Acorn Woodpecker Research Articles

Active facilitation of helper dispersal by parents and siblings in the cooperatively breeding acorn woodpecker

Offspring that delay dispersal in cooperatively breeding species have been hypothesized to gain direct fitness benefits via parental facilitation—being passively tolerated on their natal territory by their parents—thereby enjoying enhanced survival and increased probability of acquiring a breeding position in the population. Here we describe active facilitation in the acorn woodpecker (Melanerpes formicivorus) by parents and siblings assisting the dispersal of helpers in their social group. Helpers in this species compete for reproductive opportunities in “power struggles” that take place when all breeders of one sex die or disappear, creating a reproductive vacancy. Individuals compete at power struggles in coalitions of relatives, in which larger coalitions are more likely to be victorious. Based on observations of banded birds, we found that an estimated 26% of individuals competing as part of a winning coalition at a power struggle returned to their home territory at its conclusion, suggesting that they were facilitating the dispersal of kin (generally parents or siblings) that stayed to become breeders on the new territory. In at least one group, sibling facilitation was reciprocated; that is, a bird that was helped at a power struggle by a sibling joined that same sibling as part of a coalition at a subsequent power struggle. Dispersal facilitation is a novel means by which parents can nepotistically enhance the direct fitness of offspring and siblings can enhance each other’s inclusive fitness in this highly social species.Significance statementParental facilitation—being passively tolerated on the natal territory—may provide significant direct fitness benefits to helpers in cooperatively breeding species. We describe active facilitation of helper dispersal in the acorn woodpecker, where helpers compete in coalitions for reproductive opportunities at “power struggles” following the death or disappearance of all breeders of one sex. About one-fourth of individuals—including both parents and siblings—competing at power struggles were apparently facilitators who assisted related helpers by participating in the power struggle but then returned to their home territory rather than stay to breed on the new territory. In at least one group, dispersal facilitation was reciprocated; that is, a bird that was helped at a power struggle by a relative later joined that same relative as part of a coalition at a subsequent power struggle. Active dispersal facilitation by parents and siblings is an important, previously unrecognized, form of nepotistic behavior in this highly social species.

Acorn woodpecker movements and social networks change with wildfire smoke

Climate change has contributed to increased wildfires1,2. Wildfire smoke exposes wildlife to hazards and mortality from particulate matter on a scale larger than the area impacted by fire3,4. Using automated radiotelemetry, we illustrate how smoky conditions are associated with changes in behavior of acorn woodpeckers (Melanerpes formicivorus), a flagship species of oak (Quercus spp.) savannas of western North America. On smoky days, birds spent more time at their home territory and reduced visitation to others, especially to distant territories. Associations between birds decreased, and individuals were less assorted by group in co-visitation networks, suggesting less inter-individual coordination on smoky days. We show that between 2016 and 2020, ∼14% of the acorn woodpecker population in the US experienced fire, potentially exposing on average 89.42% of the range to atmospheric smoke annually. These findings highlight how potential effects of smoke on animal behavior may be widespread and exacerbate negative impacts of increasingly common "megafires", even in fire-adapted ecosystems.

Territory inheritance and the evolution of cooperative breeding in the acorn woodpecker

Territory inheritance and the evolution of cooperative breeding in the acorn woodpecker

Lifetime inclusive fitness effects of cooperative polygamy in the acorn woodpecker

Although over 50y have passed since W. D. Hamilton articulated kin selection and inclusive fitness as evolutionary explanations for altruistic behavior, quantifying inclusive fitness continues to be challenging. Here, using 30y of data and two alternative methods, we outline an approach to measure lifetime inclusive fitness effects of cooperative polygamy (mate-sharing or cobreeding) in the cooperatively breeding acorn woodpecker Melanerpes formicivorus. For both sexes, the number of offspring (observed direct fitness) declined while the number of young parented by related cobreeders (observed indirect fitness effect) increased with cobreeding coalition size. Combining these two factors, the observed inclusive fitness effect of cobreeding was greater than breeding singly for males, while the pattern for females depended on whether fitness was age-weighted, as females breeding singly accrued greater fitness at younger ages than cobreeding females. Accounting for the fitness birds would have obtained by breeding singly, however, lifetime inclusive fitness effects declined with coalition size for males, but were greater for females breeding as duos compared to breeding singly, due largely to indirect fitness effects of kin. Our analyses provide a road map for, and demonstrate the importance of, quantifying indirect fitness as a powerful evolutionary force contributing to the costs and benefits of social behaviors.

