If you touch a baby bird or nest, the parent birds will reject it because they smell human on it.
Most birds have a poor sense of smell. Parent birds typically recognise their young by sight and sound, not scent. Studies show that nestlings handled by humans are usually accepted back by parents. However, it's still better not to touch nests as it can cause stress and parasitic exposure, just not for the reason most people believe.
Reception
Sources
- Bird Sense of Smell - Audubon Society PRIMARY
- Snopes: Baby Bird Rejection REFERENCE