Caching at the Microsite Scale: P R a R U TM U LT ENT of AGRIC Pacific Northwest Region Clark’S Nutcracker Cache Site Selection, Part III 2011
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United States Department of Agriculture D E E Forest Service Caching at the Microsite Scale: P R A R U TM U LT ENT OF AGRIC Pacific Northwest Region Clark’s Nutcracker Cache Site Selection, Part III 2011 WITH EASE, nutcrackers fly into the canopies of trees, What microsites did ground caches were placed poke at the bark, and tussle bunches of lichen. Why? They’re nutcrackers select for within 9 m of a tree offering placing more than half the seeds they collect up in the seed caches in this study? possible cover. There was no canopies of living trees, a good way to keep seeds out of the significant selection for sites deep winter snow, but not good places for young trees to We observed 655 caches germinate and grow. How important is microsite, anyway? placed by 12 resident with a particular slope, aspect, nutcrackers; 450 ponderosa or ground cover type. BACKGROUND seed caches, 199 whitebark Why does it matter? We investigated habitat use, caching behavior, and seed caches, and 6 caches migratory patterns in Clark’s nutcrackers in the Pacific of an unknown seed type. The caching behavior of Northwest using radio telemetry. Over 4 years (2006– Among ponderosa and nutcrackers in autumn 2009), we captured 54 adult nutcrackers at 10 sites in the whitebark pine caches, they determines patterns of Cascade and Olympic Mountains in Washington State. cached 59 percent of seeds regeneration in whitebark We fitted nutcrackers with a back-pack style harness. above ground in the forest pine. This is because The battery life on the radio tags was 450 days, and canopy (385 caches) and the nutcrackers subsist year- we tracked nutcrackers year-round, on foot (to obtain remaining 41 percent below round on pine seeds, which behavior observations) and via aircraft (to obtain point ground in soil, gravel, or are produced only in autumn, locations). We obtained more than 6,000 telemetry point forest litter (264 caches). Most so the birds need to store locations on radio-tagged nutcrackers, and we observed aboveground caches were seeds in order to have food more than 1,000 seed-harvest events and 655 seed- placed in living trees with for winter and spring. caching events. foliage (86 percent, or 330 of Nutcrackers place the seeds in 385 caches), and caches were small “caches” of one to five Of nutcrackers captured in this study, we classified 20 placed under slips of bark, seeds and, for various reasons, nutcrackers as residents and 21 as emigrants wintering in clumps of needles, or in some caches are never on our study area. Among residents, 11 had home ranges bunches of lichen. Most below retrieved. Clark’s nutcrackers in whitebark pine stands and 9 had home ranges in ground caches were placed play a critical role in seed ponderosa pine stands. in sites with understory or dispersal for more than 10 The future of whitebark pine is of serious concern overstory cover and within 4 conifer species in western because of the species’ vulnerability to white pine blister m of a tree that could be used North America. Some species, rust, mountain pine beetle infestation, wildfires, and for cover from predation (see like the declining whitebark climate change. The Clark’s nutcracker is the primary table, next page). Even when pine, rely on nutcrackers for means of whitebark pine seed dispersal. caching seeds in open habitats all seed dispersal. like talus slopes, nutcrackers selected sites near trees. Overall, 93 percent of below TO INVESTIGATE microsite scale caching, we compared features at nutcracker cache sites to a random sample of microsites THE STUDY’S Clark’s nutcracker trap sites. available within home ranges. FACT SHEET 3 of 8 TABLE—Number of whitebark and ponderosa pine seeds placed in above- and below-ground cache sites and in different habitats by 12 resident Clark’s nutcrackers from 2006 through 2009 Whitebark pine seed caches Ponderosa pine seed caches Number Number of above- Number of below- Number of above- Number of below- Habitat type placed in ground caches ground caches ground caches ground caches habitat type Cliff/talus 83 13 23 11 36 Mixed high 75 64 5 4 2 elevation forest Mixed low elevation 183 36 32 73 42 forest Parkland whitebark 3 2 0 0 1 pine forest Parkland ponderosa 134 5 4 58 67 pine forest Whitebark pine 57 7 2 47 1 forest Ponderosa pine 108 2 4 62 40 forest Burns 6 0 0 1 5 Total 129 70 256 194 Greg Okimi Greg THIS MAP shows the placement of aboveground and below ground caches by 4 Clark’s nutcrackers in the autumns of 2006–2008. Note that most aboveground caches were placed on north-facing slopes and at high elevations (sites that receive and accumulate high amounts of winter snow) compared to below ground caches, which were placed on south-facing slopes or at lower