Fungal communities associated with acorn woodpeckers and their excavations

Wood-decay fungi soften wood, putatively providing opportunities for woodpeckers to excavate an otherwise hard substrate, yet the fungal community composition in tree cavities and the specificity of these relationships is largely unknown. We used high-throughput amplicon sequencing of the fungal ITS2 region to examine the fungal communities associated with acorn woodpeckers ( Melanerpes formicivorus ) and their cavities in mature valley oak ( Quercus lobata ) and blue oak ( Q. douglasii ) trees in an oak savannah of central coastal California, USA. Acorn woodpeckers and their excavations harbored over 1500 fungal taxa, including more than 100 putative wood-decay fungi. The fungal communities found on the birds were more similar to those found in excavated cavities than those found in trees without excavated holes. These results suggest that symbiotic associations between acorn woodpeckers and fungi are highly diverse, with low specificity. Symbiotic associations between cavity-excavators and fungi are likely more common and widespread than previously thought.

Structure and divergence of vocal traits in the Acorn Woodpecker (Melanerpes formicivorus)

Studies of geographic variation of bird vocalizations facilitate the understanding of species' divergence and evolutionary histories, as vocal traits vary in response to different factors including the environment, morphology, culture, and inheritance. The Acorn Woodpecker (Melanerpes formicivorus) is a non-passerine species of the family Picidae, and therefore its vocalizations are not acquired through learning. It is widely distributed throughout the Americas and exhibits distinctive morphological and genetic differences among the 7 allopatric subspecies, but little is known about geographic variation in the structure of its vocalizations and whether vocal variation corresponds with their genetic differences. We collected recordings throughout the species' range and assessed the frequency and temporal features of their most common calls to study geographic variation in vocalizations. Specifically, we tested whether divergence in vocal traits mirrored subspecies limits. Our results showed the formation of 2 vocal groups that do not reflect subspecies limits. The genetic divergence described in previous studies coincides with the vocal divergence found in this study, with 2 areas promoting the greatest divergence: the Isthmus of Tehuantepec and the Gulf of California. Previously described morphological variation in bill sizes also coincides with the vocal groups found in this study, in which large and small sizes are grouped separately.

Avian Abundance and Diversity on Knoxville Wildlife Area in California Following the County Fire of 2018

Wildfires can have negative impacts on wildlife during and immediately after a burn event, yet these fires are also necessary for plant succession and biodiversity. Knoxville Wildlife Area (KWA) in Napa County, California, USA is a diverse environment that contains oak woodlands, chaparral, grasslands, and riparian corridors. In July 2018, the County Fire burned 36,353 ha in the Northern California Interior Coast Ranges, including 2,429 ha on KWA. California Department of Fish and Wildlife employees used this opportunity to monitor avian abundance and diversity as the burned area revegetates. From 10 June through 14 June 2019, we began a pilot season of distance-based point counts on KWA. We conducted 5-minute point counts at 80 points spaced 1 km apart. Twenty points were within the burned area. We used the half cosine model in Program Distance version 7.2 to determine density for the 5 most frequently detected species. We detected 47 avian species; the most frequently detected species were California quail (Callipepla californica), acorn woodpecker (Melanerpes formicivorus), mourning dove (Zenaida macroura), California scrub-jay (Aphelocoma californica), and oak titmouse (Baeolophus inornatus). California quail populations were slightly denser in the burned area (0.22 birds/ha, 95% confidence interval [CI] = 0.11–0.44), compared to the nonburned area (0.19 birds/ha, 95% CI = 0.12–0.31). Acorn woodpecker density was similar in the burned (0.59 birds/ha, 95% CI = 0.34–1.02) and nonburned areas (0.58 birds/ha; 95% CI = 0.42–0.80). Mourning dove, California scrub-jay, and oak titmouse populations were slightly denser in the nonburned area; however, 95% CIs overlapped. We conducted approximately approximately 1 year post-burn, and revegetation had started, due in part to the wet winter and spring. Birds were using the burned area, though not at significantly higher densities compared to the rest of the wildlife area. The coronavirus pandemic and a second fire prevented us from surveying the site in 2020 and 2021, but we intend to conduct point counts for at least 2 more years to further monitor avian response to wildfire.

