Coadaptations of the Clark's Nutcracker and the Pinon Pine for Efficient Seed Harvest and Dispersal

Abstract

Clark's Nutcracker (Nucifraga columbiana) is one of four species of corvids that commences harvesting, transporting, and caching the seeds of pinon pine (Pinus edulis) in mid—August. Caching continues until inclement weather intervenes or the seed crop is depleted. Nutcrackers are efficient at collecting seeds from the trees because the birds discriminate edible and aborted seeds, select cones with higher than average number of good seeds and appear to concentrate on those trees that produce cones with large numbers of good seeds. The use of a sublingual pouch allows the nutcracker to carry up to 95 pinon pine seeds per trip (x = 55). A pouch full of seeds may be carried as far as 22 km from the collecting area (the pinon—juniper woodland, elevation ~ 1800 m) to communal caching areas where the seeds are buried in clumps in subterranean caches at a depth of 2 to 3 cm. These caching areas are on steep, south—facing slopes that are usually free of snow by late winter. Although solitary nesters, the birds, during the nonbreeding season, often form flocks when harvesting seeds. A flock of 150 nutcrackers cached between 3.3—5.0 X 106 pinon pine seeds in the autumn of a good cone crop. Each bird caches between 2.2 and 3.3 X its needed energetic requirements for the insect—free portion of the year. Some of the unrecovered seeds germinated and grew. The pinon pine reaches its highest density in relatively dry areas below the elevational range of other members of the genus Pinus, but ranges up to 3,100 m into the mixed coniferous forest. This pine has developed a number of traits that enhance its efficiency to be located and dispersed by the birds. The seeds are unusually large, of high energy value, and have relatively thin seed coats. The large size improves the germination potential of the seed but also attracts predators and dispersal agents. The seed coat color is markedly different for edible and aborted seeds, thus labeling them for visually oriented seed harvesters. Most pinon pine cones are oriented outward and upward which increases illumination of the seeds and tends to hold them in the cone. The pinon pine displays the seeds in its cones more readily than wind—dispersed pines by opening the seed—bearing cone scales to a uniform angle. Pinon pine seeds are retained for a long period of time in the cone because they are held in deep depressions on the cone scales by small flanges. This constellation of features indicated an evolutionary strategy on the part of the pinon pine to exploit and encourage members of the seed—caching guild to disperse the seeds. Deposition sites are presumably more favorable for germination than those obtained by simply broadcasting the seeds over a wide area. Individual seedlings potentially experience immediate intraspecific competition because the seeds are cached in clumps. Only one seedling, presumably the strongest, fastest—growing individual, survives. The benefits of this system to the nutcracker are that the bird has a supply of energy for the most unproductive months of the year. The bird also breeds in late winter or early spring, using the stored seeds for reproductive energy.