Male Territorial Aggression Does Not Drive Conformity to Local Vocal Culture in a Passerine Bird

1Male territorial aggression does not drive conformity to local vocal culture in a passerine bird

3This is a pre-review pre-print of the following published article:

4Parra, M., A. C. Dalisio , W. E. Jensen, and T. H. Parker. 2017. Male territorial aggression does

5not drive conformity to local vocal culture in a passerine bird. Ethology. 123: 800-810. doi:

610.1111/eth.12648

8Marcelina Parra1

9Anthony C. Dalisio2,3,4

10William E. Jensen3

11Timothy H. Parker1,5

131. Department of Biology, Whitman College, 345 Boyer Ave, Walla Walla, WA 99362, USA

142. Circumpolar Studies, Sterling College, Craftsbury Common, VT, 05827, USA

153. Department of Biological Sciences, Emporia State University, 1 Kellogg Circ, Emporia, KS

1666801, USA.

174. Current address: 204 Water St, Waterville, ME 04901, USA.

185. Corresponding author: [email protected]; +1.509.526.4777

20Abstract In many songbird species, young males learn songs from neighbors and then settle

21nearby, thus creating neighborhoods of conformity to local vocal culture. In some species males

22appear to postpone song learning until after dispersal, possibly to facilitate conformity to local

1 23dialects. Despite decades of study, we still lack a consensus regarding the selective pressures

24driving this delayed song learning. Two common hypothetical benefits to conformity, and thus

25delayed song learning, are rooted in male territorial interactions; males preferentially produce

26local song either to avoid detection as new arrivals (deceptive mimicry) or to be more

27effectively recognized as conspecific territory holders. The dickcissel (Spiza americana) is an

28ideal species in which to study these hypotheses. Males of this species appear to delay song

29learning until they arrive at their first adult territory, each individual sings a single song type,

30and conformity to the local song culture is high. Using playback, we contradicted both of the

31territorial hypotheses described above; male dickcissels did not respond differentially to local vs

32foreign song playback treatment. We are confident in this lack of difference because dickcissels

33clearly responded less strongly to a third treatment, neighbor song, than to the other two

34treatments, demonstrating sufficient power in our experimental design (and providing the first

35evidence of the dear-enemy effect in dickcissels). Our results raise the question of why

36dickcissels respond equally aggressively to both local and foreign songs when other bird species

37often show reduced aggression towards foreign song. If reduced aggression to foreign song is

38not ubiquitous in species that achieve conformity through delayed learning then selection from

39among-male territorial interaction seems unlikely to be a general explanation for such delayed

40learning. Reduced aggression in response to foreign songs in other species may be due to

41reduced exposure to the stimulus of foreign song or to different cost-benefit trade-offs when

42responding to songs that deviate from the local average.

2 45

46Keywords

47dear-enemy; deceptive mimicry; dickcissel, playback; recognition; song; Spiza americana

3 49Introduction

51Geographic variation in sexual signals within species is common, and is particularly conspicuous

52in the songs of many Oscine birds. These song variants, sometimes referred to as dialects

53(Baker & Cunningham 1985), can be explained at the proximate level by males learning songs

54from nearby adult conspecifics (Beecher & Brenowitz 2005). As long as individuals tend to learn

55their songs from local tutors, dispersal post-learning is limited, and new song variants emerge

56occasionally (e.g., through learning errors), song dialects should emerge (Krebs & Kroodsma

571980). Although song dialects result from cultural rather than genetic variation, this does not

58preclude a role for selection in influencing individual behaviors that promote the emergence of

59dialects. For instance, in some species individuals appear to learn song after dispersal rather

60than in their first few months of life (Payne & Payne 1997). This delay of learning until after

61dispersal promotes the development of dialects, and thus may be selected for if dialect

62conformity conveys a fitness advantage.

64A primary function of song in many Oscine species is male signaling of territorial ownership.

65Selection could favor delayed song learning, and thus conformity to a local dialect, if this

66conformity somehow aided males as they sought to establish and maintain their claim to a local

67territory. An early hypothesis, ‘deceptive mimicry’, posited that conforming to the local dialect

68could result in reduced aggression from conspecifics, allowing the newcomer to more easily

69integrate into the neighborhood than a bird with a foreign song (Payne 1981; Rohwer 1982;

70Payne 1983). If this is the case, males should respond more aggressively to a foreign dialect

4 71than a local dialect. However, this prediction is rarely met in playback studies (e.g., Lemon

721967; McGregor 1983; Podos 2007; Osiejuk et al. 2012; Bradley et al. 2013), and so the

73deceptive mimicry mechanism is unlikely (but see e.g., Baker 1982; Briefer et al. 2010).

