A friend gave me this egg today. It is the egg of the largest living bird, an Ostrich.
It weighs about 1325 gms, which is a big omelet in any ones language, equalling about 20 chickens eggs.
Now an Ostrich is a pretty big bird, here's one my son, Pete, took earlier. It weighs in the region of 100-120 Kg. The nesting habits are a bit strange with several females laying in the same nest, but it seems that a senior female probably contributes between 5 and 11 eggs with a average of eight. That means that on average an Ostrich lays eggs equivalent to about 10% of it's body weight. I suppose that this is considerably more than a woman having a baby which on average is around 5%-7% of the mums weight. However consider the poor little over worked Blue Tit in my my nest box. She weighs in at around 11 gm when she is in good condition and lays a clutch of on average 10 eggs (varies from 5-16). As each egg weighs around 1.1 gm she lays eggs from half, up to 1.6x her body weight, averaging out at about he own weight. Since she lays one egg a day the amount of her resources that egg production and laying takes up is quite fantastic.
So when you see one Blue Tit feeding another in early spring, it's not a sign of love and affection, it's the males selfish genes trying to ensure his off spring have the maximum chance of being fit and healthy.