Age-Related Differences in the use of Time and Space by Radio-Tagged Grey Herons (Ardea cinerea) in Winter

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Abstract
(1) Spatial distribution and time budgets of first-year and adult grey herons (Ardea cinerea) were analysed to answer the question whether age-related differences in the use of time and space by piscivorous predators exist, which could affect the foraging success and survival of the birds. (2) First-year grey herons, compared with adults, were radio-tracked proportionally more on a fish-farm, and on roosts around the farm and proportionally less on fish-ponds. Mean length of a stay on foraging grounds was shorter for this age-group, which, however, compensated through more daily visits to feeding sites. Data on variation in the rate of fishing success indicate that first-year grey herons, more than adults, improve their foraging efficiency by visiting a fish-farm. (3) First-year grey herons showed more variation in spatial distribution in the course of a season, between seasons and according to weather conditions than adults did. Variations in the use of time, on the other hand, were much larger for the adult birds. In the course of the day, the proportion of locations on foraging grounds did not vary much for first-year grey herons, in contrast with what was found for the adults. The latter seemed to select the most profitable periods of the day to forage, as far as fishing success is concerned. (4) Our data suggest that young birds are less flexible with regard to their use of time, but adopt more easily a change in spatial distribution. First-year grey herons seem to forage more on less typical areas and in less suitable periods of the day, due to either a lack in skill in hunting and in selecting the best time and place to forage, or due to exclusion from better periods and place to forage by adults.