Acorn Woodpecker Research Articles

Food Habits and Social Organization of Acorn Woodpeckers in Colombia

Colombian populations of Acorn Woodpeckers (Melanerpes formicivorusflavigula) inhabit the Andes and are geographically isolated from Central American populations. Their diverse food habits include acorns from Quercus humboldtii, the only oak in the Colombian Andes. Unlike birds from North America, however, Colombian Acorn Woodpeckers do not store acorns or rely heavily upon them. Instead, they feed on insects, sap, and fruit year-round. Although oak woodlands seem to be their preferred habitat, Colombian Acorn Woodpeckers are found residing permanently and breeding cooperatively in other kinds of habitat. The limited availability of storage facilities (habitat saturation hypothesis) does not seem to be adequate to explain cooperative breeding in these populations.

Incestuous Mating in a Communal Bird: A Family Affair

In the past, observations of animal dispersal has been taken as evidence of inbreeding avoidance. Reduced dispersal by the young of communal breeders makes them ideal for investigating the possibility of inbreeding and/or inbreeding avoidance. Communally breeding pukeko are especially suitable for such a study because they exhibit extreme philopatry, have a polygynandrous mating system, and engage in readily observed copulations. Results from a 7-yr study show that most pukeko remain and breed in their natal territory and incestuous matings are common. In contrast, incidents of dispersal out of the natal territory were infrequent, involving only males. Outbreeding appeared less common than inbreeding. Dispersal by breeding males that had mated previously with their offspring could not be logically linked to incest avoidance, but movements of nonbreeding subadults might have been related to either incest avoidance or intersexual dominance. Except for young nonbreeding males that tend to be subordinate to breeding females, male pukeko dominate females. A few subadult males that were stopped from copulating by the aggressive behavior of their mothers subsequently dispersed, whereas others showed reduced sexual behavior in the presence of their mothers. Both incest avoidance by the mothers (but not the sons) and parental dominance could explain these observations. Parental dominance seems to be a viable explanation because there were no examples of daughters avoiding matings with their fathers or sisters avoiding matings with brothers. Results from other studies of communally breeding birds also show various degrees of incestuous mating. Data from one in-depth study on acorn woodpeckers, from which incest-avoidance mechanisms have been postulated, is considered in detail; the results appear equivocal and do not exclude alternative explanations. Additional factors, such as a level of fecundity markedly higher than that of survival and a long history of communal (and presumably incestuous) habits in pukeko, lead us to question whether inbreeding among pukeko is likely to have deleterious genetic effects. The possibility of nonrandom fusion of gametes would also support this view. Overall, pukeko appear to demonstrate higher levels of incestuous matings than current research and theories on the evolution of vertebrate mating systems would have predicted. We argue that this conflict arises as a product of the unquestioning assumption by many workers that behavior leading to inbreeding has been selected against or that inbreeding-avoidance mechanisms have been selected for in animal populations.

Territory Quality and Dispersal Options in the Acorn Woodpecker, and a Challenge to the Habitat-Saturation Model of Cooperative Breeding

The central feature of cooperative breeding systems is the presence of one or more nonbreeding birds in a social group that devote time and energy to help raise the offspring of other group members. The most widely accepted model for the evolution of this behavior is that it arises when there is some ecological constraint to independent breeding by young birds that otherwise would disperse from their natal groups. Among territorial species, this constraint is usually hypothesized to be saturation of suitable habitat by sedentary established groups. We use data collected over a 10-yr period to test this model for a population of acorn woodpeckers (Melanerpes formicivorus) in central New Mexico. Acorn woodpeckers store large quantities of mast, and territories vary greatly in the amount of storage facilities present. Storage facilities significantly affect the survival and reproductive success of groups occupying those territories. Most helpers occur in high-quality territories; low-quality territories are frequently unoccupied and thus available for colonization. We estimate lifetime direct and indirect offspring production for yearlings that help in territories of differing quality and for varying numbers of years. Birds that forgo individual reproduction for one or more years in high-quality territories but eventually breed there can have greater lifetime fitness than those that disperse at 1 yr of age and breed immediately in low-quality territories. Had we combined data from all territories and all groups, these findings would have been obscured; rather, the resulting calculations would have erroneously indicated that, for yearlings, dispersing and breeding is always the preferred option. Our analyses indicate that habitat saturation or some other constraint on the opportunity to breed successfully is not required to explain the existence of cooperative breeding in this population of woodpeckers. Instead, we suggest that competition for space may be the outcome of factors that promote philopatry, such as access to a critical resource and/or mutualistic benefits of living in a group and eventually breeding in the natal territory. If this scenario is correct, habitat limitation is a result of other factors favoring cooperative breeding rather than the basis for this kind of social system.

