How old should ducklings be to be introduced to chickens?

[MadisonOde]

Hey all,
We have 3 4 week old khaki campbell ducklings. They have outgrown the heat lamp and now live in a temporary pen in the chicken run. They sleep in a dog kennel in the coop at night. At what age should I be able to introduce them to the chickens? Thanks!

 

[Duck Lover88]

I would introduce them when they are the same or almost the same size as the chickens...