How Nutcrackers Find Their Seed Stores

-The Clark's Nutcracker (Nucifraga columbiana) stores conifer seeds in the late summer and fall of each year. During winter and spring, seeds from buried caches are the major food of nutcrackers and their young. A bird must find more than a thousand seed caches each year. The alternative means by which nutcrackers may locate seed stores are (1) memory of cache site and (2) trial and error search. While searching for stored seeds, a Clark's Nutcracker makes a series of prod holes with its bill. A hole from which a cache was recovered is usually distinguished by a pile of broken seed coats. This allows an estimate of a nutcracker's success rate in locating seed caches. Theft of seed caches by rodents and removal of intact seeds from cache sites by nutcrackers may confuse this estimate. A field technique based on deductive reasoning is presented to determine how nutcrackers recover their caches. In the eastern Sierra Nevada I gathered field data on the success rate and nearest neighbor distances among prod holes in spring and summer of 1975. Data indicating the proportion of caches taken by rodents were obtained in 1978. Analyses of these data suggest that nutcrackers find most of their caches by means of memory. A search based on memory implies that seed stores are not communal and that nutcrackers must remain in the area where they stored seeds until the following summer. Theft of seed stores by rodents must be an important selective pressure on nutcracker caching behavior. By caching seeds in storage areas which become covered with deep snow, nutcrackers may reduce their losses to rodents. Many birds of the family Corvidae store food for future use. Food storage is a major event in the annual cycles of the genera Garrulus, Aphelocoma, Gymnorhinus and Nucifraga (Tur6ek and Kelso 1968). Both species of Nucifraga, Clark's Nutcracker (N. columbiana) and the Eurasian Nutcracker (N. caryocatactes), store conifer seeds in late summer and fall (Bibikov 1948, Reimers 1953, Kuznetsov 1959, Vander Wall and Balda 1977, Tomback 1977a). These stores are the major food of the nutcrackers from winter to mid-summer (Kuznetsov 1959, Mezhenny 1961, Holtmeier 1966, Tomback 1977a). Both species breed in early spring, raising young almost exclusively on stored seeds (Mewaldt 1956, Swanberg 1956, Reimers 1959a). Stores made by nutcrackers consist of many small clusters of seeds hidden in a variety of sites throughout the montane habitat (Bibikov 1948, Swanberg 1951, Reimers 1959b, Kishchinskii 1968, Tomback 1977a). I will use the term "cache" to denote one cluster of seeds; seed stores consist of many caches. The means by which a nutcracker finds these stored seeds has not been satisfactorily explained and remains an intriguing problem in animal behavior. The most direct approach to this problem would be xperimentation on captive birds under controlled conditions. R. P. Balda (in press) recently concluded a series of controlled investigations of seed recovery behavior in the Eurasian Nutcracker. His findings concur with previous field observations (Swanberg 1951, Mezhenny 1964, Crocq 1977, Mattes 1978) and also present new information. However, it is possible that the limited space and predictable stimuli of experimental enclosures can bias results. Before similar experimental work is undertaken on the Clark's Nutcracker, confirmation of laboratory findings with field data is desirable. In this paper I present the results of a field investigation of how Clark's Nutcrackers recover their seed stores. THE NATURE OF THE PROBLEM Vander Wall and Balda (1977) calculated that a single Clark's Nutcracker may store between 22,000 and 33,000 pifion pine seeds (Pinus edulis) in the late summer and fall of each year in northern Arizona. This qua tity represents between 2.2 and 3.3 times the energetic requirement of the nutcracker. I calculated (Tomback 1977b, unpubl. data) that each year in the eastern Sierra Nevada one Clark's Nutcracker may store as many as 32,000 whitebark pine