Autumnal Breeding Acorn Woodpeckers in Southern New Mexico

Autumnal Breeding Acorn Woodpeckers in Southern New Mexico

Acorn Woodpecker Predation on Cliff Swallow Nests

Unlike many other woodpeckers that feed on woodboring insects and larvae, Acorn Woodpeckers (Melanerpesformicivorus) feed primarily on acorns, insects caught by flycatching, and sap (Bent 1939, MacRoberts 1970, MacRoberts and MacRoberts 1976). They also supplement their diets with an occasional small lizard (Koenig, pers. comm.), fruit, oak catkins, and wild oat seeds (MacRoberts and MacRoberts 1976). Acorn Woodpeckers are also known to eat eggs that have been removed from two-female communal nests (Mumme et al. 1983). Other unusual predatory behavior includes a single report of predation on a Western Wood-Pewee (Contopus sordidulus) nest (Bryant 1921) and the mutilation of two Red-breasted Sapsucker (Sphyrapicus ruber) nestlings (Shuford 1985). These incidents, however, appear to be opportunistic; the Acorn Woodpecker does not regularly prey on other vertebrates. We report here on an instance when nest predation by Acorn Woodpeckers may have harmed a colony of Cliff Swallows (Hirundo pyrrhonota) nesting in the Stanford Quadrangle at Stanford University, Stanford, California. The colony consisted of 56 mud nests concentrated under overhanging shingles from the roof of the east-facing main archway of the quadrangle. An additional 10 to 15 nests were dispersed throughout the colony. The nests were constructed in corners bordered by roof supports and shingles, ranging from 3 to 8 m off the ground. We observed eight Acorn Woodpecker visits to this colony during the week of 28 May through 3 June 1985, between 08:00 and 09:00. Though attempts at predation sometimes failed when mobbing Cliff Swallows induced the Acorn Woodpecker to flee, in four (50%) of these visits the woodpecker succeeded in stealing an egg from a nest. In another instance, an Acorn Woodpecker also appeared to have taken a nestling. Acorn Woodpeckers were nesting in two palm trees less than 25 m from the Cliff Swallow colony. After removing an egg, the woodpecker would fly towards its nesting site with the egg still intact in its beak. Unfortunately, we were unable to discover what the woodpeckers did with the Cliff Swallow eggs, though ' Received 4 December 1985. Final accepta ce 27 October 1986. SHORT COMMUNICATIONS 177

Poor Acorn Crop, Dominance, and Decline in Numbers of Acorn Woodpeckers

Une mauvaise glandee en 83-84 dans la reserve d'Hastings (Californie) altera la dynamique et le comportement social d'une population de Melanerpes formicivorus

A guide to the literature on aggressive behavior

A guide to the literature on aggressive behavior

Hummingbird Association with Acorn Woodpecker Sap Trees in Colombia

Journal Article Hummingbird Association with Acorn Woodpecker Sap Trees in Colombia Get access Gustavo Kattan, Gustavo Kattan Department of Zoology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: Oxford Academic Google Scholar Carolina Murcia Carolina Murcia Department of Zoology, University of Florida, Gainesville, Florida 32611 Search for other works by this author on: Oxford Academic Google Scholar The Condor, Volume 87, Issue 4, November 1985, Pages 542–543, https://doi.org/10.2307/1367956 Published: 01 November 1985 Article history Received: 27 October 1983 Accepted: 13 May 1985 Published: 01 November 1985

Replacement of breeders and within-group conflict in the cooperatively breeding acorn woodpecker

Breeding males or females were removed from groups of the cooperatively breeding acorn woodpecker to study (1) competition among individuals from different groups to fill the breeding vancancies, (2) conflict between parents and offspring within groups over replacement of the removed bird, and (3) conflict between parents and offspring for breeding status. Intense fights (“power struggles”) among nonbreeders from other groups to fill the breeding vacancies developed after the removals. In groups containing only helpers of the opposite sex as the removed bird, replacement was rapid, but in groups containing helpers of the same sex as the removed bird replacement was delayed substantially. In these latter groups parent-offspring conflict developed: helpers could not breed because their parent of the opposite sex was present, and they attacked potential replacements, while their remaining parent advertised for a new mate by calling. Once replacement had occurred, dominance interactions between parents and offspring and among siblings appeared to prevent some individuals from breeding. We conclude that the potential for competition among close kin is high in cooperative species because breeding opportunities are limited.

