Foraging plasticity by a keystone excavator, the white-headed woodpecker, in managed forests: Are there consequences for productivity?

Abstract

Information on the foraging ecology of animals is important for conservation and management, particularly for keystone species whose presence affects ecosystem health. We examined foraging by an at-risk cavity excavator, the white-headed woodpecker (Picoides albolarvatus). The foraging needs of this species are used to inform management of ponderosa pine (Pinus ponderosa) forests in some areas of western North America. Past observational studies indicated that white-headed woodpeckers forage predominately on cones and trunks of large-diameter (>68cm) pines in old-growth stands, although habitat selection while foraging has not been formally examined. We used radio telemetry to track forage substrate use among 37 adult, breeding woodpeckers for 176h (10,576min) in forest stands that had been recently thinned and/or burned with prescribed fire. We used discrete choice models to examine forage site selection and multinomial regression to examine consequences of foraging on nest productivity. Woodpeckers foraged on more than ten individual substrates and switched substrates seasonally, presumably to take advantage of prey availability. Dead wood and fir foliage were used commonly in the nesting period (86% and 68% of foraging, respectively), whereas pine foliage and trunk foraging dominated in the fledgling (66% of foraging) and post-fledgling periods (73% of foraging). Average size of used trees was 49cm (±20cm) and pine cones were rarely used (4% of foraging). During the nesting period, substrate use (χ2=1.49, df=4, P=0.83) and distances traveled from nests for foraging did not affect productivity (F(3,16)=0.61, P=0.62), which was high even for birds with the longest (2.1km) and shortest (0.39km) maximum forage distances. Habitats selected for foraging matched substrate use, and woodpeckers selected areas with low basal areas of live trees in the nesting period, but high basal areas in the post-nesting period. The variable foraging that we observed suggests that white-headed woodpeckers are plastic in their foraging in managed forests, and this plasticity has no negative consequences for productivity.