Long-Distance Delivery

Photo by Darren Swim / Creative Commons

In the world of trees, it is difficult for seeds and seedlings to take root far from their parent plants, yet that’s a key factor in seedling survival. “The closer you are to mom, the more you’re competing with her, the more specialized predators will find you, the more likely you are to pick up diseases specific to that tree,” explained biologist Mario Pesendorfer. “There are tremendous advantages to traveling far.”

That’s where blue jays and other birds in the corvid family come in. The large seeds of pines, oaks, and other trees are served up by trees as food for the jays, but because of the birds’ proclivity for storing seeds – many more than they will ever eat – for later consumption, they help the trees disperse their seeds, sometimes great distances.

Pesendorfer, a postdoctoral researcher at Cornell University, reviewed dozens of research studies about the “scatter hoarding” behavior of jays and crows, and he found that the birds play an unheralded role in maintaining the health and productivity of the world’s forests. He noted, for instance, that the birds are picky about what seeds they select – ignoring rotting acorns and those containing weevils or fungi – thereby increasing the likelihood that viable seeds will be dispersed. A bird transports the seeds in its bill or crop, ensuring that the seeds remain unharmed; other forms of seed require a rigorous trip through an animal’s gut. And the seeds are often hidden in the ground at a depth that is ideal for germination.

“It’s a mutualistic interaction,” Pesendorfer said, “one that blue jays in the Northeast are particularly good at.” During the most recent ice age, the treeline was pushed far to the south, but as the ice retreated, the treeline moved northward at the unexpectedly high rate of several hundred meters per year. “Once the climate conditions were suitable, the treeline rebounded very quickly, and this was likely due to seed dispersal from blue jays,” he said.

Blue jays have been seen transporting acorns up to four kilometers, and in one study, a population of blue jays collected and dispersed more than 130,000 acorns in one year from just 11 oak trees. Ninety percent of these acorns were stored in habitats suitable for the growth of new oak trees.

Pesendorfer believes that forest managers should consider taking advantage of birds’ ability to disperse seeds by manipulating the birds into helping restore habitats. It’s already happening in Germany and several Mediterranean islands: in years when acorn production is low, foresters there set out baskets of acorns for the jays to disperse.

“Some techniques, such as seed addition, the creation of space that jays use for caching, and the creation of habitat islets as a seed source, have long been used by foresters and managers,” wrote Pesendorfer in the journal Condor. “Nonetheless, we believe that there is greater potential for the use of corvid ecosystem services and engineering in the habitat maintenance and restoration context.”

 
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