74Conversely, sharing song types with territorial neighbors could result in increased recognition as

75a conspecific and thus aid a newcomer in defending a territory against established males

76(discussed in Baker 1994) or facilitate longer territory tenure in established males (Beecher et

77al. 2000; but see Nelson & Poesel 2013). If this is the case, males should respond more

78aggressively to playback of an unfamiliar individual singing the local dialect than to playback of

79a foreign dialect. This prediction holds for many species of birds (e.g., Lemon 1967; McGregor

801983; Podos 2007; Osiejuk et al. 2012; Bradley et al. 2013). Of course the existence of

81heightened response to local versus foreign playback does not demonstrate that this selection

82pressure caused the evolution of local song learning and thus dialects. Conformity to local

83dialects may not be favored by selection stemming from interactions among males. It could be,

84for instance, that female choice drives dialect conformity (e.g., Searcy & Andersson 1986;

85Danner et al. 2011; Nelson & Poesel 2013). Alternatively, dialect conformity may be nothing

86more than an epiphenomenon of learning songs from conspecifics and limited post-learning

87dispersal.

89Dickcissels (Spiza americana) are songbirds in which individuals produce a single song type and

90conform to striking song dialects that change over the scale of a few km (Schook et al. 2008).

91They are common breeders on the Great Plains of North America (Temple 2002). Circumstantial

92evidence strongly suggests that male dickcissels delay song learning until they arrive at their

5 93first adult territory after returning from wintering in northern South America (Schook et al.

942008). One researcher at our study site banded 317 nestlings that later fledged and only one of

95the banded male nestlings was detected as a returned adult (B. Sousa, personal

96communication). This observation, combined with the fact that our site encompasses multiple

97local dialects, leads to the conclusion that a male settling on our site likely did not fledge in the

98area of the dialect he sings as an adult, or even in any of the surrounding dialect areas. We

99cannot rule out the possibility that males roam and learn multiple dialects their first summer

100and then choose to sing which ever song matches the dialect of their adult territory (Marler &

101Peters 1982; Nelson & Marler 1994). However, given subtle changes in dialects detectable on

102the scale of 100’s of meters and the high degree of conformity to local dialects (Schook et al.

1032008), this mode of learning with attrition would substantially limit dispersal options. Further,

104given the readiness with which dickcissels disperse to occupy newly available habitats (Temple

1052002), limited dispersal appears unlikely. Finally, young males tend to remain within a few

106hundred meters of their nest in the weeks after fledging, often into the period when adult

107males cease defending territories and disperse (B. Verheijen, personal communication). Thus,

108most young males are likely exposed to only a few local dialects at best before singing subsides

109and dialects dissolve in late summer.

110

111Because dickcissels appear to show delayed song learning that promotes high dialect

112conformity, we set out to test hypotheses explaining dialect conformity as an adaptation to

113influence male-male interactions. If dickcissels conform to the local dialect to reduce aggression

114from conspecifics (deceptive mimicry), we expected dickcissels would respond more

6 115aggressively to playback of a foreign dialect than to playback of song from the local dialect. If

116dickcissels conform to the local dialect to be more readily recognized as conspecific, we

117expected that dickcissels would respond more aggressively to playback of an unfamiliar

118individual’s song from the local dialect than to song from a foreign dialect.

119

120

121Methods

122We conducted a playback experiment on male dickcissels in June and July 2014 at Konza Prairie

123Biological Station (KPBS) (39°05’ N, 96°35’ W, approximately 420 m elevation) near Manhattan,

124KS, USA.

125

126We recorded male dickcissel song onto digital media using a Marantz PMD680 recorder and a

127Sensheiser ME66 shotgun microphone. We recorded dickcissels from a distance of

128approximately 10 m for at least three song repetitions to increase the chance of recording at

129least one clear song from each bird with minimal background noise for use in playback trials.

130We edited songs using Raven 1.4 (Cornell Laboratory of Ornithology, Ithaca, NY, USA).

131

132Our primary goal was to compare responses of territorial male dickcissels between playback of

133local dialect song and highly divergent foreign dialects, but we also included a third treatment

134of playback of song from an adjacent neighbor. We added this third treatment because we had

135strong a priori expectation of a differential response to neighbor song versus non-neighbor song

136(e.g., Brooks & Falls 1975; Godard 1993; Wilson & Vehrencamp 2001; Hyman 2005; Briefer et

7 137al. 2008; Wei et al. 2011), and so this treatment could serve to establish the sufficiency of our

138protocol to detect differential responses to playback.

139

140We delineated each target male’s territory one to three days before a trial by repeatedly

141flushing him from song perches (Zimmerman & Finck 1989), recording GPS coordinates, and

142noting landmarks on a paper map. When repeatedly flushed in this manner, territorial male

143dickcissels reliably move among favored song perches on their territory and typically continue

144with singing with almost no pause. We did not band birds, as luring dickcissels into nets

145requires playback of song, so banded birds have previous experience with playback that might

146influence their response. Instead we carefully mapped each territory and returned within one

147or two days. Our (THP, WEJ) observations of within-season site fidelity of banded birds on the

148site over six field seasons demonstrated that males typically remain within the same territory

149over the course of a season. The daily apparent survival rate during the breeding season for

150territorial males on our site is 0.93 (unpublished data), meaning that that there is a 93% chance

151that a male will persist from one day to the next. Thus, we are confident that in nearly all cases,

152the bird receiving the playback treatment was the one whose territory was mapped during the

153previous three days. We scouted birds in clusters of three or four adjacent territories. The

154minimum closest distance between clusters was 205 m, and the mean closest cluster distance

155was 355 m.