ARTHROPOD FOOD OF SOME FOREST-INHABITING BIRDS

AbstractThe diets of 25 species of forest-inhabiting birds were examined in California from 1962 to 1968. Coleoptera and Hymenoptera comprised the bulk of the diets. Coleopterans were, in decreasing amounts, the most common food of the three-toed woodpecker, Steller's jay, American robin, brown creeper, hairy woodpecker (both sexes), dark-eyed junco, downy woodpecker (both sexes), olive-sided flycatcher, western wood-pewee, and white-headed woodpecker (both sexes). Hymenopterans, mainly ants, were the most common food of the northern flicker, Williamson's sapsucker, red-breasted sapsucker, and acorn woodpecker. White-headed woodpecker (both sexes) also ate ponderosa-pine seeds in the fall. Hairy woodpecker (both sexes) fed on the seeds of ponderosa pine and sugar pine. Detailed estimates of foods consumed by item and volume from analyses of stomach contents of 241 specimens are presented.

Shared paternity in the acorn woodpecker (Melanerpes formicivorus)

We investigated multiple paternity in acorn woodpeckers (Melanerpes formicivorus) from central New Mexico by starch-gel electrophoresis. Seven of thirty-one presumptive genetic loci were polymorphic. Multiple paternity was revealed in one of three multi-male groups sampled on the basis of offspring and adult electrophoretic phendtypes, and was suggested in the two other groups. This study provides the first documentation of multiple paternity for any avian cooperative breeder.

Genetic Variation and Parentage in a California Population of Acorn Woodpeckers

-We examined genetic variation in a California population of the highly social Acorn Woodpecker (Melanerpes formicivorus) using starch-gel electrophoresis of 580 blood samples collected between 1975 and 1982. Eighteen genetic loci were resolvable from blood, 2 of which (Mpi and Np) were polymorphic. One additional locus (Est-2) showed variation that was not easily interpreted as an outcome of simple Mendelian inheritance. Average heterozygosity (0.032) was comparable to that of other bird species. Both variable loci showed a slight but not significant deficiency of heterozygotes relative to Hardy-Weinberg expectation. Of 186 nestlings from 62 nests, the genotypes of 4 nestlings (2.2%) from 2 nests (3.2%) were inconsistent with the genotypes of the group breeding males, thus indicating kleptogamy (cuckoldry) in our study population. The low level of genetic variation and the high degree of relatedness between males within groups prevented us from making definitive conclusions concerning the patterns of within-group paternity. Nonetheless, at 1 one-female nest the brood was probably, but not certainly, multiply sired. Received 23 January 1984, accepted 13 September 1984. THE genetic structure of natural populations of social vertebrates is a topic of much current research. Of particular interest is the question of whether social structuring results in decreased effective population size and increased inbreeding, which could in turn promote more rapid rates of evolutionary divergence (Wilson et al. 1975, Bush et al. 1977, Schwartz and Armitage 1980) or the evolution of cooperative or altruistic traits (Brown 1974, Johnson and Brown 1980). Electrophoretic and other genetic markers have proved useful in investigating alternative reproductive strategies. For example, Hanken and Sherman (1981) found that multiple matings by female Belding's ground squirrels (Spermophilus beldingi) result in multiple paternity of most litters. Gowaty and Karlin (1984) suggest that extrapair copulations and intraspecific nest parasitism result in frequent kleptogamy (Gowaty 1982) in the Eastern Bluebird (Sialia sialis). Joste (1983) used electrophoretic 1 Present address: Department of Biology, Memphis State University, Memphis, Tennessee 38152

