The Value of a hoard: not just energy

We present a stochastic dynamic programming model of the contributionof stored food to winter survival. Using the acorn woodpecker(Melanerpes formtcivorus) as a model organism, we demonstratethat a hoard of small energetic value can impart considerableimprovement in the probability of surviving the winter and soimprove fitness. With this model we hope to resolve Koenig andMumme's paradox, that acorn woodpeckers expend much time andeffort to create and maintain hoards of acorns which provideno more that 16% of their energetic needs over the period inwhich they are used. We further demonstrate that the contributionof hoarded acorns to survival depends on the variability inforaging outcome, independently of the energetic value of thehoard in absolute terms. We point out that the results applyin principle to all hoarding animals and suggest a number offurther elaborations of the model.     

Genetic monogamy in single-male groups of acorn woodpeckers, Melanerpes formicivorus

Acorn woodpeckers have one of the most complex social systems of any bird species. Breeding units range in size from monogamous pairs to groups of 15 birds that include multiple breeding males and females as well as nonreproductive helpers-at-the-nest. Groups form when young remain at their natal nest to help their parents breed or when single-sex coalitions of siblings disperse to fill a reproductive vacancy on another territory. Plural breeding and helping behaviour are thought to be favoured through indirect fitness benefits for individuals that would otherwise be unable to breed due to a shortage of reproductive vacancies on territories with acorn stores. We report the results of multi-locus DNA fingerprinting of 51 offspring from 18 nests of 16 socially monogamous pairs of acorn woodpeckers. If socially monogamous females mate outside the pair-bond, indirect fitness benefits for cobreeders and helpers will be significantly reduced. Monogamous pairs accounted for all but one of the 51 offspring we tested; the single exception was apparently sired by the putative father, but the putative mother was excluded from maternity. Our results indicate that individuals remaining on their natal territories as helpers are generally the genetic offspring of the pair they help. They also suggest that single-sex coalitions offspring dispersing together from nests of socially monogamous pairs will be full-siblings.     

Red-Headed Woodpecker Nest-Habitat Thresholds in Restored Savannas

Abstract: The red-headed woodpecker (Melanerpes erythrocephalus) is a widespread, adaptable species that continues to decline across North America. We examined stand, nest-tree, and cavity characteristics of red-headed woodpeckers in restored savannas within the Necedah National Wildlife Refuge, Wisconsin, USA, during 2002 and 2003. Based on availability, red-headed woodpeckers selected snags and trees with greater dead limb length. Red-headed woodpeckers nested in areas with greater basal area, cavity density, snag density, limb-tree density, and total dead limb length. Red-headed woodpeckers exhibited a decadent-tree threshold that was most accurately measured by the number of trees with dead limbs around (0.04 ha) nests. We found that the probability of a red-headed woodpecker nest being present greatly increased above the decadent-tree threshold. Woodland managers throughout the red-headed woodpecker's extensive breeding range can use our results and recommendations to guide decadent-tree retention for this species.     

Evolution of polyandry in a communal breeding system

The evolution of polyandry requires an asymmetrical factor thatfavors more matings per breeding female than per breeding male,thus reversing Bateman's principle. Here a model is presentedfor the evolution of avian cooperative polyandry. The modelshows that polyandry can evolve if communal breeding is initiallyadvantageous and if increasing clutch size beyond an optimumis detrimental. The advantage of communal breeding favors theaddition of more breeders (either males or females) and thusselects against breeding as single pairs (monogamy). The optimaldutch size creates the asymmetry that favors adding male breeders(polyandry) over adding female breeders (polygyny). Adding femalesis detrimental because females must lay eggs to reproduce andcan therefore increase the clutch size of the group. On theother hand, males can reproduce by sharing paternity withoutincreasing dutch size. It is shown that cooperative polyandryevolves either because it maximizes both male and female fitnessor because polygyny and monogamy are behaviorally unstable.Data from acorn woodpeckers support the assumptions of the modeland suggest that cooperative polyandry evolved because it isbehaviorally more stable. The persistence of monogamy and polygynyin acorn woodpeckers (at a lower incidence than polyandry) isalso examined. Polygyny in these birds represents a case ofthe Prisoner's Dilemma     

Seed removal in a tropical North American desert: an evaluation of pre‐ and post‐dispersal seed removal in Stenocereus stellatus

• To determine seed removal influence on seed populations, we need to quantify pre‐ and post‐dispersal seed removal. Several studies have quantified seed removal in temperate American deserts, but few studies have been performed in tropical deserts. These studies have only quantified pre‐ or post‐dispersal seed removal, thus underestimating the influence of seed removal. We evaluated pre‐ and post‐dispersal seed removal in the columnar cactus Stenocereus stellatus in a Mexican tropical desert.
• We performed selective exclosure experiments to estimate percentage of seeds removed by ants, birds and rodents during the pre‐ and post‐dispersal phases. We also conducted field samplings to estimate abundance of the most common seed removers.
• Birds (10–28%) removed a higher percentage of seeds than ants (2%) and rodents (1–4%) during pre‐dispersal seed removal. Melanerpes hypopolius was probably the main bird removing seeds from fruits. Ants (62–64%) removed a higher percentage of seeds than birds (34–38%) and rodents (16–30%) during post‐dispersal seed removal. Pogonomyrmex barbatus was probably the main ant removing seeds from soil.
• Birds and ants are the main pre‐ and post‐dispersal seed removers in S. stellatus, respectively. Further studies in other S. stellatus populations and plants with different life forms and fruit types will contribute to evaluate seed removal in tropical American deserts.