156

157After birds were scouted, we determined whether each potential target bird showed a high

158degree of dialect conformity by qualitatively evaluating its song recordings in Raven. We

8 159considered males conformers if, based on careful examination of song spectrograms, a large

160majority of notes in their song closely matched, in terms of frequency, duration, and order,

161notes in the songs of its neighbors. We only used conformers in the experiment. This qualitative

162method of classifying song similarity is simple and highly repeatable in dickcissels (Schook et al.

1632008). More than 75% of birds we evaluated met this high standard of dialect conformity within

164their neighborhood. The songs that did not meet this standard were nearly all still quite similar

165and were readily identifiable as members of the broader cultural group found on our site. This

166stands in contrast to the foreign songs we recorded at other sites which differed dramatically

167from local songs.

168

169Our local song treatment was designed to mimic an unfamiliar individual singing a song that

170conformed to the local dialect. Because dickcissel dialects changes over relatively small

171geographic scales, we assumed that within an area of high conformity it was possible that any

172given male had heard the song of every other male. Thus to find an unfamiliar rendition of the

173local dialect, we planned to use songs recorded the previous decade because we assumed that

174these would likely have been sung by unfamiliar individuals. However, dialects had changed

175sufficiently that we could not find songs amongst old recordings that showed high conformity

176to modern dialects. Therefore we instead opted to create our local songs by combining the

177songs of multiple different individual males in a composite. Because we were comparing local

178songs to foreign songs and we wished to minimize any differences between these two

179treatments except song origin, we also created composite foreign song using the same method.

180

9 181When we created composite songs for our local and foreign song playback treatments (Fig. 1),

182we always combined songs from males whose songs had identical or nearly identical series of

183notes within phrases. Note conformity within phrases is very high among neighboring

184dickcissels, though the spacing of phrases is somewhat variable (Schook et al. 2008) (Fig. 1).

185When creating a composite song, we adopted the phrase spacing of one of the males whose

186song we were using, and replaced a subset of phrases and parts of phrases with the

187corresponding phrase portions from one or two other males. No stimulus male had its song

188used more than once in a given treatment category (neighbor, local, or foreign) to prevent

189pseudoreplication (Kroodsma 1989), though some songs used as a neighbor treatment in one

190cluster of males also contributed a phrase to a local treatment in another cluster of males. The

191parts of the local composite songs were from the same dialect as the target bird, but from sites

192on average 414 m away, and at least 100 m away (with one exception where one third of local

193song came from a non-adjacent territory ~60 m away) from the target bird’s territory. We

194expected that by using non-neighbor song, the focal birds would follow the pattern found in

195many species of treating playback of non-neighbor song more aggressively than playback of

196neighbor song, presumably because non-neighbor song is treated as a territorial invasion.

197Further, by creating composite songs, we hoped to reduce the chance that the focal male

198would recognize the song as coming from a particular known individual and thus treat that song

199differently than an unfamiliar local song. This precaution is rarely taken in playback studies

200comparing responses of local to foreign song, and this makes our method particularly robust to

201any bias that could stem from recognition of a local song as belonging to a particular familiar

202individual. Further, even without this precaution, studies usually detect greater aggression

10 203towards local song than towards foreign song (e.g., Lemon 1967; McGregor 1983; Podos 2007).

204As stated above, to control for effects of using composite local songs, we created composite

205foreign treatment song by splicing together songs of two different individuals (of the same

206foreign dialect from the same distant site) recorded previously between 2005 and 2011. The

207closest location of a foreign song recording to our study site was 22 km and the mean distance

208was 158 km. At this distance, songs show many dramatic differences (Schook et al. 2008) (Fig

2091). We recorded neighbor songs from males with territories adjacent to focal playback males.

210We included the neighbor song treatment only to assess whether males could respond

211differentially to different playback songs that were expected to induce a differential response

212(e.g., Godard 1993; Wilson & Vehrencamp 2001; Hyman 2005; Wei et al. 2011) and not because

213we wished to test hypotheses about the effects of neighbor songs per se. We did not

214manipulate neighbor song by cutting and splicing because we wanted to be sure the neighbor

215song was recognized as a song produced by a particular individual. We did, however,

216manipulate all recordings by eliminating sounds from other birds or from insects, and by

217filtering background noise below 2 kHz.

218

219To each target bird we played two of the three possible treatments, each on a different day. We

220initiated all trials between 06:00 and 09:00 CST. We did not conduct trials if it was raining or if

221wind appeared to be influencing male behavior (reduced perching and singing) or prevented us

222from making a clear audio recording of the trial. For all treatments, we placed the speaker near

223the target bird’s territorial boundary but within the estimated territory and facing the territory

224center. For birds receiving the neighbor treatment, we placed the speaker adjacent to the

11 225territory of the neighbor whose recorded song we were using for the playback. The observer

226stood at least 20 m behind the speaker to get the best view of the entire territory. We used an

227AmpliVox SW720 integrated amplifier and speaker and played each song to obtain peak volume

228of 90 dB as detected with a RadioShack Sound Level Meter at 1 m in front of the speaker. We

229tested the volume of each recording in the field, but >300 m from test territories, prior to

230playback. This produced song with high fidelity and an absence of noticeable distortion. We

231observed the target bird for one minute to confirm its presence on its territory, then played

232either a local, foreign, or neighbor song for two minutes. The song played at a rate of once

233every five seconds, the typical song rate for dickcissels (Schook et.al 2008). We continued to

234observe the target bird for two minutes after playback. Within 15 second intervals before,

235during, and after playback, we estimated the closest approach to the speaker, tallied the

236number of flights, and recorded an estimated average distance of the bird from the speaker.