Spacing in Acorn Woodpeckers

foraging in a bumblebee, Bombus terricola Kirby. Journal of Comparative Physiology 77:49-64. Herrera, C. M. 1981. Are tropical fruits more rewarding to dispersers than temperate ones? American Naturalist 118: 896-907. 1982. Seasonal variation in the quality of fruits and diffuse coevolution between plants and avian dispersers. Ecology 63:773-785. Hiebert, S. M., and W. A. Calder III. 1983. Sodium, potassium, and chloride in floral nectars: energy-free contributors to refractive index and salt balance. Ecology 64:399402. Hulme, A. C. 1971. The biochemistry of fruits and their products. Volume 2. Academic Press, New York, New York, USA. Inouye, D. W., N. D. Favre, J. A. Lanum, D. M. Levine, J. B. Meyers, M. S. Roberts, F. C. Tsao, and Y-Y. Wang. 1980. The effects of nonsugar nectar constituents on estimates of nectar energy content. Ecology 61:992-996. Mantovani, G. 1974. Dry substance in sugar products other than sugars. Pages 157-167 in Report of the Proceedings of the 16th Session of the International Commission for Uniform Methods of Sugar Analysis. ICUMSA, Peterborough, England. Martin, A. C., H. S. Zim, and A. L. Nelson. 1951. American wildlife and plants. McGraw-Hill, New York, New York, USA. Milton, K., and F. R. Dintzis. 1981. Nitrogen-to-protein conversion factors for tropical plant samples. Biotropica 13:177-181. Moermond, T. C., and J. S. Denslow. 1983. Fruit choice in neotropical birds: effects of fruit type and accessibility on selectivity. Journal of Animal Ecology 52:407-420. Percival, M. S. 1961. Types of nectar in angiosperms. New Phytologist 60:235-281. i g in a bumblebee, Bombus terricola Kirby. Journal parative Physi logy 77:49-64. era, C. M. 1981. Are tropical fruits mo rewarding to sers than temperate ones? American Naturalist 118:

Individual Contributions To Cooperative Behaviour in the Acorn Woodpecker: Effects of Reproductive Status, Sex, and Group Size

AbstractAcorn woodpeckers (Melanerpes formicivorus) are communal breeders that live in permanently territorial family groups of 2-15 individuals. All group members participate in several forms of cooperative behaviour, which we define as the investment of resources in a common interest shared by other group members (CHASE, 1980). Acorn woodpecker group interests include the caching of acorns in a specialized storage tree (the granary), maintenance of the granary, and defence of the granary against intra- and interspecific intruders. Because of a strong positive relationship between the availability of stored acorns and group reproductive success, investments in acorn storage, granary maintenance, and granary defence have a direct positive effect on the individual fitness of breeders. Non-breeding helpers, however, do not realize a direct reproductive reward, and therefore profit less directly from investment in these forms of cooperative behaviour. Thus, we predicted that breeders should invest more heavily in cooperative behaviour than should helpers. The prediction was upheld: for nearly every form of cooperative behaviour examined, breeders contribute significantly more than do helpers. On the other hand, while established breeders only rarely intrude on neighbouring territories, nonreproductive helpers frequently do so. These dispersal "forays" constitute attempts by helpers to locate ecologically restricted breeding vacancies. The effect that group size has on individual contributions to cooperative behaviour is unclear. Individual rates of acorn storage decrease significantly with increasing group size, but granary attendance and granary maintenance do not. Male and female acorn woodpeckers do not differ in their contributions to acorn storage, granary maintenance, or interspecific defence. Females, however, play a significantly greater role in intraspecific defence because of a female-biased intruder sex ratio and sex-specific defensive behaviour.

Cooperative Breeding in the Acorn Woodpecker

Cooperative Breeding in the Acorn Woodpecker

The Breeding System of the Acorn Woodpecker in Central Coastal California

Abstract and SummaryThe breeding system of the acorn woodpecker in California is detailed, and evidence supporting the occurrence of both cooperative polygamy (polygynandry) and nonbreeding by group offspring still living in their natal territory (helping) is presented. Both these phenomena occur independently and often simultaneously within the population. Although groups almost invariably consist of close relatives, incest is avoided, apparently by the recognition of close relatives. However, 10 cases of known or probable inbreeding have been recorded, resulting both from philopatry and from breakdowns in the usual pattern of incest avoidance. The latter appears to occur when the cost of attracting an unrelated mate is unusually high; the overall extent and significance of inbreeding in the acorn woodpecker remain to be determined.ZusammenfassungEichelspechte leben in Gruppen, die aus einem “Brüterzentrum” (bis zu 4 Männchen und 2 Weibchen) bestehen und den nichtbrütenden Helfern; das sind bis zu 10 Nachkommen beiderlei Geschlechts aus mehreren Vorjahresbruten der Gruppe. Brutgruppen entstehen, indem sich eine Gruppe von Schwestern und eine Gruppe ihnen verwandtschaftlich nicht nahestehender Brüder zusammenschließen. Hinzu kommen dann Zuwanderer und die eigenen Nachkommen. Diese brüten in der Familie nicht, es sei denn ein andersgeschlechtiger Elter stirbt und wird durch einen Fremden ersetzt. In diesem Falle können Nachkommen brüten, obwohl ihr gleichgeschlechtiger Elter noch in der Gruppe ist. Sterben alle Brüter eines Geschlechts, wandern andere Individuen dieses Geschlechts ein; Zuwanderer gibt es aber auch zusätzlich.Zuwanderer sind 1. Geschwister von Brütern, 2. Gruppennachkommen, die nach einem erfolglosen Brutversuch außerhalb wieder daheim Heifer werden, 3. Gruppennachkommen, die kein Auswandererglück hatten und im heimatlichen Revier brüten, nachdem ihr andersgeschlechtiger Elter starb. Als Folge dieser Brüt‐Ordnung wird Inzest weitgehend vermieden, offenbar zum Teil durch Verwandtschaftserkennen. Dennoch sind 10 Fälle von Inzucht beschrieben; 5 davon nach einem Zusammenbruch des Verwandtschaftserkennens, 5 aus Ortstreue. Inzucht scheint aufzutreten, wenn die Kosten für das Anwerben eines unverwandten Partners ungewöhnlich hoch sind. Wie häufig das vorkommt und welche Folgen es für das Sozialverhalten hat, bleibt zu untersuchen.