elevations. The photo shows the terrain depicted in the map, with the steep, south-facing slope used for most below ground caches and the higher elevation, north-facing whitebark pine stand used for aboveground caches. How specifically did we found that most were placed What does habitat-scale therefore, they are likely to study ‘microsite scale’ at the bases of logs or trees. selection have to do with select cache sites that will selection? Dimmick (1993) observed microsite-scale selection? enable easy access during We first followed radio-tagged 911 caches and found that We found that the selection winter months. Although nutcrackers to their cache 60 percent of caches were of a microsite depended they are capable of digging sites, marking the locations placed near trees that could be highly on the cache habitat through snow to retrieve of all observed caches using used for cover. These results type (see table, facing page). seed stores, it is unlikely portable GPS units. Then we suggest that nutcrackers most In high-elevation whitebark that they are capable of noted whether the cache was commonly cache seeds below pine and mixed forest types, digging through the 3-m- placed below ground (in the ground and near living trees. the nutcrackers we observed deep snowpacks typical of soil, forest litter, or gravel) However, these studies did placed 90 percent of all seeds the higher elevation forest or above ground (in stumps, not measure the availability in aboveground cache sites. types in our study area. By logs, or tree branches). of microsite features, so there In low-elevation forests (or selecting aboveground cache For all accessible below is no information on whether south-facing slopes), they sites in those habitat types, ground caches, we revisited features were selected (used placed only 56 percent of nutcrackers place the seeds the cache sites in summer. in greater proportion to caches aboveground in trees well above the level of winter We estimated the following their availability), avoided and favored below ground snowpack, where they can parameters within a 1-m- (used in lesser proportion to caches. Why? Nutcrackers retrieve them. radius circle centered on each their availability), or simply typically retrieve their seed cache site: used in proportion to their stores in winter and spring; availability (Manley et al. 1. Percentage canopy cover; 2002). 2. Percentage understory cover; IN OUR study of cache 3. Percentage ground site selection, nutcrackers cover by rock, soil, litter placed most seed caches in (usually pine needles), and aboveground sites. Nutcracker vegetation; #312 is shown here, caching ponderosa pine seeds under 4. Slope; a slip of bark on a Douglas-fir 5. Aspect; branch. 6. Distance to nearest tree that could be used for cover from predation; and 7. Patch type. We then measured these same features at one random site located within 30 m of the cache site. We used logistic regression to evaluate whether features were selected or WHEN CACHING seeds avoided relative to their in below ground locations, availability (Manly et al. nutcrackers selected concealed 2002). sites, presumably because this minimized predation risk. This How does this compare photo shows nutcracker #632 caching ponderosa pine seeds to past studies? in a site shaded with nearly 100 Previous studies revealed percent understory cover. that nutcrackers select a wide variety of microsites for caching. Among 24 observed caches, Tomback (1978) THE BOTTOM were placed in suitable some locations (i.e., a natural FURTHER READING LINE: MANAGER’S sites—that is, in whitebark regeneration approach). Dimmick, C.R. 1993. Life PERSPECTIVE pine stands or on talus slopes Our results show that history and the development (suitable habitats) and below nutcrackers disperse relatively Can we count on of cache-recovery behaviors nutcrackers to restore ground (suitable microsites). few whitebark pine seeds in Clark’s nutcracker. Ph.D. whitebark pine This number seems very to suitable sites. Although dissertation, Flagstaff, AZ: populations? low. Because nutcrackers nutcrackers evolved as seed Northern Arizona University. are co-evolved mutualists of dispersers for whitebark pine, We quantified seed dispersal whitebark pine, we expected they are likely most effective Lorenz, T.J.; Sullivan, K.A.; effectiveness of Clark’s that most seeds would be only when there are mast Bakian, A.V.; Aubry, C.A. nutcrackers by determining placed in suitable microsites. crops. 2011. Cache-site selection in the numbers of whitebark But most seed dispersal Clark’s nutcracker (Nucifraga pine seeds placed in locations Given that whitebark pine columbiana). Auk. 128(2): effectiveness achieved at the seed production has dropped that were favorable for the landscape and habitats scales 237−247. establishment of mature, due to tree mortality caused was compromised at the Manly, B.F.J.; McDonald, L.L.; reproductively active trees.