanerpes formicivorus), a group-living,
 cooperatively breeding bird, with playbacks of current group members,
 former group members still living nearby, former group members that had
 died or left the study area, and familiar non-group members. Subjects
 responded more quickly to the calls of non-group members than to the calls
 of current group members or former group members still living in the study
 area but did not discriminate between non-group members and former group
 members that had died or disappeared. This suggests that acorn woodpeckers
 can vocally recognize both current group members and former group members
 that have dispersed to nearby groups, and that they either forget former
 group members that no longer live in the vicinity, or classify them
 differently from former group members that still live nearby. This study
 suggests an important role for vocal recognition in maintaining valuable
 relationships with social affiliates post-dispersal.
description: MATERIAL AND METHODS Study site and
 population monitoring All data were collected on wild
 acorn woodpeckers at Hastings Natural History Reservation in central
 coastal California, USA (36.387ºN, 121.551ºW). This population has been
 the subject of a long-term study since 1968 (MacRoberts and MacRoberts
 1976; Koenig 1981b), and &gt;95% of the individuals are color-banded.
 Most individuals are banded in the nest at 21 days of age, and unbanded
 adults immigrating into the population are captured and banded whenever
 possible. As of 2019, there are approximately 50 social groups within the
 study area, and each group is censused approximately every 8–10 weeks
 using spotting scopes to re-sight color banded individuals.
 Experiment 1: vocal discrimination of current, former, and
 non-group members We conducted Experiment 1 14 Apr–19
 Jun 2017 and 2 May–14 Jun 2018. Subjects were 7 females and 7 males from 8
 social groups, and all but one were of breeder status. We presented each
 subject with 3 different playback stimuli on different days: (1) a call of
 a current group member; (2) a call of a former group member that had died,
 left the group, or remained on the natal territory after the subject had
 dispersed 1.1–6.4 years prior to the experiment (median = 2.8 years); and
 (3) a call of an unrelated individual from a nearby territory that had
 never lived in the same group as the subject. The distance between the
 territory centroids of the subject and the non-group member caller was
 40–862 m (median=167 m), and in 9 of 14 non-group member trials the caller
 and subject shared a territorial boundary. As acorn woodpeckers make
 frequent forays to other territories with a mean foray distance of 500–600
 m, subjects were likely familiar with all of the non-group member callers
 (Barve et al. 2020). The order of the presentation was
 balanced according to a Latin square design (Table 1). The 3 stimuli
 played to a given subject were always recorded from callers of a single
 sex, such that each subject received either 3 male or 3 female calls. The
 sex of the subject matched the sex of the callers in 7 of 14 cases.
 Successive playbacks to the same group or to groups &lt; 250 m from
 each other were spaced by 6.0 ± 4.8 days on average (minimum 3 days to
 same group, 2 days to groups closer than 250 m).
 Testing the difference in response to callers living in the study
 area (hereafter “nearby”) vs. callers that had died or left the study area
 (hereafter “absent”) was not an <i>a priori </i>goal of
 Experiment 1. However, because of the difficulty of obtaining
 playback-quality recordings from known individuals, we used the call of an
 individual that was no longer observed on the study area at the time of
 the experiment as the former group member stimulus for two female and four
 male subjects (Table 1). None of the former group members that were
 classified as “absent” had been observed on the study site for at least a
 year prior to the experiment and none were seen at least a year
 post-experiment (as of Sep 2019). For two of the males that received a
 former group member stimulus from an absent caller, the non-group member
 stimulus was also from an absent caller (not seen for 7 months prior to
 the experiment). Among the subjects that received the call of a nearby
 former group member, the distance between the territory centroids of the
 subject and the caller was ­­­121–1587 m (median = 228 m), and in 5 of 8
 nearby former group member trials, the caller and subject shared a
 territorial boundary Experiment 2: vocal
 discrimination of nearby and absent former group members
 We conducted Experiment 2 6 Apr–12 Jul 2019, using 5 female and 6
 male subjects from 8 different groups (Table 2). Six individuals were used
 as subjects in both Experiment 1 and Experiment 2, and among the 11
 subjects used in Experiment 2, all but 2 were members of a social group
 that was exposed to playbacks in Experiment 1. Experiment 2 was designed
 as a follow-up to Experiment 1 to investigate whether acorn woodpeckers
 could vocally discriminate between nearby and absent former group members.