237We determined number of songs by listening to an audio recording of the trial. We placed

238marking flags at four to eight points 10 m and 20 m away from the speaker to help estimate

239distance of the target bird from the speaker. We do not expect that playback has long-term

240detrimental impacts on dickcissel males. In our previous work that used playback to attract

241males to nets for capture, playback often lasted much longer than the two minutes imposed in

242our current study, and banded birds in those prior studies rarely abandoned territories and

243often returned to the site in later years. All song recording of focal birds, song classification and

244processing, and field trials were implemented by one researcher (M. Parra), and so it was not

245possible to blind the observer to the treatment category of the individual trial.

246

12 247We anticipated high variance among males in strength of response that would be unrelated to

248playback treatment, so each male received two treatments to allow us to partition variance

249resulting from differences among males in their tendency to respond to playback. We

250systematically distributed treatments across dates and thus dates did not differ among the

251three treatments (ANOVA: F2,61 = 0.09, p = 0.91). We also randomized treatment order amongst

252individual males and thus order (first vs. second) did not differ amongst the three treatments

2 253(GLM with binomial error: Wald χ 2= 0.09, p = 0.95). Because part of the among-male variance

254could come from positions of the speaker and the observer, we marked the position of the

255speaker and the observer with flags to be consistent for the second trial, which took place 1 to

2562 days after the first trial.

257

258We used generalized linear mixed models (SPSS version 23) to analyze responses to our

259playback experiment as measured by the number of songs, number of flights, and mean

260distance from the speaker. Because of the anticipated importance of partitioning variance

261among territories, we excluded the few trials in which the corresponding second trial on the

262given territory was never completed and we included individual territory identity as a random

263effect in all analyses. For the count data (# songs, # flights), we assumed a negative binomial

264distribution. However, the average distance to speaker was skewed right, so we performed a

265square root transformation which produced a distribution that did not differ substantially from

266normal (during playback: Kolmogorov-Smirnov statistic = 0.101, df = 64, p = 0.117; after

267playback: Kolmogorov-Smirnov statistic = 0.119, df = 64, p = 0.024), and thus is reasonable to

268analyze assuming normal error (McDonald 2014). We report 2-tailed p-values for all tests. In

13 269our first round of analyses, we assessed whether territorial male dickcissels appeared to

270respond to the song playback by comparing behavior among the three experimental time

271periods: before, during, and after the playback. For this comparison, we used a standardized

272per-two-minute rate. This meant doubling the one minute pre-playback counts. We limited our

273pre-playback counts to one minute to minimize the chance that we would lose track of the

274target individual before it was time to begin the trial. For both the count variables, values

275differed significantly between the during-playback time period and both before and after, but

276the before and after periods were not significantly distinguishable from each other (see

277Results). Thus we elected to compare responses to the different playback treatments (foreign,

278local, neighbor) with both count variables during, but not after, playback. Average distance to

279the speaker was significantly shorter during and after playback than before playback, but did

280not differ between the periods during and after playback (see Results). Thus we also chose to

281compare among the experimental treatments based on average distance to speaker, and we

282chose to use data from both during and after the playback. In the analyses of response to the

283three experimental treatments, we also included trial number as a fixed effect to account for

284any possible habituation effects in each male’s second trial. The analysis of the response of

285song rate to experimental treatment during playback was unsuccessful because the final

286Hessian matrix was not positive definite and so did not produce reliable results. In an attempt

287to create a successful statistical model assessing song response, we combined the during- and

288after-playback data and included trial identity nested within experimental treatment as a

289random effect and during versus after playback as a fixed effect. However, this model failed for

290the same reason and so we were unable to assess response to the experimental treatments as

14 291measured by song. We wanted to base our inferences on strengths of effects (Nakagawa &

292Cuthill 2007), and we wished to avoid the elevated risk of type I error that occurs when

293assessing a hypothesis with multiple comparisons. Thus, for all pairwise-comparisons of means,

294we calculated Hedge’s d, a standardized mean difference commonly used to assess strength of

295effect in meta-analysis. A common set of benchmarks is that Hedge’s d of approximately 0.2 is a

296weak effect, 0.5 is a moderate effect, and 0.8 is a strong effect (Rosenberg et al. 2013). We then

297calculated average Hedge’s d values and meta-analytical 95% confidence limits (controlling for

298non-independence among measures) for each set of comparisons testing the same general