Constructing social networks from automated telemetry data: A worked example using within‐ and across‐group associations in cooperatively breeding birds

Abstract Advances in data‐logging technologies have provided a way to monitor the movement of individual animals at unprecedented spatial and temporal scales. When used in conjunction with social network analyses, these data can provide deep insight into the structure and dynamics of animal social systems. Emergence of these new technologies demands concomitant progress in workflows to translate data streams from automated systems to social networks, based on biologically relevant metrics. Here we outline key considerations for constructing social networks from automated telemetry data. We highlight the need for paying particular attention to the spatial arrangement of receiver stations with respect to the ecology of study system and developing appropriate criteria for quantifying associations. We provide a case study for constructing social networks from automated telemetry data collected over 1 month during a study of acorn woodpeckers Melanerpes formicivorus, a cooperatively breeding bird. The data consisted of detections of known birds near receiver stations placed within core areas of group territories. We use this system to demonstrate how to build social networks to investigate biological questions about patterns of associations between group members and territory visitors across the landscape.

Lifetime reproductive benefits of cooperative polygamy vary for males and females in the acorn woodpecker (Melanerpes formicivorus).

Cooperative breeding strategies lead to short-term direct fitness losses when individuals forfeit or share reproduction. The direct fitness benefits of cooperative strategies are often delayed and difficult to quantify, requiring data on lifetime reproduction. Here, we use a longitudinal dataset to examine the lifetime reproductive success of cooperative polygamy in acorn woodpeckers (Melanerpes formicivorus), which nest as lone pairs or share reproduction with same-sex cobreeders. We found that males and females produced fewer young per successful nesting attempt when sharing reproduction. However, males nesting in duos and trios had longer reproductive lifespans, more lifetime nesting attempts and higher lifetime reproductive success than those breeding alone. For females, cobreeding in duos increased reproductive lifespan so the lifetime reproductive success of females nesting in duos was comparable to those nesting alone and higher than those nesting in trios. These results suggest that for male duos and trios, reproductive success alone may provide sufficient fitness benefits to explain the presence of cooperative polygamy, and the benefits of cobreeding as a duo in females are higher than previously assumed. Lifetime individual fitness data are crucial to reveal the full costs and benefits of cooperative polygamy.

Are you my baby? Testing whether paternity affects behavior of cobreeder male acorn woodpeckers

Abstract Natural selection is expected to favor males that invest more in offspring they sire. We investigated the relationship between paternity and male behavior in the acorn woodpecker (Melanerpes formicivorus), a cooperative breeder that lives in family groups including offspring that remain on their natal territory, sometimes for years, and cobreeders of both sexes. Regardless of group composition, only one communal nest is attended at a time. Whereas cobreeding females share maternity equally, one male usually sires the majority of young in the group’s communal nest. Copulations are rarely observed, and thus it has not been possible to link paternity to sexual behavior. There were no differences among cobreeder males that did or did not sire young in their propensity to roost in the nest cavity at night. However, cobreeder males that attended females continuously prior to egg-laying were more likely to successfully sire young than males that did not, and the relative share of feeding visits and time spent at the subsequent nest were positively related to a male’s realized paternity. These differences in male behavior were partly due to differences among males and partly to plasticity in male behavior covarying with paternity share. Feedings by males successfully siring young also involved a larger proportion of nutritionally valuable insect prey. Males are aware of their paternity success, apparently because of their relative access to females prior to egg laying, and provide more paternal care at nests in which they are more likely to have sired young.

