NOT a good idea!
The following is based on research of chickens but it also applies to ducks and other avian species.
2 and 3 month old birds are way too young to be on a diet of 4% calcium. 4 months is the absolute minimum age for layer feed. That may still be too young in a backyard situation whereas commercial flocks grown in blackout housing and a strict lighting program insure the birds are all laying by 5 months or so, yet they still have mortality due to urolithiasis.  A backyarder has no guarantee when their birds will start laying and needing to build egg shells. Until they do, 1% calcium is sufficient.
Excess calcium will cause mineralization of all organs but especially kidneys. Chickens have 2 kidneys, each with 3 divisions. Continued ingestion of excess calcium will cause those divisions to stop functioning  Only 2 functioning divisions are needed for sustained egg production and the continued appearance of health. Damaged kidneys cause uric acid to build up in the blood and deposits calcium crystals in the liver, heart, air sacs and joints but particularly the kidneys. Atrophied or missing kidney portions are compensated by enlargement of the remaining viable portions to maintain adequate kidney function.
Bilateral kidney failure causes visceral gout. Birds with gout rarely show any symptoms until within 24 hours of death.
 
http://nhjy.hzau.edu.cn/kech/synkx/dong/2bao/UrolithiasisChina.pdf
http://ps.oxfordjournals.org/content/67/12/1694.abstract
http://www.hyline.com/aspx/redbook/redbook.aspx?s=5&p=36
http://www.neobio.biz/database/n-5data5-2/n-5data5-2-7.pdf
Even when ducks stop laying in the fall, they should come off the layer feed and switched to a maintenance feed.
http://www.forthebirdsdvm.com/pages/care-and-feeding-of-pet-ducks
 
The species doesn't matter.
http://www.2ndchance.info/goutpollocAvianrenaldis.pdf
 
Usually when people have birds die, they don't get a necropsy so they have no idea what killed them.
A friend of mine recently lost her pullets. This is the content of an e-mail I received.
 
"We got the pathology back a few days ago. Dr. ******* said our chickens had died of visceral gout. We are guessing they ate too much of the big girl food with the calcium in it before they were actually laying eggs and this caused kidney problems. Or they weren’t getting enough water because we had just moved them with the big girls and maybe the bog girls were running off a couple of the little girls from the water. We just don't really know for sure. But Dr. ******* said that it was this visceral gout that showed up on the pathology.