 We presented each subject with the call of a former group member living on
 a neighboring territory within the study area (nearby), the call of a
 former group member that had not been observed in the study area for
 1.1–7.3 years before the experiment (median absence =3.0 years), and the
 call of an unrelated individual from a nearby territory that had never
 lived in the same group as the subject. The
 interterritorial distance of subjects and nearby former group members was
 102–257 m (median = 132 m), and all nearby former group members shared a
 territorial boundary with the subject. The interterritorial distance of
 subjects and non-group members was 102–734 m (median = 257 m), and 5 of 11
 non-group members shared a territorial boundary with the subject. Thus, as
 in Experiment 1, subjects were likely familiar with all nearby former
 group member and non-group member callers. As of Sep 2019, none of the
 absent former group members had been observed on the study area since
 their last sighting 1.1–7.3 years before the experiment.
 We ensured that the amount of time since the subject and caller
 last lived together did not statistically differ between nearby former
 group member and absent former group member playback stimuli (Paired
 <i>t</i>-test,
 <i>t</i><sub>10</sub>=-1.2,
 <i>P</i>=0.25). As in Experiment 1, order of presentation was
 balanced according to a Latin square design (Table 2), and the 3 playback
 stimuli presented to a given subject were recorded from 3 callers of a
 single sex, which matched the sex of the subject in 6 of 11 cases.
 Successive playbacks to the same group or to groups &lt;250 m apart
 were separated by 6.6 ± 6.5 days (minimum 2 days).
 Playback stimuli In both experiments, the
 calls used as playback stimuli were <i>waka</i> calls, an
 individually distinctive, affiliative call typically produced when members
 of the same group approach one another after a short period of separation
 (MacRoberts and MacRoberts 1976; Yao 2008). All playback stimuli were
 recorded at Hastings Reservation 19 Mar 2015–26 May 2017 using a
 Sennheiser ME67 or ME62 microphone (Wedemark, Germany) and a Marantz
 PMD661 (Kanagawa, Japan), Fostex FR-2 (Akishima City, Tokyo, Japan), or
 Roland R26 (Hamamatsu, Shizuoka, Japan) digital recorder (48 kHz, 16 or 24
 bits). Prior to constructing the playback stimuli, the calls were
 high-pass filtered (200 Hz cut-off, 6 dB roll off) and normalized to -3 dB
 in Audacity® 2.1.1, and any calls originally recorded at 24 bits were
 converted to 16 bits. In Experiment 1, the playback
 stimuli consisted of 60 sec of background noise with a 10-sec fade-in,
 followed by a single <i>waka</i> call, followed by 30 sec of
 background noise, followed by the same <i>waka</i> call,
 followed by a final 10 sec of background noise with a fade-out applied to
 all 10 sec. Repeating the call increased the likelihood that the subjects
 would respond to the playback, and the 30-sec interval between calls
 followed previously published protocols (Yao 2008; Pardo et al. 2018).
 While natural <i>waka </i>calls are most commonly produced
 singly, they are sometimes repeated at an interval close to 30 sec (M.
 Pardo, unpublished data). The playback stimuli for Experiment 2 were
 constructed in the same way except that the initial period of background
 noise only lasted 30 sec. We made this change to reduce the chance that
 the subject would fly away before the call began.
