A lot depends on the age of the ducklings. New-hatched will need some supplemental heat. I use a brooder plate so they can have darkness to sleep in, but if you like to use a regular light bulb, that may work.
You just need to watch them and see how they're doing. They'll huddle together as far as they can get away from you (you're seriously terrifying), but try to observe from farther away if you can. They shouldn't be constantly peeping (unless the light is keeping them from sleeping). Put the light on one side of the brooder and their food & drink on the other. If they stay huddled under the light, crying, you may need it lower (closer). If they never use it or they get as far as possible from it, raise it. After a week or less, start raising it, always sensitive to their behavior. They likely won't need it very long in your weather conditions.
I have 2 wk old goslings (three of them) that I put out in a mobile coop last night. I turned off their brooder plate at one week. They snuggled in the tractor coop's nest area and did just fine. Last night's low was in the low 50s. The high today was in the 70s.
