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The nesting behaviour of the Great Blue Heron ( Ardea herodias L., 1758) was studied in western Canada in 1998 and 1999 to (i) investigate how individual parents assess risk when repeatedly exposed to a disturbance stimulus (an investigator) and (ii) experimentally test in the field whether any variation in their nest defense behaviour was due to experience with the disturbance stimuli or the stage of the nesting period. Heron response declined through the nesting period and the level of response varied among colonies, suggesting different perceptions of risk among groups of herons in the study. It was experimentally shown that variation in the response of herons through one nesting period was due to both behavioural habituation and the stage of the nesting period. Response was stronger in 1999 than in 1998, indicating that habituation did not hold between nesting periods. In general, habituation in herons may bode well for their potential to persist in areas with light human use. But irrespective of habituation, stimuli early in the nesting period and large or novel events may still cause herons to abandon their nests owing to the effects of the stage of the nesting period.

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