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Eastern gray squirrels (Sciurus carolinensis) are considered to be a pest by many bird-lovers because they take significant quantities of seed from birdfeeders. None of the available methods of protecting birdseed against squirrels is completely effective. We assessed the efficacy of treating birdseed with capsaicin oleoresin as a means of deterring squirrels. Consumption of treated and untreated whole, black-oil sunflower seed was compared by carrying out one-choice feeding trials at 3 sites near Ithaca, New York, from 11 May to 24 June 1999. The heat strength of the treated seed was 40,000 Scoville Heat Units (SHUs) (2,424 ppm) on the shell and 2,000 SHUs (121 ppm) on the heart. At each site, we provided 600 g of seed at a feeding station for one 3-hr session each day, and recorded the weight of seed consumed. Observations of feeding behavior by squirrels, birds and Eastern chipmunks (Tamias striatus) were recorded throughout the 3-hr session on 2 days per week at each site during most weeks. Untreated seed was provided in weeks 1,2, and 4; capsaicin-treated seed was offered in weeks 3, 5, and 6. We concluded that treatment with capsaicin significantly reduced both the amount of seed taken by squirrels and the total time squirrels spent feeding. The reduction in squirrel feeding time was primarily due to a decrease in the duration of feeding visits. Visitation rates by birds were unaffected by seed type at 2 sites, and increased with the treated seed at the third site. Seed type had no effect on the species composition of the birds visiting the feeder. The treated seed was not effective in deterring Eastern chipmunks from taking the seed.