Nutrition, Energy and Passage Time: Experiments with Fruit Preference in European Blackbirds (Turdus merula)

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Abstract
The patterns of fruit consumption by European blackbirds (T. merula) were examined in relation to in vitro nutritional values, the per cent apparent digestibilities, per cent metabolizable energies, metabolizable energies, the pulp and seed passage rates and intake rates of 6 fruit spp. which occur in Wytham Wood, Oxfordshire, [England, UK]. The in vitro nutritional values, per cent apparent digestibilities, per cent metabolizable energies and seed and pulp passage rates of the fruit species differed significantly. Blackbird preferences among winter and spring-ripening fruits were positively related to the in vitro energy, per cent metabolizable energy, metabolizable energy, per cent apparent digestibility and seed passage rates of fruits. Preferences of birds for autumn-ripening fruits were related to seed passage rates only. Seed passage rates may play an important role in determining perference, particularly if nutritional and other properties (e.g., accessbility) of fruit species are similar. Calculations show that birds obtain a high rate of energy gain by consuming fruits whose seeds are regurgitated. This is because gut volume may place a constraint on fruit uptake. Seed regurgitation results in a rapid elimination of non-nutritional seed ballast and creates space in the gut for additional food. Fruit species containing seeds which are defecated have lower rates of energy gain because the seeds remain (and take up space) in the gut for much longer periods of time. Fruit intake rates did not appear to constrain energy intake and did not correspond with blackbird preferences.