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

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

Repeat this for the whole time the bird is laying the eggs, when she is at the 5th or 6th egg, you can then take away the fake eggs and introduce 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 equal in size and have the same chances as being fed and cared for with less loss of hatchlings.

http://www.windycityparrot.com/Planit-USA-Canary-Eggs-6-pc-set-breeding-canaries.html