 Playback protocol Playback trials for both
 experiments followed a similar protocol to Pardo et al. (2018). In brief,
 we placed a Yamaha PDX 11 loudspeaker (Hamamatsu, Shizuoka, Japan)
 characterized at 100.1 ± 1.3 dB re 20 µPa at 1 m in a tree 1–1.5 m off the
 ground and 40 m away from a tree near the center of the group’s territory
 (“center tree”). This volume was at the upper end of the range of natural
 <i>waka</i> calls produced by a captive adult male acorn
 woodpecker (Pardo et al. 2018). The speaker was always placed in the same
 location during successive trials to a given group. Once the subject was
 located in the center tree, an observer began filming the subject using
 either a Canon PowerShot SX510 digital camera (Ota City, Tokyo, Japan) or
 a Sony Handycam DCR-SX45 Camcorder (Minato, Tokyo, Japan), and immediately
 played the appropriate playback file. Measuring
 response to playback Based on video and audio
 recordings of each playback trial, we measured the following aspects of
 the focal bird’s response: latency to the first “reaction” (defined as
 vocalizing, flying up to a higher vantage point, or flying toward the
 speaker), latency to the first “positive” flight (defined as flying up to
 a higher vantage point or toward the speaker), latency to the first
 approach to the speaker, latency to the closest approach to the speaker,
 distance of the first approach to the speaker, and distance of the closest
 approach. For the latency variables, if the focal bird did not exhibit the
 behavior of interest within 3 min after the start of the playback, latency
 was assigned the maximum possible value of 180 sec and marked as censored.
 Distances were estimated by eye to the nearest 5 m using landmarks of
 known distance to the speaker, measured before the experiment with a 50-m
 tape. The authors involved in scoring the videos were blind to the
 experimental condition in each trial until all scoring was
 complete. Statistical analyses We
 conducted statistical analyses in R 3.6.1 (R Core Team 2019). Because of
 limited sample sizes, we pooled the results of both experiments. For
 Experiment 1, we rescored each former group member trial as either nearby
 former group member or absent former group member, according to whether
 the caller was still living in the study area at the time of the
 experiment. Thus, in the pooled dataset there were four treatment
 categories (current group member, nearby former group member, absent
 former group member, and non-group member), and each subject only received
 3 of these 4 treatments within a given experiment. We used 13 unique call
 exemplars from 13 different callers as playback stimuli for the current
 group member category (n=14), 17 unique exemplars from 13 different
 callers for the nearby former group member category (n=19), 12 unique
 exemplars from 8 different callers for the absent former group member
 category (n=17), and 19 unique exemplars from 17 different callers for the
 non-group member category (n=25). We limited our model
 set to <i>latency to react</i>, <i>latency to positive
 flight</i>, and <i>distance of first approach</i> as
 response variables, as all other responses measured were highly correlated
 (Pearson’s <i>r</i>&gt;0.75) with at least one of these.
 We analyzed <i>latency to react </i>and <i>latency to
 positive flight </i>using Cox regression in the R package
 <i>coxme</i> (Therneau 2018) to account for the fact that some
 of the latencies were censored. We analyzed <i>distance of first
 approach</i> using linear mixed models in the R packages
 <i>lme4</i> (Bates et al. 2015) and
 <i>lmerTest</i> (Kuznetsova et al. 2017). For each response
 variable, we ran a model with <i>treatment</i>,
 <i>subject sex, caller sex, </i>and the interaction of
 <i>subject sex </i>by <i>caller sex </i>as fixed
 effects, and <i>individual ID </i>as a random effect. As the
 goal of the experiment was to determine whether subjects could
 discriminate current and former group members from non-group members, we
 compared the responses to non-group members with the responses to each
 remaining treatment category using Dunnett’s method in package
 <i>emmeans</i> to adjust for multiple comparisons (Lenth
 2018). We also conducted pairwise comparisons for the interaction of
 <i>subject sex</i>*<i>caller sex </i>using Tukey’s
 method in <i>emmeans</i>. To determine if
 time since the subject last lived with a former group member affected the
 subject’s response to that former group member’s call, we ran an
 additional model for each response variable using only the present and
 absent former group member trials. These models included <i>time
 since together </i>as a fixed effect and <i>individual ID
 </i>as a random effect.
description: Definitions of all the variables are included in a separate
 file