If she stays on the nest at night instead of roosting with the other chickens, chances are she is broody. Other signs of broodiness include-
1. Not coming off the nest during the day, except for one or two short periods of time where she puffs up at all the other chickens and squawks bloody murder if she thinks one of them gets a little too close, finds something to eat in a mad dash, grabs a few drinks and rushes back to the nest.
2. Not wanting you near the nest, puffing up at you and squawking loudly.
3. Not wanting any other chickens near the nest.
4. She stops laying eggs.
Good luck! Call ducks are neat birds.
Edited to add- If the nest is deep, she can incubate more eggs. I'm not sure how big Call duck eggs are, so I don't know how many she can brood.