     

Preburn Characteristics and Woodpecker Use of Burned Coniferous Forests

ABSTRACT Large wildfires are common in many western coniferous forests, and these fires can affect woodpecker reproduction and habitat use. Our objectives were to examine nesting densities, reproductive parameters, and species-specific habitat selection of woodpeckers in a recently burned region of the Black Hills in South Dakota, USA, between 2001 and 2004. Postfire nesting densities were greatest in areas dominated by high prefire canopy cover, and reproductive success averaged >70%. For some species of woodpeckers, factors such as diameter at breast height, burn severity, and distance to unburned patches were important for nest-site selection. Our data indicated that nesting densities of many woodpeckers in the Black Hills were lower than what has been recorded elsewhere following recent, large wildfires in ponderosa pine (Pinus ponderosa) forests. Management activities that simulate mixed-severity fire effects and retain higher numbers of large snags are likely to benefit cavity nesters in this region.     

Acorns,insects, and the diet of adult versus nestling Acorn Woodpeckers

ABSTRACT The diet of Acorn Woodpeckers (Melanerpes formicivorus) in central coastal California consists of a mixture of high‐quality (insects) and low‐quality (stored acorns) food during the spring breeding season. We used stable isotope ratios obtained from blood samples to estimate possible differences in the proportion of these items being fed to nestlings and being consumed by breeding adults, as well as the extent to which the diet of nestlings shifts between insects and acorns during the nestling period. Based on both feeding observations and items recovered from nestlings, older nestlings were fed 28% acorns. Using this value as a baseline for the isotope analyses, the diet of breeding adults was estimated to consist of 90% acorns. Based on repeated samples from the same nestlings over time, the estimated proportion of acorns in their diet increased from 19% in 9‐ to 12‐d‐old nestlings to 42% in 23‐ to 26‐d‐old nestlings. There was a significant correlation between the isotope values of adults and the nestlings they fed, indicating that different groups of Acorn Woodpeckers have significantly different diets. Although important for successful reproduction in this species, stored acorns are not the primary food resource used to feed young. Instead, the availability of stored acorns allows adult Acorn Woodpeckers to provide nestlings with more protein‐rich insects while maintaining themselves on relatively protein‐poor, low‐quality acorns. Differences in the diets of adults and their nestlings are likely widespread among species that feed, at least in part, on low‐quality foods, such as fruit and seeds, during the breeding season.     

Habitat-Suitability Models for Cavity-Nesting Birds in a Postfire Landscape

ABSTRACT Models of habitat suitability in postfire landscapes are needed by land managers to make timely decisions regarding postfire timber harvest and other management activities. Many species of cavity-nesting birds are dependent on postfire landscapes for breeding and other aspects of their life history and are responsive to postfire management activities (e.g., timber harvest). In addition, several cavity nesters are designated as species at risk. We compare the ability of 2 types of models to distinguish between nest and non-nest locations of 6 cavity-nesting bird species (Lewis's woodpecker [Melanerpes lewis], black-backed woodpecker [Picoides arcticus], hairy woodpecker [P. villosus], northern flicker [Colaptes auratus], western bluebird [Sialia mexicana], and mountain bluebird [S. currucoides]) in the early postfire years for a ponderosa pine (Pinus ponderosa) forest in Idaho, USA. The 2 model sets consisted of 1) models based on readily available remotely sensed data and 2) models containing field-collected data in addition to remotely sensed data (combination models). We evaluated models of nesting habitat by quantifying the model's ability to correctly identify nest and non-nest locations and by determining the percentage of correctly identified nest locations. Additionally, we developed relative habitat-suitability maps for nesting habitat of black-backed and Lewis's woodpeckers from the best models. For all species except Lewis's woodpeckers, model performance improved with the addition of field-collected data. Models containing remotely sensed data adequately distinguished between nest and non-nest locations for black-backed woodpecker and Lewis's woodpecker only, whereas models containing both field-collected and remotely sensed data were adequate for all 6 species. Improvements in the availability of more accurate remote sensing technology would likely lead to improvements in the ability of the models to predict nesting locations. External validation with data from other wildfires is necessary to confirm the general applicability of our habitat-suitability models to other forests. Land managers responsible for maintaining habitat for cavity-nesting birds in postfire landscapes can use these models to identify potential nesting areas for these species and select areas in burned forests where postfire salvage logging is most likely to have minimal impacts on cavity-nesting bird habitats.     

Fall movements of Red‐headed Woodpeckers in South Carolina

Fall migration of Red‐headed Woodpeckers (Melanerpes erythrocephalus) can be erratic, with departure rates, directions, and distances varying among populations and individuals. We report fall migration departure dates, rates, and routes, and the size of fall home ranges of 62 radio‐tagged Red‐headed Woodpeckers in western South Carolina. Rates of fall migration differed among years; all radio‐tagged woodpeckers migrated in 2005 (15 of 15), none (0 of 23) migrated in 2006, and 54.2% (13 of 24) migrated in 2007. Of 28 woodpeckers that left their breeding territories, we relocated eight either en route or on their fall home ranges. These woodpeckers migrated short distances (4.3–22.2 km) south along the floodplain forest of a large creek. The variable migration patterns we observed indicate that Red‐headed Woodpeckers may best be described as facultative migrants. We determined the home range sizes of 13 woodpeckers in both seasons, regardless of whether they migrated, and fall home ranges were smaller (mean = 1.12 ha) than summer home ranges (mean = 3.23 ha). Fall‐winter movements of Red‐headed Woodpeckers were concentrated on mast‐producing oak (Quercus spp.) trees, which may have restricted home range sizes. The partial migration we observed in 2007 suggests that factors other than mast crop variability may also influence migration patterns because woodpeckers that year responded to the same annual mast crop in different ways, with some migrating and some remaining on breeding season home ranges during the fall.