299hypothesis (Nakagawa & Santos 2012). Our calculation of confidence limits accounted for non-

300independence by accounting for the correlations between pairs of different measures of

301dickcissel response. For instance, the greater the correlation between number of flights and

302distance to speaker across trials, the less independent information these two response

303variables contribute to the calculation of the confidence limits. In cases in which no correlation

304existed because the measures were based on different sets of trials (for instance, when

305comparing number of flights in response to neighbor song vs. number of flights in response to

306local song) we made the conservative choice to assign the highest correlation value in our

307correlation matrix thus reducing the degree to which those two measures contributed

308independent information to the calculation of our confidence limits. We calculated average

309Hedge’s d and associated confidence limits for (1) comparisons of before-playback behaviors to

310during-playback behaviors to assess the overall hypothesis that male dickcissels did respond to

311playback, (2) for comparisons of response to first versus second trials, (3) for comparisons of

312responses to playback of foreign versus local song, and (4) for comparisons of playback of

15 313neighbor song versus the other two treatments. When calculating averages we assumed that

314more flights and a lower average distance to the speaker indicated greater aggression. For

315songs, we assumed that a reduced rate during the trial and an elevated rate after the trial

316indicated greater aggression. We made this assumption based on pilot data from 2007 in which

317we observed dickcissel males reduce their song rate in response to playback and then elevate it

318relative to pre-playback levels as soon as the playback ceased. We observed a similar pattern in

319our 2014 data, though the post-playback song rate was not significantly greater than the pre-

320playback rate (see Results). Our averages of Hedge’s d provide robust tests of these hypotheses

321without elevated type I error from multiple comparisons.

322

323

324Results

325

326Territorial male dickcissels responded strongly to song playback treatments in their territory by

327approaching the speaker and remaining near it for the duration of the trial (Table 1, 2). Males

328also made more frequent flights and decreased their song rate during the two minutes of

329playback (Table 2). The absolute value of the effect size averaged across these response

330variables was moderate to large and convincing with very small confidence limits (Fig 2). During

331the two minutes after the playback ceased, males remained in close proximity to the speaker,

332but returned to approximately pre-playback rates of flight and song (Table 2).

333

16 334Male dickcissels responded less strongly, based on approach distance and number of flights,

335during the second playback trial relative to the first (Table 3, 4). The averaged effect size was

336moderate with small confidence limits that do not approach zero (Fig 2).

337

338Territorial male dickcissels responded almost identically to the local vs. foreign song playback

339treatment according to the three response variables assessed (Table 5). The average effect size

340was very close to zero and confidence intervals encompassed zero (Fig 2).

341

342However, focal males responded more strongly to the local and foreign treatments than to the

343neighbor treatment as measured by the number of flights and the mean distance to the

344speaker (Table 6). The averaged effect size was moderate to large with confident limits well

345clear of zero (Fig 2).

346

347

348Discussion

349

350Our results contradict the applicability of both the deceptive mimicry hypothesis and the

351recognition hypothesis in the dickcissel. The lack of differential response to local and foreign

352playback treatments suggests the apparent tendency of male dickcissels to selectively learn the

353local dialect does not result from selection related to territorial interactions. Thus, some other

354hypothesis is necessary to explain the delayed song learning that promotes dialect

355development in this species. Our results also raise important questions about why male

17 356dickcissels respond equally aggressively to local and to foreign dialects while territorial males in

357so many other species (e.g., Lemon 1967; McGregor 1983; Podos 2007; Osiejuk et al. 2012;

358Bradley et al. 2013) respond more aggressively to local song than to foreign song.

359

360We are confident in our observed lack of difference in response to local and foreign song

361playback for two reasons. First, dickcissels responded strongly to playback and thus were not

362simply ignoring both treatments. Second, we readily detected significantly reduced aggression

363towards playback of neighbor song relative to local and foreign dialect treatments. The dear-

364enemy effect (Brooks & Falls 1975) explains reduced aggression between individuals who have

365mutually established a territorial boundary. This is a widely recognized effect in birds (Godard

3661993; Wilson & Vehrencamp 2001; Hyman 2005; Wei et al. 2011), especially during the peak of

367the breeding season (Briefer et al. 2008) as in our study, and it is an effect not limited to birds

368(e.g., Davis 1987). Besides increasing confidence that we had the power to detect difference in

369response among playback treatments, the results of our comparisons of responses to neighbor

370versus stranger song are the first indication of the dear-enemy effect in the dickcissel. We

371would like to point out that our neighbor song treatment songs were unmanipulated whereas

372the two songs from our primary hypothesis tests were both spliced together from multiple

373individuals. As explained in the methods, we avoided manipulating the neighbor song because

374we wanted to be sure the neighbor was recognized as a particular individual. Because of this

375difference in song preparation, we cannot exclude the possibility that the difference in

376responses to the neighbor songs and the two other treatment categories was due to the

377difference in manipulation. However, this seems highly unlikely. Manipulation of the song

18 378should, if done poorly, weaken its effectiveness as a signal. Thus, we would have expected a

379weaker response to the manipulated songs if the manipulation itself were impacting their

380effectiveness as a signal. However territorial male dickcissels responded most strongly to the

381two manipulated song treatments.

