Why just ducks? How about chickens?

[Powergirl]

As shown in much research, egg mass is shown to increase over the incubation period because the life form, otherwise known as the chick, is growing and developing greatly. As the mass of the chick is increasing the overall mass of the egg is increasing with it.

 

[Speceider]

I believe you have it wrong....egg mass declines in a linear manner throughout incubation.

Clint