While observing a bird's behaviour the observer moved too close and the bird flew away. This is an example of what effect?

Correct answer: The observer's presence altered the behaviour

Direct answer: Option A, the observer's presence altered the bird's behaviour. In natural observation, the aim is to see what an organism normally does without disturbance. The observer came too close, the bird detected a possible threat, and it flew away; therefore the recorded behaviour was influenced by the observer. Option A is correct and illustrates observer or observer-effect bias. Option B is wrong because nothing in the event shows that the bird's mass increased, and mass does not explain the flight. Option C is wrong because no measurement unit changed; the problem concerns behaviour. Option D is meaningless and scientifically unsupported: a question cannot prove itself. To reduce the effect, observe from a safe distance, remain still, hide from view, use a camera, or observe indirectly. Memory cue: if the observer changes what is observed, the observation is not fully natural.