382

383Although a failure to support the deceptive mimicry hypothesis is not a surprise, our results are

384in contrast to the many studies that have found reduced aggression to foreign songs (e.g.,

385Lemon 1967; McGregor 1983; Podos 2007; Osiejuk et al. 2012; Bradley et al. 2013) as predicted

386by the recognition hypothesis. Our failure to support the recognition hypothesis in one species

387does not disprove the applicability of this hypothesis more widely, but the recognition

388hypothesis has occasionally been contradicted in other species (e.g., Hansen 1984; Colbeck et

389al. 2010), and it does raise the possibility that the recognition hypothesis is not a general

390explanation for the evolution of dialect conformity.

391

392If the selection pressure promoting delayed song learning in dickcissels does not arise from

393recognition by conspecific males, selection from female choice may drive conformity (Danner et

394al. 2011). In some systems, female preference for males conforming to the local dialect could

395serve to promote female mating with locally adapted males (Searcy et al. 2002). This

396mechanism is unlikely in the dickcissel, however, where both males and females typically

397disperse beyond the range of a typical dialect, males appear to learn their song after dispersal,

398and the geographic extent of dialects is almost certainly much smaller (Schook et al. 2008) than

399the extent of any locally adapted gene complexes. However, degree of conformity to the local

19 400dialect could be a function of male learning ability and thus could signal some aspect of male

401quality. In this scenario, selection could favor females who discriminate among males based on

402dialect conformity (Nowicki et al. 1998). Even if dialect conformity is not a signal related to male

403quality, it could still be a target of female aesthetic preference and thus could act as a sexual

404ornament (Prum 2010), or the clustering of similar signals could itself be attractive to females

405(Wagner 1998). We have not yet investigated the relationship between female choice and male

406dialect conformity in dickcissels.

407

408We also cannot rule out the possibility that delayed song learning in male dickcissels is not

409currently adaptive. The dickcissel is in a monotypic genus and so we cannot compare patterns

410of song learning in congeners, but in the closely related genus Passerina (Klicka et al. 2007),

411males appear to also exhibit delayed song learning (reviewed in Payne 2006; Greene et al.

4122014). Thus, it may be that delayed song learning was favored in the past and has persisted as

413a phylogenetic legacy in the absence of modern selection.

414

415Although our original goal was to assess how selection might act to favor learning of local

416songs, we might also ask about the selection pressures leading males to respond aggressively to

417various types of song playbacks. Some researchers who have detected greater responses of

418territorial males to local than to foreign song playbacks have hypothesized that this differential

419response may be an adaptation to invest more in aggression towards males who pose the

420greatest territorial threat (Tomback et al. 1983; Wright & Dorin 2001; Nicholls 2008; Turcokova

421et al. 2011). If foreign birds are just passing through or are unlikely to attempt to settle, then

20 422investing in challenging those individuals might be selected against. However, in the dickcissel it

423is not uncommon for a male with a foreign dialect to settle in a neighborhood otherwise

424occupied by dialect conformers (our personal observations). These non-conformers are likely

425those males, well documented in dickcissels (Schartz & Zimmerman 1971; Zimmerman & Finck

4261989), who have abandoned unsuccessful territories elsewhere. Although dramatically non-

427conforming dickcissels are, in our experience, a minority (when both moderate and dramatic

428non-conformers were tallied together, they constituted 5.5% of 183 monitored territories this

429site), they may be common enough to provide a selection pressure that favors a territorial male

430who directs equal amounts of aggression to any conspecific male he encounters, regardless of

431dialect.

432

433Another component of this selection environment comes from the risk of mistakenly

434responding to the song of a heterospecific (Hamao 2016). If males commonly encounter other

435species with similar songs, and if responding aggressively to heterospecific singers carries a

436cost, then selection may push males to constrain the acoustic space that triggers a response in

437such a way that reduces the risk of responding to heterospecifics. This selection could then lead

438to reduced response to foreign conspecific dialects as a by-product (Hamao 2016). Dickcissels

439are the sole species in their genus, and sing a song that is, to our ears, quite distinct from the

440songs of other birds in the region. Thus it may be that dickcissels simply do not face much

441selection from mistaken response to heterospecific song, and thus have evolved to respond to

442song from a relatively broad range of acoustic space.