Nest cavity reuse by the cooperatively breeding Acorn Woodpecker

Abstract Although primary cavity-nesting species are capable of excavating new cavities, they often reuse old ones. To determine potential factors driving such reuse, we studied nest-cavity reuse in the Acorn Woodpecker (Melanerpes formicivorus), a cooperatively breeding species that reuses old cavities for 57.2% of nests at Hastings Reservation in central coastal California, USA. We found no evidence for significant fitness costs or benefits of cavity reuse compared to using newly constructed cavities. In contrast, several lines of evidence supported a role for constraints on both cavity reuse and on new cavity construction. The main constraint on reuse was cavities failing to survive from one year to the next, usually because the limb fell apart, filled with water, or was usurped by another species. Evidence that constraints on new cavity construction may be important included more frequent cavity reuse when groups renested and use of artificial cavities when they were experimentally provided. Nest-cavity reuse in this population appears to be driven primarily by constraints, including the energetic costs and time required to excavate a new cavity, rather than fitness consequences, even though Acorn Woodpeckers regularly excavate small holes in trees for acorn storage and the energetic costs of new cavity construction are apparently insufficient to significantly depress reproductive success. Constraints play a significant role in cavity reuse and may affect both the intraspecific and interspecific frequency of cavity reuse among facultative excavating species.

Experimental evidence that acorn woodpeckers recognize relationships among third parties no longer living together

Abstract Triadic awareness, or knowledge of the relationships between others, is essential to navigating many complex social interactions. While some animals maintain relationships with former group members post-dispersal, recognizing cross-group relationships between others may be more cognitively challenging than simply recognizing relationships between members of a single group because there is typically much less opportunity to observe interactions between individuals that do not live together. We presented acorn woodpeckers (Melanerpes formicivorus), a highly social species, with playback stimuli consisting of a simulated chorus between two different individuals, a behavior that only occurs naturally between social affiliates. Subjects were expected to respond less rapidly if they perceived the callers as having an affiliative relationship. Females responded more rapidly to a pair of callers that never co-occurred in the same social group, and responded less rapidly to callers that were members of the same social group at the time of the experiment and to callers that last lived in the same group before the subject had hatched. This suggests that female acorn woodpeckers can infer the existence of relationships between conspecifics that live in separate groups by observing them interact after the conspecifics in question no longer live in the same group as each other. This study provides experimental evidence that nonhuman animals may recognize relationships between third parties that no longer live together and emphasizes the potential importance of social knowledge about distant social affiliates.

Acorn woodpeckers vocally discriminate current and former group members from nongroup members

Abstract 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 nongroup members. Subjects responded more quickly to the calls of nongroup members than to the calls of current group members or former group members still living in the study area but did not discriminate between nongroup 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 postdispersal.

Wandering Woodpeckers: Foray Behavior in a Social Bird

Nonbreeding helpers in several cooperative breeders birds make cryptic extraterritorial movements to discover dispersal opportunities before leaving their natal territories. Studying the ecology of these secretive forays has been a long-time challenge. By tracking group-living acorn woodpeckers (Melanerpes formicivorus) fitted with solar-powered radiotags, we studied how often breeders and helpers undertake extraterritorial forays. Surprisingly, we discovered that both breeders and helpers undertake multiple extraterritorial forays each day throughout the year. Our results suggest that while helpers may benefit from finding dispersal opportunities, forays may be used more generally by acorn woodpeckers as a strategy to gather social information, such as tracking changes in group membership of neighboring territories. These photographs illustrate the article “Wandering woodpeckers: foray behavior in a social bird” by Sahas Barve, Natasha D.G. Hagemeyer, Russell E. Winter, Samuel D. Chamberlain, Walter D. Koenig, David W. Winkler, and Eric L. Walters published in Ecology. https://doi.org/10.1002/ecy.2943

Wandering woodpeckers: foray behavior in a social bird.

