Volume 30 Number 17 September 7, 2020 Volume 30 Number 17 Pages R000–R000; 000–000 September 7, 2020

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Correspondence generations) elicit violent fi ghts or ‘power consumption (Figure 1A). Stored acorns struggles’, among multiple same-sex are consumed by adults when food is Tracking the warriors coalitions from neighboring groups. Here, scarce and are also fed to nestlings. using an automated radio-telemetry Granaries are pilfered by intra- and and spectators of system, we found that individuals in interspecifi c competitors and are coalitions competing for breeding thus zealously defended by all group acorn woodpecker vacancies — the ‘warriors’ — invested members. Large-granary territories are wars up to ten hours per day on successive often controlled by polygynandrous days before one coalition emerged groups consisting of multiple male and victorious. Power struggles also attracted female breeders and their non-breeding 1,4, 1 Sahas Barve *, Ally S. Lahey , ‘spectators’— acorn woodpeckers not offspring (‘helpers’). Same-sex co- 2 Rebecca M. Brunner , eligible to fi ll the breeding vacancy. breeders are closely related to each other 3 1 Walter D. Koenig , and Eric L. Walters Apparently present only to gain social but unrelated to breeders of the opposite information, spectators travelled from sex [3]. In addition to within-group Although intergroup confl ict is territories as far as over three kilometers dynamics, acorn woodpeckers recognize widespread in vertebrates, simultaneous away. Our study reveals the complexity associations among individuals outside agonistic interactions among several of acorn woodpecker social group their group and track membership groups are rare [1]. Acorn woodpeckers networks, demonstrating the signifi cant changes in surrounding territories [4]. (Melanerpes formicivorus) are effort of both warriors and spectators to This information transfer is likely to occur cooperatively breeding birds that defend pursue fi tness benefi ts and obtain social via numerous daily off-territory forays to acorn storage facilities (‘granaries’), information. neighboring territories [5]. which provide signifi cant survival and Acorn woodpecker groups live A typical way that non-breeding reproductive benefi ts to breeders in on year-round territories defi ned by helpers obtain a breeding position is the social group [2]. Breeder vacancies granaries, trees with hundreds to by fi lling a breeder vacancy in a non- in high-quality territories (i.e., large thousands of holes, drilled by the natal territory. In our California study granaries accrued over multiple birds, where they store acorns for later population, same-sex coalitions of

A B C

600 600

400 400 * 200 200 Minutes present Time present (min)

0 0 0 1000 2000 3000 Warriors Spectators Distance travelled (m) D E 3000 5

2000 * 3 N days 1000

1

Distance from home territory (m) 0 Warriors Spectators Warriors Spectators

Figure 1. Acorn woodpecker behavior at power struggles. (A) ‘Spread-wing’ display by a female acorn woodpecker on a granary (photo: © Bruce Lyon). (B–E) Variation in individual investment in power strug- gles. Black points represent the mean for each variable; error bars denote standard errors. Asterisks indicate statistically signifi cant differences. (B) Birds from >3 km away visited power struggle sites. Colored points indicate time spent (min) attending power struggles by each sex-status cat- egory as a function of distance to home territory for warriors (yellow = helper females), and spectators (purple = breeder females, green = breeder males, red = helper males). (C) Helper female warriors spent signifi cantly longer times (P < 0.05) at power struggles compared to spectators (birds present but not competing). (D) Warriors attending power struggles came from groups signifi cantly closer than spectators. (E) There was no differ- ence (P > 0.05) in the duration of attendance (days) by individual warriors and spectators at a power struggle.