443

21 444Another non-exclusive alternative explanation is that males learn to recognize features of

445conspecific songs through interactions with conspecifics, and individuals who have encountered

446few dialects less readily recognize foreign dialects as representing a threat (Wright & Dorin

4472001). In many species, for instance those which do not migrate or in which individuals from

448different dialects migrate to different wintering grounds, individuals may have very little

449exposure to foreign songs and thus when they hear playback of a foreign song it may serve as a

450less effective stimulus of territorial aggression. In contrast, a species in which individuals

451frequently encounter singing conspecifics from many dialects during migration or winter may

452respond to a more diverse array of dialects when back defending a territory on the breeding

453grounds (Colbeck et al. 2010). Dickcissels appear likely to fall into this latter category. They

454aggregate in flocks of many thousands in winter and are reported to sing in these flocks

455(Temple 2002). However, it is important to point out that recognition of foreign dialects does

456not appear to require prior experience with the particular dialect in question even in species

457which show reduced aggression to foreign vs. local dialects. First, in those cases where

458response to playback of foreign conspecific dialects versus heterospecific song has been

459investigated, males typically show greater aggression to foreign conspecific song than to

460heterospecific song (Slabbekoorn & Smith 2002). Thus, there is some recognition of these

461foreign dialects, just a reduced response relative to the local dialect. Further, we know that in

462some species, naïve juveniles preferentially learn conspecific song over song of other species

463(Marler & Peters 1977) or even other sub-species (Nelson 2000). Thus, it seems that ability to

464recognize unfamiliar conspecific song may be widespread. So, if familiarity with foreign dialects

465increases the strength of response to foreign dialects, this might not be based on recognition

22 466per se but it could still result from some form of learning. For instance, individuals might learn

467to react more strongly if they have previously interacted with males singing a particular dialect.

468Or, individuals may not require experience with specific dialects, but exposure to various

469dialects may expand the multivariate acoustic space that induces aggressive responses, even if

470the particular dialect they are confronting is novel. However, in one of the few species where

471there is published evidence relevant to this hypothesis, migratory Puget Sound white-crowned

472sparrow (Zonotrichia leucophrys pugetensis) individuals not only recognize foreign song but also

473can temporarily borrow foreign phrases during migration (DeWolfe & Baptista 1995), and yet

474birds from this population still react more strongly to local than to foreign song (Nelson & Soha

4752004).

476

477Because dickcissels experience both territorial threats from males singing foreign songs and

478exposure to many foreign dialects in the non-breeding season, and because they likely face

479little selection from mistaken response to heterospecifics, we cannot rule out any of these

480potential explanations outlined above explaining why dickcissel males respond equally

481aggressively to foreign and local dialects while most other species respond more aggressively to

482local dialects. An effective comparison of these hypotheses could come from a cross-species

483meta-analysis of strength of response to local and foreign song playbacks combined with

484sufficient natural history information regarding the similarity of heterospecific songs, the

485tendency for males to disperse across dialect boundaries, and the tendency to hear a diversity

486of conspecific dialects.

487

23 488We have found evidence consistent with the existence of the dear-enemy effect in dickcissels,

489ruled out two adaptive hypotheses that might explain delayed song learning in this species, and

490we have raised important questions regarding the factors that lead male dickcissels to respond

491equally aggressively to both foreign and local dialect playback. Although meta-analysis could

492help answer questions regarding why some bird species show reduced response to foreign song

493versus local song, there remain major unanswered questions across songbirds and within

494dickcissels. Despite studies in a variety of species showing that subsets of songs can signal

495species identity and thus facilitate identification of conspecifics, the processes producing

496recognition and classification of song, and leading to differential responses to these different

497songs, remains poorly known across songbird species and within dickcissels.

498

499Acknowledgments

500Support for this project came from a Whitman College Perry Award and the Kansas State

501University Biology National Science Foundation (NSF) Research Experience for Undergraduates

502(REU) program (DBI-0552930 and DBI-1156571). Access to research sites was provided by Konza

503Prairie Biological Station. We would like to thank B. Sandercock and the members of his lab for

504their generous support of our dickcissel research over the past decade. B. Sandercock and B.

505Snyder both generously facilitated our participation in the REU program. Additional foreign

506songs were recorded by K. Ballinger, C. Foo, H. Kahl, E. Ross and P. Williams. S. Nakagawa

507provided assistance with the calculation of average effect sizes. This work was approved by the

508Whitman College Institutional Animal Care and Use Committee (protocol number: 042114wctp)

509and complies with the current laws of the United States.

24 510

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627

29 628

629

30 630Table 1. Tests from generalized linear mixed models for the overall effects of time period (pre-

631playback, playback, post-playback) on each of the following three response variables, each in a

632separate analysis: number of songs, number of flights, and average distance from the speaker.

633Each analysis included territory identity as a random effect.

634

response variables F df df p # songs 6.19 5 189 0.002 # flights 26.40 2 189 <0.001 average distance 19.93 2 189 <0.001

random effect of territory Z p

(from tests of): # songs 2.11 0.035 # flights 1.92 0.056 average distance 2.97 0.003 635

636

31 637Table 2. Comparisons of mean measures, derived from generalized linear mixed models, of

638dickcissel response (number of songs, number of flights, and average distance from the

639speaker) among three time periods (before playback, during playback, after playback). We

640assigned positive values to Hedge’s d for cases of greater aggression during playback than

641before or after playback, or greater aggression after playback than before playback.

