Influence of large snow depths on Black Woodpecker Dryocopus martius foraging behaviour

Abstract

Large snow depths are assumed to constrain the Black Woodpecker's (Dryocopus martius) ability to feed on carpenter ants (Camponotus herculeanus) in stumps and logging debris following clearcutting . To document foraging behavior in a snow-rich area, we radio-marked three Black Woodpeckers in Nordmarka, southcentral Norway, and compared the results with data from a nearby snow-poor area at Varaldskogen (Rolstad et al . 1998). At snow depths below ca . 1 m, birds were feeding on carpenter ants in stumps and dead downed wood. At snow depths above 1 mbirds increasingly fed on carpenter ants in the base of trunks of infested living trees, and on bark beetles (Ips typographus) and beetle larvae in dead standing trees. Home ranges at Nordmarka were markedly larger (mean = 449 ha, SD = 71, n = 3) than at Varaldskogen (mean = 226 ha, SD = 109, n = 23). One female at Nordmarka, killed by a pine marten (Martes martes), had lost 20% of body mass due to starvation . The results indicate that winter food limits Black Woodpecker populations in snow-rich managed forests, and we suggest that arboreal feeding on bark beetles renders the birds more vulnerable to goshawk (Accipiter gentilis) predation.