Reproductive competition in the communal acorn woodpecker: sisters destroy each other's eggs

Among group-living vertebrates, highly cooperative or apparently altruistic behaviours usually occur between close relatives1,2. In contrast, highly competitive or selfish behaviours, such as the killing of conspecific young, are often directed towards unrelated individuals3,4. These findings are consistent with predictions of inclusive fitness theory5, and have led to the suggestion that kin should receive favoured treatment in the form of not only cooperation but also reduced aggression or competition2,6–9. However, selfish, competitive and manipulative behaviours can evolve among close relatives if the gain in direct (individual) fitness10 exceeds the loss in indirect fitness5,11–14. We report here an extreme example of reproductive interference between close relatives in the acorn woodpecker, in which communally nesting sibling females in this species destroy their sister's eggs prior to laying their own.

Mate guarding in the acorn woodpecker: Within-group reproductive competition in a cooperative breeder

California populations of acorn woodpeckers ( Melanerpes formicivorus) show two distinct types of cooperative breeding: Cooperative Polygamy, whereby two or more individuals of the same sex share breeding status and mates, and Helping, whereby non-reproductives assist in rearing young. Because the presence of more than one male within a single social unit could greatly increase the threat of kleptogamy (cuckoldry), we quantified the degree to which males maintain surveillance of their females prior to and during egg laying. In polyandrous groups with two breeding males and one breeding female, both males closely guard the female early in the breeding season. Guarding begins 7–25 days before the first egg is laid and terminates 1–2 days before clutch completion. However, our data indicate that mate guarding is absent in groups that contain only a single male. This suggests that mate guarding in the acorn woodpecker is an expression of within-group reproductive competition, rather than a means by which males prevent their mates from couplating with extra-territorial males. The presence of male helpers, usually older offspring of the breeding female, has a positive effect on the mate-guarding behaviour of male breeders, even though male helpers themselves do not guard females. Female helpers, usually older offspring of the breeding males, do not lay eggs and are not guarded.

Are Acorn Woodpecker Territories Aggregated?

Etude statistique critique des conclusions de Burgess et col. (ibid., 63, 575-578) sur la repartition spatiale de Melanerpes formicivorus et reponse de l'auteur

Intragroup dynamics of a cooperative breeder: An anlysis of reproductive roles in the acorn woodpecker

The acorn woodpecker is a cooperative breeder generally thought to breed promiscuously within groups, but alternative patterns of reproductive investment can be expected and remain to be examined. With this feature of mating system in mind, we studied the reproductive roles of individuals in a single group of this species in central coastal California over a three-year period. Particularly detailed observations were made in 1979, when the group consisted of two potentially breeding sibling males and a single breeding female. One male (298) contributed significantly more than ♂ 297 in virtually all aspects of nest and fledgling care. These include (1) feeding nestlings, (2) nest sanitation, (3) nocturnal incubation and brooding, (4) feeding fledglings, and (5) escorting fledglings. In the few observed and rarely occurring dominance interactions, ♂ 298 prevailed. These results suggest that ♂ 298 had a higher probability of fathering the group's two 1979 offspring than did his brother. Thus, the two males differed substantially in their parental roles within the group, and very likely in reproductive roles, also. At present stage of knowledge, monogamy in groups with one female and more than one potential reproductive male is no less likely than promiscuity. These findings are discussed with respect to the evolution of cooperative breeding in this species.