642

variable mean SD n mean SD n t df p Hedge’s variance

d of d before playback during playback # songs 10.477 11.200 64 7.263 7.912 64 2.2 189 0.03 0.330 0.032 # flights 1.365 1.800 64 4.621 5.120 64 3.2 189 <0.001 0.843 0.034 distance 6.064 1.704 64 4.900 1.704 64 5.5 189 <0.001 0.679 0.033 before playback after playback # songs 10.477 11.200 64 12.643 13.416 64 1.2 189 0.24 0.174 0.031 # flights 1.365 1.800 64 1.677 2.128 64 1.0 189 0.30 0.157 0.031 distance 6.064 1.704 64 4.924 1.704 64 5.4 189 <0.001 0.665 0.033 during playback after playback # songs 7.263 7.912 64 12.643 13.416 64 3.2 189 0.002 0.485 0.032 # flights 4.621 5.120 64 1.677 2.128 64 4.7 189 <0.001 0.746 0.033 distance 4.900 1.704 64 4.924 1.704 64 0.1 189 0.91 0.014 0.031 643

644

645

32 646Table 3. Tests with generalized linear mixed model analyses of the effects of treatment (foreign,

647local, and neighbor song) and trial number (first or second trial for each male) on number of

648flights during playback, and mean distance to the speaker (during and after playback). Each

649analysis included territory identity as a random effect.

X F d.f. p # of flights during playback overall model 5.18 3, 60 0.003 treatment 3.88 2, 60 0.026 trial # 8.65 1, 60 0.005 territory ID (random) Z = 0.77, p = 0.441 mean distance during playback overall model 4.36 3, 60 0.008 treatment 4.81 2, 60 0.012 trial # 3.86 1, 60 0.054 territory ID (random) Z = 2.02, p = 0.044 mean distance after playback overall model 2.84 3, 60 0.046 treatment 2.80 2, 60 0.069 trial # 3.21 1, 60 0.078 territory ID (random) Z = 1.00, p = 0.315 650 651

652

33 653Table 4. Comparisons of mean measures of dickcissel response (number of flights during

654playback, average distance from the speaker during playback, and average distance from the

655speaker after playback) to a first playback trial versus a second playback trial. We assigned

656positive values to Hedge’s d for cases of greater aggression during the first trial.

657

first second variable mean SD N mean SD N t df p Hedge’s variance

d of d # flights 5.909 4.435 32 3.415 2.834 32 2.8 60 0.006 0.662 0.066 distance during 4.642 1.375 32 5.169 1.375 32 2.0 60 0.054 0.379 0.064 distance after 4.588 1.680 32 5.268 1.680 32 1.8 60 0.08 0.400 0.064 658

659

660

34 661Table 5. Comparisons of mean measures of dickcissel response (number of flights during

662playback, average distance from the speaker during playback, and average distance from the

663speaker after playback) to playback of foreign song versus non-neighbor local song. We

664assigned positive values to Hedge’s d for cases of greater aggression to foreign song.

665

foreign local variable mean SD N mean SD N t df p Hedge’s variance

d of d # flights 5.701 4.278 22 5.218 3.991 21 0.4 60 0.69 0.115 0.093 distance during 4.550 1.332 22 4.625 1.329 21 0.2 60 0.83 0.055 0.093 distance after 4.643 1.670 22 4.551 1.668 21 0.2 60 0.85 -0.054 0.093 666

667

35 668Table 6. Comparisons of mean measures of dickcissel response (number of flights during

669playback, average distance from the speaker during playback, and average distance from the

670speaker after playback) to playback of neighbor song versus playback of either foreign (F) or

671non-neighbor local (L) song.

672

foreign or local neighbor variable mean SD N mean SD N t df p Hedge’s variance

d of d # flights (F) 5.701 4.278 22 3.046 2.584 21 2.5 60 0.01 0.733 0.099 # flights (L) 5.218 3.991 21 3.046 2.584 21 2.1 60 0.04 0.634 0.100 distance during (F) 4.550 1.332 22 5.541 1.329 21 2.8 60 0.007 0.723 0.097 distance during (L) 4.625 1.329 21 5.541 1.329 21 2.6 60 0.01 0.676 0.101 distance after (F) 4.643 1.670 22 5.591 1.668 21 2.0 60 0.054 0.558 0.097 distance after (L) 4.551 1.668 21 5.591 1.668 21 2.1 60 0.04 0.612 0.100 673

674

36 675Figure legends

676Figure 1. Examples of playback songs used in a single neighborhood. Note the high note

677conformity within phrases but the variability in phrase number and phrase spacing between the

678unmanipulated ‘neighbor’ songs, as is common in dickcissels. Note also the close match in

679notes between the neighbor and local treatments and the radical differences when compared

680to the foreign songs.

681

682Figure 2. Forest plot of average standardized differences (Hedge’s d, ± 95% confidence limit)

683between mean dickcissel responses (number of songs [in first comparison only], number of

684flights, closest approach distance) before versus during playback, during first versus second

685trials, during and after playback of foreign versus local song, and during and after playback of

686neighbor versus either foreign or local song. Hedge’s d values between 0.5 and 0.8 are

687considered moderate to large. Thus, dickcissels responded strongly to playback trials, though

688more strongly in first than in second trials. Further, there was no detectable difference in

689response to playback of foreign versus local songs, though both foreign and local songs elicited

690much more vigorous responses than did playback of neighbor song.

691

692

37 693Fig 1

694

38 695

696Fig 2

697