An article in a forthcoming issue of The Condor: Ornithological Applications may make you look at corvids — the nearly cosmopolitan group of birds that includes ravens, crows, and jays — in a new light.
The article, a review of scientific literature and an examination of case studies from around the world, explores how oaks and pines depend on corvids to reproduce and spread — and suggests that the birds may be the key to helping the trees weather the challenges of habitat fragmentation and climate change.
The review was written by Mario Pesendorfer, of the School of Biological Sciences at the University of Nebraska–Lincoln, and three colleagues from the Smithsonian Migratory Bird Center, Cornell Lab of Ornithology, and Nature Conservancy.
Corvids store seeds in small caches spread across the landscape, a behavior called scatter-hoarding. The birds typically cache more seeds than they eventually eat. When the overlooked seeds subsequently sprout, scatter-hoarding becomes seed dispersal, a process that helps trees colonize new areas.
Arizona birder Charles J. Babbitt described two North American scatter-hoarders in our June 2014 issue — Pinyon Jay and Clark’s Nutcracker. Both species are omnivores, but they eat a wide range of foods and prefer pine seeds when available. In winter, each is almost entirely dependent on the seeds it collected and cached the previous autumn.
Their caches, write Pesendorfer and his team, are essential to establishment and maintenance of ponderosa pine populations in the western United States.
The reviewers also describe scatter-hoarding by Eurasian Jays, which are associated with oak and pine habitat throughout their range, extending from England to Japan. The jays love to eat acorns, and, more often than not, they cache the seeds in abandoned fields, roadside areas, mowed grass, and other disturbed areas. These areas often contain low densities of oaks.
“As habitat fragmentation and climate change affect European hardwoods,” conclude the authors, “Eurasian Jays are likely to be an increasingly valuable ally for the oaks.”
Familiar Blue Jays play an important seed-dispersing role, too. According to the review, they speed forest-fire recovery here in North America by increasing their caching effort after fires and selecting canopy gaps as cache sites. The birds are known to transport acorns remarkable distances — as much as 4 km, almost 2.5 miles.
Harnessing this behavior may aid habitat restoration, writes Pesendorfer. Europeans have been aware of the relationship between jays and oaks for centuries. Indeed, managers in western Europe are planting small stands of seed-source trees and then relying on corvids to help disperse their seeds across the landscape.
Read the paper
Mario B. Pesendorfer, T. Scott Sillett, Walter D. Koenig, and Scott A. Morrison, 2016, Scatter-Hoarding Corvids as Seed Dispersers for Oaks and Pines: A Review of a Widely Distributed Mutualism and Its Utility to Habitat Restoration, The Condor: Ornithological Applications, Volume 118, pp. 215–37.
Among the case studies recounted in Pesendorfer’s review, the most promising comes from Santa Cruz and Santa Rosa, the two largest islands in California’s Channel Islands National Park, south of Santa Barbara.
Likely aided by the endemic Island Scrub-Jay, one of the rarest and most range-restricted passerine species in the United States, oak and pine vegetation on Santa Cruz Island has more than doubled since livestock removal began in the late 1980s.
By contrast, recovery of oak and pine habitats on nearby Santa Rosa Island, which lacks Island Scrub-Jays, has been slow. Restoration, write the reviewers, will require either an enormous investment of capital and human labor or a very long time, perhaps centuries.
What to do? Send in the scrub-jays, say Pesendorfer and his co-authors.
“Translocation of Island Scrub-Jays to Santa Rosa Island,” they speculate, “is a potential strategy to both abate threats associated with the jays’ small population size and restricted range and accelerate restoration of the island’s oak and pine habitats.”
Pesendorfer is now a postdoctoral associate at the Cornell Lab of Ornithology. The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society.
Read Hidden Today, Found Tomorrow, Charles J. Babbitt’s 2014 article about Pinyon Jay and Clark’s Nutcracker.
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