Cache-Site Selection in Clark's Nutcracker (Nucifraga Columbiana)

Abstract

Clark's Nutcracker (Nucifraga columbiana) is one of the most specialized scatter-hoarding birds, considered a seed disperser for four species of pines (Pinus spp.), as well as an obligate coevolved mutualist of Whitebark Pine (P. albicaulis). Cache-site selection has not been formally studied in Clark's Nutcrackers, which are considered effective seed dispersers for pines because past studies have found that they harvest and store large quantities of seeds. Although many seeds are placed in sites suitable for germination and establishment, information is lacking on the proportions of seeds placed in suitable versus unsuitable sites. We used radiotelemetry to investigate cache-site selection and evaluate the suitability of selected cache sites for establishment of Whitebark and Ponderosa (P. ponderosa) pines. On a landscape scale, Clark's Nutcrackers cached seeds centrally within home ranges, even though this required them to transport seeds up to 32.6 km. They selected low-elevation forests for caching, presumably because these sites accumulated little snow. When caching at high elevations, the birds placed most seeds in aboveground microsites. Only 15% of Whitebark Pine seed caches (n = 155 caches) were placed below ground and in habitats where seeds could germinate and seedlings grow. For comparative purposes, 42% of Ponderosa Pine seed caches were placed in suitable habitats and below ground. Although Whitebark Pine is an obligate mutualist of Clark's Nutcracker, our study suggests that Clark's Nutcrackers in some populations may be more effective seed dispersers for Ponderosa Pine than for Whitebark Pine.