Myth: Handling bird eggs and chicks in the nest is the cause for the parent abandoning them.
This is what you hear most often then not, that the eggs or chicks were abandoned due to human touch. It is a Myth and not a Fact.
It is very important to understand that the birds have very poor sense of smell. So it is not possible to smell the human smell.
In the wild the nest is visited by many creatures and it does not affect the nest. I have read some where that some have experimented by spraying sunk spray on the eggs and chicks with no adverse effect.
Birds in general do not abandon their eggs or chicks that easily. They try their best in defending them.Sometimes while defending the eggs are broken or trampled to death.
Other reason is when they are unable to defend and leave the nest they come under stress, and do not return for a while, but in rare cases they do not return to the nest thus abandoning the eggs or chicks. It is because of stress and not because of smell.
I can give you a one example from my personal experience of a Lorikeet breaking its egg during the nest inspection few years back. When I opened the nest box. I found an egg. I put my hand to remove it. One of the bird jumped in and tried to pull the egg away me but accidentally pecked the egg.
But there are many questions that need to be answered If birds can not smell or taste, Then how do they avoid eating toxic monarch butterflies? How do hummingbirds distinguish water from sugar water?