Blue Jay: Acorn Planters

Tuesday, January 12, 2010

Nature Notes

by Bob Thomas

As I do most mornings, I filled my bird feeders, including putting a couple of handfuls of whole peanuts in a platform feeder on my deck. As I settled in with a cup of coffee, birds began to arrive. The peanuts are there for the blue jays (Cyanocitta cristata), and this morning about six raucous individuals took advantage of the free meal.

One blue jay made an effort to stuff a peanut into its expandable throat, as it does with acorns. Each bird typically picked up several peanuts, seemingly looking for the right size. Most peanuts were too big and could not pass into the throat, so the bird grabbed one with its beak and flew away. Some of the peanuts were eaten immediately on top of the fence or a limb overhead. Most jays, however, flew away over nearby houses with their treasures.

Watching blue jays eat peanuts and acorns is neat. As in other New World jays, they have a specialized joint between the lower jaw and skull that prevents unhinging of the two when the lower jaw is used like a jackhammer to split seeds open. This “buttress complex” is composed of a variety of marvelous bony adaptations that allow the lower jaw to move, but remain connected to the skull as the bird uses it to pound seeds with considerable force. The rhamphotheca (the black, horny covering of the mandibles – what we see as the “bill”) is shaped like a chisel at the tip. The blue jay holds a seed with its feet against the substrate, slightly opens the mouth, and uses the lower jaw only as a battering ram.

There have been a number of interesting studies about blue jays and their kin harvesting, transporting, and storing seeds from oaks, beeches, chestnuts, pecans, and more. An article by W. Carter Johnson and Curtis S. Adkisson (1986, Airlifting the oaks, Natural History 10: 40-47) summarizes our knowledge of this fascinating behavior.

Each fall, large numbers of blue jays may gather in healthy stands of trees. Johnson and Adkisson found that individuals consistently gathered seeds during their hundreds of visits to the forest during the fall. The jays ate about 25% of the crop to fuel their trips and transported the remaining useful seeds back to their territories as far as five miles away.

The authors believe that, if the birds make ten round-trips per day, each may fly 1,500 miles or more per month.

On each trip, the blue jays fill their expandable throats, and then usually add one more acorn by holding it in their beak. The researchers found that the jays can carry about seven pin oak acorns per trip. Pin oak acorns are slightly smaller than our local southern live oak acorns.

A study in Virginia revealed that a community of 50 blue jays moved and cached about 150,000 acorns harvested from 11 pin oak trees during one season. At the same location, nearly all acorns that were not harvested were either consumed by the blue jays or destroyed by weevils.

Upon arriving at their territories, the blue jays disgorge the seeds from their beaks and throats. They then systematically hide the seeds by burying them in the soil or by pushing them under vegetation one at a time.

Blue jays make choices about which trees are harvested, and which and how many nuts are gathered. The birds then make choices about where and how the seeds are cached. These facts suggest that blue jays play an important role in determining forest structure and quality.

Carter and Adkisson found a germination rate of 88% in beechnuts taken by blue jays, whereas there was a 10% germination rate among beechnuts the researchers randomly collected from the same trees. Blue jays appear to test the nuts by holding them in their beaks or shaking them. Those that they deem unworthy are simply dropped to the ground.

Blue jays tend to cache a huge surplus of nuts which thereby allows some acorns to be eaten by small mammals. Many are left to germinate and repopulate the forest.

As in all endeavors, there are costs and benefits associated with such behavior in blue jays.

Benefits are obvious: immediate food, cached food available over the winter, or buried food awaiting their return to their territories from winter migration.

The costs include the energy they expend collecting and moving the seeds, as well as their increased exposure to predation during longer flights required between collecting sites and their territories. The latter is mediated by birds avoiding open grasslands. Instead they fly along tree lines and fencerows where they may dart into cover if attacked by a raptor such as a Cooper’s hawk.

In summary, blue jays are not only attending to their own needs by their seed caching behaviors, but they also attend to the needs of the trees by selectively planting the most viable seeds produced by trees, thus ensuring a greater chance of germinating success. Also, by spreading the acorns over a wide area in the flock’s territories, blue jays enhance the forest’s richness of genetic variability, thus ensuring greater adaptability across diverse landscapes.

An interesting application of this knowledge involves the reforestation of North America over the past 10,000 years following the end of the Ice Age. The development of the massive ice sheets and their movement south of todays Chicago area displaced many species, including the trees discussed in this story. For decades, zoogeographers have wrestled with understanding how those and related tree species migrated north and reformed the enormous Austroriparian forest of eastern North America. It is theorized that before the pilgrims reached America, a squirrel could climb a tree in Missouri and travel to the Atlantic shore without touching the ground, illustrating the continuity and density of the forest. The question: how did those heavy-seeded trees like oaks, beeches, and chestnuts move so quickly back to their northern environs as the glaciers melted and retreated. In fact, data suggest that the trees made a mad dash northward, undoubtedly aided by none other than blue jays. Historical information indicates that the oak forests moved northward at an average pace of 380 yards per year. Carter and Adkisson’s studies show that blue jays transporting nuts up to several miles from host trees may well result in an average spread of germinating trees of several hundred yards per year.

Blue jays are incredible instruments of ecological change.

by Shannon Fronberry