In many cooperatively breeding taxa, nonbreeding subordinates, or helpers, use extra-territorial forays to discover dispersal opportunities. Such forays are considered energetically costly and foraying birds face aggression from conspecific members of the territories they visit. In contrast, breeders in cooperatively breeding taxa are expected to foray seldomly. We used novel tracking technologies to follow 62 acorn woodpeckers (Melanerpes formicivorus), a cooperatively breeding bird, to study extra-territorial foray behavior. Both helpers and breeders engaged in extra-territorial forays routinely and often several times per day. Helpers forayed earlier in the day and invested more time when foraying to high-quality territories. Unexpectedly, breeders forayed as often and as far as helpers. Breeders from high-quality territories forayed closer to their home territories than breeders from low-quality territories, reflecting a potential trade-off between foraying and territory defense. Such a routine pattern of extra-territorial forays in both helpers and breeders suggests that the motives behind forays differ by sex and social status and involve more than simply searching for dispersal opportunities.

Does Helping-at-the-Nest Help? The Case of the Acorn Woodpecker

Cooperative breeding groups often involve “helpers-at-the-nest”; indeed, such behavior typically defines this intriguing breeding system. In few cases, however, has it been demonstrated that feeding nestlings by helpers, rather than some other behavior associated with helpers’ presence, leads to greater reproductive success. One prediction of the hypothesis that feeding behavior per se is responsible for the fitness benefits conferred by helpers is that there should be close congruence between the patterns of helping-at-the-nest and the fitness effects of helpers. Here we look for such a relationship in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) in order to begin to identify the behaviors of helpers that drive the increased fitness benefits they confer. In terms of young fledged, a helper male confers approximately the same fitness benefits to a group as does a helper female; more dramatically, the effects of helper males increases with increasing food supply, most importantly the prior year’s acorn crop on which this species depends, whereas that of helper females does not. These patterns do not match the nest-feeding patterns of helpers, which are greater for females than males and do not increase with a larger acorn crop the prior autumn. In contrast, the proportion of time helpers spend tending acorn-storage facilities (granaries) and are present in or near their home territory is greater for males than females and, at least for males, positively related to the size of the acorn crop. These results fail to support the hypothesis that the primary benefit conferred by helpers is feeding young in the nest; rather, they suggest that behaviors such as territorial defense and predator detection are more important. Understanding exactly what those behaviors are in this, and most other cooperatively breeding systems, remain to be determined.

Habitat Saturation Results in Joint-Nesting Female Coalitions in a Social Bird.

Joint nesting by females and cooperative polyandry-cooperatively breeding groups with a male-biased breeder sex ratio-are little-understood, rare breeding systems. We tested alternative hypotheses of factors potentially driving these phenomena in a population of joint-nesting acorn woodpeckers (Melanerpes formicivorus). During periods of high population density and thus low independent breeding opportunities, acorn woodpecker females formed joint-nesting coalitions with close kin. Coalitions were typically associated with groups with a male bias. We found strong evidence for both inter- and intrasexual conflict, as joint nesting conferred a fitness benefit to some males, a significant fitness cost to females, and no gain in per capita reproductive output for either sex. Such conflict, particularly the cost to females, may be an important reason why joint nesting is rare among cooperatively breeding taxa.

Causes of seasonal decline in reproduction of the cooperatively‐breeding acorn woodpecker

