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Bird repellents that are effective have tended to be toxic, while those that are relatively nontoxic have tended to be ineffective. There is a need for repellents that work well and safely. Interest has focused on the natural chemical defenses used by plants to defend themselves from herbivores. Preferences of bullfinches in orchards for different pear cultivars were correlated with biochemical differences between cultivars. A class of plant secondary compounds has been isolated and shown to be physiologically active against bullfinch and pigeon gut enzymes, and also to deter feeding in the laboratory. The physiological and biochemical mechanisms responsible for their repellency are under investigation.