Most birds are very shy during the breeding season, especially during incubation and when the young arc newly hatched. Remember that tits, especially the great tit, which during the winter may even be so bold as to eat from one's hand, are very sensitive to disturbance during the breeding season. Disturbance may shock the young into leaving the nest a little too soon, resulting in them falling easy victim to predators. Therefore you must never look into nest boxes during the breeding season.
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