Clutch size and reproductive success decline seasonally in a wide range of temperate avian taxa. Two competing hypotheses have been proposed to explain such declines: the ‘timing’ hypothesis, which states that conditions affecting reproduction decline intrinsically with date, and the ‘quality’ hypothesis, which proposes that high‐quality individuals or individuals in high‐quality situations breed earlier. We contrasted the relative importance of these two hypotheses using a long‐term dataset of the cooperatively‐breeding acorn woodpecker Melanerpes formicivorus in central coastal California (USA). This population exhibits an 11% seasonal decline in clutch size, a 60% seasonal decline in fledging success, and a 77% seasonal decline in fledgling overwinter survival. Clutch size tracks seasonal availability of flying insects, which are a likely ecological driver of the seasonal decline in reproduction and, because of the nonlinear relationship of flying insects with date, constitute a likely factor constraining even earlier nesting. By parsing lay date data into within‐female and between‐female components, we found that only the within‐female component had a statistically significant direct effect on clutch size, supporting the timing hypothesis. For both fledging success and overwinter survival, however, both within‐ and between‐female effects were highly significant, with between‐female effects (differences in female quality) being stronger than within‐female effects. These results suggest that timing per se is a key factor affecting the seasonal decline in clutch size, but that both differences in female quality and an intrinsic seasonal decline in conditions drive the even more dramatic seasonal declines in fledging success and overwinter survivorship of fledglings.

Wild acorn woodpeckers recognize associations between individuals in other groups.

According to the social intelligence hypothesis, understanding the cognitive demands of the social environment is key to understanding the evolution of intelligence. Many important socio-cognitive abilities, however, have primarily been studied in a narrow subset of the social environment-within-group social interactions-despite the fact that between-group social interactions often have a substantial effect on fitness. In particular, triadic awareness (knowledge about the relationships and associations between others) is critical for navigating many types of complex social interactions, yet no existing study has investigated whether wild animals can track associations between members of other social groups. We investigated inter-group triadic awareness in wild acorn woodpeckers (Melanerpes formicivorus), a socially complex group-living bird. We presented woodpeckers with socially incongruous playbacks that simulated two outsiders from different groups calling together, and socially congruous playbacks that simulated two outsiders from the same group calling together. Subjects responded more quickly to the incongruous playbacks, suggesting that they were aware that the callers belonged to two different groups. This study provides the first demonstration that animals can recognize associations between members of other groups under natural circumstances, and highlights the importance of considering how inter-group social selection pressures may influence the evolution of cognition.

Does aggression avoidance drive oak tree attendance by corvid scatter-hoarders?

We investigated the role of species- and community-wide seed production by a community of oaks (Quercus spp.) in influencing tree attendance and aggression among California scrub-jays (Aphelocoma californica), corvids that are seed dispersal mutualists, and acorn woodpeckers (Melanerpes formicivorus), near-exclusive seed predators. We tested the hypothesis that scrub-jays reduce their attendance in response to seed crop size on Quercus lobata, the species preferentially attended by acorn woodpeckers, and therefore reduce the risk of potential injury from aggression when community-level seed production enables them to forage on other trees (the “aggression-avoidance” hypothesis). Results indicated that scrub-jay attendance correlated positively with Q. lobata crops during the year when community-level seed production was low, resulting in high levels of aggression. In contrast, during the 2 years when acorns of other species were abundant in the landscape, scrub-jays reduced attendance at Q. lobata and experienced less aggression. Similarly, we found that the scrub-jays experienced equally low rates of aggression at Q. douglasii trees that were attended by fewer acorn woodpeckers. These results support the aggression-avoidance hypothesis and illustrate how resource and social dynamics may interact to determine the attendance of mutualistic seed dispersers at oak species competing for seed dispersal. Furthermore, they show that intra- and interspecific social interactions at the seed source, as well as at caching locations, affect the scatter-hoarding behavior of wild corvids. SIGNIFICANCE STATEMENT: Food-hoarding animals play a central role in determining the seed fate of large-seeded trees, both in the role of predators and dispersers. Tree attendance and seed hoarding, however, are often context-dependent, varying with population- and community-level seed production, and with intra- and interspecific social dynamics. In California oak communities, acorn woodpeckers—seed predators—compete with California scrub-jays—high-quality seed dispersers—for acorns on preferred trees, resulting in considerable levels of aggression. Using behavioral observations at two species of oaks, one of which is preferred by the woodpeckers, we determined that aggression avoidance explained tree attendance behavior of the scrub-jays, but only when landscape-wide seed production allowed them to forage elsewhere. Our results show for the first time that seed dispersal by hoarding animals can be affected by intra- and interspecific aggression at the seed source.