Hoarding food and Finding hidden caches

[Beatriz Cazeneuve]

Hoarding Food

Birds living in environments that produce food in abundance during one season of the year, and relatively little (or none) during others, may cope with the uneven supply in several different ways. One strategy is to leave the area for the periods of shortage, as many North American birds do in migrating from barren, cold, temperate, or polar regions to the relatively food-rich, warm tropics for the winter. The hazards of migration are accepted in return for continuity of food supply. Another strategy is to stay through the times of shortage and scrounge hard for the food that is available, as do ptarmigan and even chickadees, although throughout the northern winter the latter remain daily poised on the razor's edge of starvation.

A third strategy is to harvest more food than is needed during periods of seasonal abundance and to store it. This strategy, too, has its dangers; the store may be robbed or may spoil. Long-term storage -- what might be called hoarding -- is practiced in North America by various woodpeckers, some jays, Clark's Nutcrackers, and nuthatches. Techniques of storing differ from species to species. Groups of Acorn Woodpeckers place surplus acorns in one or two storage sites near the center of the group territory -- dead trunks and limbs are studded with acorns jammed into shallow holes. Up to 50,000 acorns may be stored in a single snag (standing remains of a dead tree), which is known as a granary. The woodpeckers aggressively defend their acorns, which are so tightly hammered into the holes that it is difficult for potential robbers to retrieve them without being detected or, in many cases, to remove them from the holes at all. Other woodpeckers use cracks and cavities to store acorns and various other nuts. Interestingly, Lewis' Woodpeckers may (for unknown reasons) husk acorns before storing them, and individuals of at least one woodpecker species, the Red-bellied, apparently do not defend their stores.

Nutcrackers and jays tend to store acorns, nuts, seeds, pine cones, and other items in the soil or under loose litter, as well as in elevated niches and cracks. A Clark's Nutcracker may store more than 30,000 pinyon pine seeds during a single season, placing them in caches with 4-to-5 seeds each. The trees are not always passive victims in this game, however. For instance, pinyon and other pines are coevolving with Clark's Nutcrackers and Pinyon jays, with the trees affecting the birds' evolution, and vice versa. The pines appear to have evolved cone and seed structures and fruiting times that increase the chance for their seeds to be buried (planted) by the birds. For example, whitebark pines have been shown to be dependent on Clark's Nutcrackers for their reproduction. Other animals harvest and feed on their seeds, but only the nutcrackers cache them in places favorable to the growth of the tree and at an appropriate depth for seedling establishment.

The nutcrackers and jays, in turn, have evolved to exploit this resource. Clark's Nutcracker has evolved both a long, sharpbill for use in prying the pine seeds from their cones, and a special pouch under the tongue for carrying them. One individual was found with 38 seeds, weighing a total of more than an ounce, in its pouch. A powerful flier, the nutcracker can carry its burden of seeds a long distance. It can start breeding as early as February by exploiting its hoarded seeds.

North American jays have not proceeded quite as far down that evolutionary path as the nutcracker. Pinyon jays store seeds in an expandable esophagus, but are unable to transport as large a load as the nutcracker. Steller's jays have a less specialized bill, and an esophagus of smaller capacity, and Scrub jays (which are the least dependent of all on stored food in the winter) can carry only one seed at a time.

Both birds and pines benefit from the relationship. The birds get fed and the trees get millions of their seeds buried, not just in the immediate vicinity, but in adjacent areas for miles around. Movement by the birds promotes rapid colonization by the pines of new areas made suitable for them as a result of climatic change. In contrast to these mutually beneficial relationships, Acorn Woodpeckers seem to be predators on oaks, because generally they do not place the acorns in sites suitable for germination. Thus the oaks gain little, if any, benefit from the relationship.

Some birds, especially parids (titmice, chickadees), some raptors, owls, shrikes, and (occasionally) woodpeckers, store animal prey, but this seldom extends beyond short-term storage of a few days, because such prey soon decay. Exceptions may be the victims of northern owls and parids, for fall-stored mammals and insects may be preserved by cold and used over a longer period. This is indicated by the recent discovery that Great Horned, Boreal, and Saw-whet Owls thaw frozen prey by incubating them as they would eggs! All the thoroughly studied, long-term-storing birds use conifer seeds, acorns, and other nuts that are energy rich and yet have a hard, impervious coat that retards spoilage. Some acorns apparently can be stored for up to a year or more.

Careful observations are needed of nuthatch hoarding behavior, about which little is know. Also desirable would be information on the elapsed time before hoards are recovered. Marking of the stores and repeated observations should help to solve the question of their "shelf life" and their relative importance to the birds.



Finding Hidden Caches

How does a long-term hoarder like Clark's Nutcracker recover stored seeds when it needs them to feed its young? Ornithologists at first thought that the food was stored only in certain kinds of areas, and that the birds rediscovered it by later foraging in the same areas. But recent experiments with nutcrackers in aviaries, done by Stephen Vander Wall of Utah State University, have shown clearly that individuals are able to recall where they have cached seeds. The birds remember where the seeds are in relation to certain landmarks, such as rocks. If the landmarks are moved, the areas the birds search are displaced an equivalent amount.

The results of these experiments confirmed observations by behaviorist Diana Tomback, who examined the distribution of beak marks in earth and snow where nutcrackers had searched for caches. The marks were not random, but rather unsuccessful probes were clustered in the vicinity of successful ones (indicated by the presence of pinyon-seed coats, which the birds removed before eating the seeds, next to the hole). In addition, early in the year before rodents started to find the caches, the birds found caches on about two out of three attempts, far more frequently than one would expect if they were searching at random. This capacity to remember the sites of stored food seems to be an evolutionary enhancement of a spatial memory that is more widespread in birds. For example, titmice often store food in diverse places when it is abundant. They show excellent short-term (hours-days) memory for the storage locations, as elegant experiments by British avian behaviorist John Krebs and his colleagues have shown.