Preferences and apparent digestibilities of sugars by fruit damaging birds in Japan

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SummaryGray starlings Sturnus cineraceus, azure‐winged magpies Cyanopica cyana and brown‐eared bulbuls Hypsipetes amaurotis are among the main bird pests in commercial fruit orchards in central Japan. Recently Brugger & Nelms (1991) suggested that developing high‐sucrose fruit cultivars could reduce crop damage, because some pest birds lack the enzyme sucrase and can develop an aversion to sucrose. Preferences for, and digestibilities of, the monosaccharides glucose and fructose and the disaccharide sucrose by these pests species were therefore studied to assess whether this idea would be applicable in central Japan. Gray starlings and brown‐eared bulbuls were able to detect glucose, fructose, a mixture of glucose and fructose, and sucrose at a concentration of 12% w/v. Azure‐winged magpies also detected glucose and fructose, but failed to detect sucrose at the same concentration. In pairwise preference trials gray starlings and azure‐winged magpies selected the monosaccharides over sucrose, but brown‐eared bulbuls did not. To estimate the digestibility of the sugars the apparent assimilated mass coefficient, AMC*, was calculated for each species eating each sugar by measuring intake and faecal output. Monosaccharides had mean AMC*s of 0.77, 0.96 and 0.92 when consumed by gray starlings, azure‐winged magpies and brown‐earned bulbuls respectively. AMC* values for sucrose were 0.82 and 0.49 for brown‐eared bulbuls and azure‐winged magpies respectively, but gray starlings were shown to be unable to digest sucrose. As AMC* values varied from 0.75 to 0.97, consumption rates of sugars increased as digestibility decreased. Although increasing sucrose contents of commercial fruits may deter sucrase‐deficient birds such as gray starlings from depredating fruits, it may also lead to increased crop damage by species such as the azure‐winged magpie and brown‐eared bulbul which may have to consume more of the less digestible fruit in order to meet their energy requirements.