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In page Yellow-billed stork:

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Nestlings show remarkable behavioural transformations at 3 weeks of age. During the constant parental attendance before this time, the young show little fear or aggression in response to intruders (such as a human observer), but are found to merely crouch low and quietly in the nest.[1] After this time, when both parents go foraging and leave the young in the nest, a nestling shows strong fear in response to an intruder. It either attempts to climb out of the nest to escape or acts aggressively toward the intruder.[1]