R982 Current Biology 30, R963–R983, September 7, 2020 © 2020 Elsevier Inc. ll Magazine helpers will fi ght against a dozen or more expected, warriors spent the most time ACKNOWLEDGMENTS competing coalitions (40 or more birds) at the power struggles: helper females for a breeding vacancy, with winners co- (total N = 13) attended power struggles We thank David Winkler for help with the breeding at the new territory [6]. Typically, for nearly 113 minutes longer per day radio-telemetry technology. Russell Winter, a quarter of the about 50 groups we (mean ± SE 112.8 ± 28.5 min) than Emily Goldberg, Megan Massa and Hannah follow have a breeding vacancy in a spectators (N = 23, P < 0.001, Figure 1C). Horowitz assisted with fi eldwork. We thank given year; although long, violent power During one power struggle, two helper Dr. Brandt Ryder and an anonymous reviewer struggles that attract a large number of female coalition members returned over for constructive comments. We thank the National Science Foundation for funding birds tend to happen at territories with four consecutive days, staying over ten (grants IOS-1455881 and IOS-1455900). big granaries [7]. Such power struggles hours each day (Figure 1C), but did not can last for several days and involve win the power struggle; an untagged spread-wing displays (Figure 1A), female coalition ultimately won the REFERENCES incessant calling, and intense physical — breeding position. Such a continuous 1. Kitchen, D.M., and Packer, C. (1999). Complexity sometimes fatal — fi ghts [6]. A key presence at these confl icts demonstrates in vertebrate societies. In Levels of Selection to understanding the factors driving a remarkable willingness to expend in Evolution, L. Keller, ed. (Princeton: Princeton the success of a coalition in achieving intense short-term effort for potential University Press), pp. 176–196. 2. Koenig, W.D., Walters, E.L., Stacey, P.B., breeding status involves quantifying the access to the long-term benefi ts of Stanback, M.T., and Mumme, R.L. (2020). Acorn effort expended by coalition members a breeding position at a high-quality Woodpecker (Melanerpes formicivorus), version 1.0. In Birds of the World, P.G. Rodewald, and that travel to, and fi ght at, power territory. B.K. Keeney, eds. (Ithaca, NY: Cornell Lab of struggles. However, visually monitoring Spectators spent almost an hour Ornithology), https://doi.org/10.2173/bow. behavior at power struggles — especially per day attending power struggles acowoo.01 3. Koenig, W.D., Walters, E.L., and Haydock, J. individual investment — is diffi cult due to (mean ± SE = 52.1 ± 10.4 min/day, (2016). Acorn woodpeckers: helping at the the chaotic nature of these confl icts. range 1–462 min; Figure 1B,D). This nest, polygynandry and dependence on a variable acorn crop. In Cooperative Breeding in Using an automated radio-telemetry suggests that maintaining current Vertebrates: Studies of Ecology, Evolution and system [5], we tracked 36 acorn information within the acorn woodpecker Behavior, W.D. Koenig, and J.L. Dickinson, eds. woodpeckers that attended three power social network is worth leaving a home (Cambridge: Cambridge University Press), pp. 217–236. struggles (2018: May, Aug.; 2019: territory unattended for considerable 4. Pardo, M.A., Sparks, E.A., Kuray, T.S., Apr.). Because each power struggle periods of time. Warriors came from Hagemeyer, N.D., Walters, E.L., and Koenig, W.D. (2018). Wild acorn woodpeckers was triggered by a female breeder group territories that were signifi cantly recognize associations between individuals in vacancy, we expected helper females closer (mean ± SE = 644 ± 136 m) than other groups. Proc. R. Soc. B 285, 20181017. to invest the most effort as warriors spectators (1432 ± 167 m) (P < 0.001, 5. Barve, S., Hagemeyer, N.D., Winter, R.E., Chamberlain, S.D., Koenig, W.D., Winkler, D.W., [6,7]. Females with a breeding position Figure 1D). Additionally, there was no and Walters, E.L. (2020). Wandering at another group, as well as any males, difference in the number of days warriors woodpeckers: foray behavior in a social bird. Ecology 101, e02943. were considered spectators, since and spectators visited any one power 6. Hannon, S.J., Mumme, R.L., Koenig, W.D., and such individuals were not relevant to struggle site (P > 0.05, Figure 1E). Pitelka, F.A. (1985). Replacement of breeders the female vacancy. Although not the Spectators are thus clearly willing to and within-group confl ict in the cooperatively breeding acorn woodpecker. Behav. Ecol. case with tagged birds in this study, it repeatedly travel considerable distances, Sociobiol. 17, 303–312. is possible that such individuals were apparently even farther than those 7. Koenig, W.D. (1981). Space competition in the acorn woodpecker: power struggles in a assisting helper coalitions from their competing for the vacancy, to gather cooperative breeder. Anim. Behav. 29, 396–409. home groups (their own offspring). social information. 8. Schliehe-Diecks, S., Eberle, M., and Given the tradeoff between gaining Our study not only demonstrates Kappeler, P.M. (2012). Walk the line—dispersal movements of gray mouse lemurs (Microcebus information at a power struggle vs. the signifi cant effort invested by some murinus). Behav. Ecol. Sociobiol. 66, 1175–1185. defending a home territory [5], we did individuals to ensure long-term fi tness 9. Ryder, T.B., Dakin, R., Vernasco, B.J., Evans, B.S., Horton, B.M., and Moore, I.T. not expect to detect many spectators at benefi ts, but also reveals that social (2020). Testosterone modulates status-specifi c power struggles. We used linear mixed birds — including those that already patterns of cooperation in a social network. Am. models (Supplemental information) to have a breeding position — foray well Nat. 195, 82–94. 10. Papageorgiou, D., Christensen, C., Gall, G.E., test whether a bird’s role as a warrior beyond their home territory to gather Klarevas-Irby, J.A., Nyaguthii, B., Couzin, I.D., or spectator explained variation in time social information [8]. Automated radio- and Farine, D.R. (2019). The multilevel society of a small-brained bird. Curr. Biol. 29, R1120– spent at power struggles (i.e., number telemetry is thus a powerful tool that R1121. of minutes a tag was detected by a can help reveal individual investment receiver at the granary), and distance in complex social behaviors like power 1 traveled to reach power struggles from struggles; future studies should link such Department of Biological Sciences, Old Dominion University, Norfolk, VA 23529, USA. home territories. social events to the fl ow of information 2Department of Environmental Science, Policy, The three power struggles attracted through social networks [9,10]. and Management, University of California, about a third (31 ± 7%) of all radio- Berkeley, CA 94720, USA. 3Hastings tagged birds in our study area SUPPLEMENTAL INFORMATION Natural History Reservation, University (N = 41/61/73) at the time of each event. of California Berkeley, Carmel Valley, CA 4 Some birds visited power struggles from 93924, USA. Current address: Division of Supplemental Information includes one fi gure Birds, Department of Vertebrate Zoology, over three kilometers away, close to the and experimental procedures and can be Smithsonian National Museum of Natural maximum distance between any two found with this article online at https://doi. History, Washington, DC 20560, USA. groups in our study area (Figure 1B). As org/10.1016/j.cub.2020.07.073. *E-mail: [email protected]

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