March 5, 2012
In the coming weeks, students in a Cornell University course studying bird banding and tracking techniques will be contributing a series of blog posts about recent research focused on species that visit feeders in North America. In this first installment in the series, sophomore Kathryn Grabenstein takes a closer look at a study focused on the relationship between Clark’s Nutcrackers and trees in the mountains of the west.
By: Kathryn Grabenstein, Cornell University class of 2014
Recent research demonstrates how Clark’s Nutcrackers help regenerate the forests upon which they rely for their own survival. A member of the Corvidae family, which includes ravens, crows, and jays, Clark’s Nutcrackers occasionally visit bird feeders in the Western half of North America where they can often be seen leaving with a beak full of seed for later consumption. Nutcrackers are scatter hoarders, meaning that they store seeds in many different locations with the intent of returning to eat the seeds at a later date. These seed storage sites are known as ‘caches’—some caches are never retrieved and these seeds may germinate and grow into new plants. In the Western pine forests, nutcrackers play an important role in seed dispersal, caching up to 98,000 seeds per year and spreading them nearly 500 times farther than seed dispersing mammals, such as chipmunks, and nearly 20 times farther than the wind. For one tree species—the whitebark pine—nutcrackers are obligate mutualists, meaning that nutcrackers and the trees depend upon on another. The nutcrackers require the food resource (seeds) provided by the trees, and the trees require the seed dispersal services of the nutcrackers. Whitebark pines, however, are suffering a rapid decline due to invasive insect pests, parasite outbreaks, and fire suppression. A recent study by researchers in Washington State sought to better understand the relationship between nutcrackers and whitebark pine trees.
Researchers followed 12 birds for four years in the eastern Cascade Mountains using radio telemetry, a technique in which a small radio transmitter is attached to a harness on the back of the birds. Radio waves from the transmitters are picked up by a receiver through an antenna, much like a typical AM/FM radio. Each bird is assigned a separate frequency and researchers can flip back and forth between birds just like flipping through channels on a radio. The radio signals allow researchers to pinpoint the location of the birds and track their movements. All 12 of the birds were followed both on foot and by an airplane as they were caching seeds in an effort to locate their cache sites.
Researchers found that nutcrackers transported seeds as far as 32 kilometers (20 miles) from the harvest location and almost always cached inside their home ranges, even when they harvested the seed from locations beyond the borders of their typical home range. Birds tended to cached more seeds at low elevations than at high elevations. At lower elevations, nutcrackers were more likely to cache below ground, whereas at the higher elevations, 59% of caches were above ground.
This phenomenon of caching most seeds above ground may be found in wetter, colder climates where seeds stored underground may be inaccessible during snowy weather or may deteriorate quickly in wet weather. Whatever the reason, the above ground caching in no way benefits the whitebark pine because it inhibits germination. Although most seeds may be consumed or otherwise prevented from germinating, nutcrackers may be effective seed dispersers simply because of the sheer numbers of seeds that they disperse. Approximately 15% of whitebark pine seed caches were stored in locations that were suitable for germination. Considering that tens of thousands of seeds are stored each autumn, nutcrackers are dispersing thousands of seeds to viable locations, helping to “plant” future generations of the trees. The researchers note that even if only 15% of seeds are cached in habitats where they could germinate and survive, that equates to an impressive minimum of 4,800 seeds dispersed to germination-friendly locations per nutcracker each year.
Kathryn Grabenstein is a sophomore Biology major at Cornell University concentrating in Neurobiology and Behavior. She works at the Lab of Ornithology analyzing paternity in the Red-backed Fairywren, a small insectivorous bird in Northern and Eastern Australia. During winter break 2011-2012, Kathryn had the opportunity to travel Down Under to conduct behavioral observations on this species. She writes that her trip was, “an amazing educational and scientific experience, which included two sightings of Albert’s Lyrebird!”
Source: Lorenz, T.J., Sullivan, K.A., Bakian, A.V., & Aubry, C.A. (2011). Cache-Site Selection in Clark’s Nutcracker (Nucifraga columbiana). The Auk 128:237-247.