Parental investment is a key topic in avian ecology, and many authors have focused on nestling-feeding behaviour to analyse this issue. Surprisingly, most studies have based their results on feeding patterns recorded over periods of only one or two hours, possibly leading to over generalizations regarding temporal-dependent behavioural patterns. Irrespective of nestling age or brood size, if we use observations from such short periods as conclusive evidence we must assume that parents behave consistently across time and that the window of time selected is representative of parental effort. To test this assumption, we analysed the time consistency of nestling provisioning rates and prey composition of 32 breeding pairs of Mediterranean Great Tits Parus major, from dawn to midday (7 hours recording). Regardless of a parallel decrease in the intensity of work for both sexes, we found that hourly provisioning rates per nestling correlated strongly with the mean number of feedings per nestling and per hour recorded over the whole 7 hours of recording. Weather conditions and nestling age had no effect on hourly provisioning rates per nestling, although parents with older nestlings worked relatively less hard. We also observed that the peak of morning activity was higher in nests with small clutches. Prey proportions showed a high degree of temporal repeatability, but nestling diet composition should be studied with caution. Although prey composition was stable over time, we recorded a strong decrease in the number of prey items delivered by parents throughout the day, along with an increase in prey size. We thus recommend using a wider time window to obtain reliable results when studying prey composition. In any case, considering our results, one hour of recording may be sufficient to describe nestling-feeding behaviour of Great Tit parents.
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