What are fake canary eggs used for?

Canaries lay eggs 5 to 6, one egg a day. When they have a full clutch the hen will continue to sit on them until they hatch and feed them. Since they were laid a day apart, by the time the last egg hatches, the first baby is 3-5 times the size of the newest hatchling.

The baby birds intinctually shove the smaller birds out of the nest or get in the way of the feeding of the smaller birds. This can result in the loss of 1 or 2 birds.

The artificial eggs are used as decoys for the real eggs. The hen will sit on the fake ones too. When the hen lays one egg, take a spoon and carefully remove the real egg from the nest and put in a safe place, on a bed of cotton, tissue or toweling. 

Repeat this for the entire time the bird is laying the eggs, when she is at the 5th or 6th egg, you can then remove the fake eggs and replace the real ones in the nest. She will sit back on them and they will all develop and hatch the same day. 

Thus all the babies will be the same size and have the same chances with being fed and cared for with less loss of hatchlings.