Do my Ducklings need heat?

[jawst]

I had 2 little Aylesbury duckling chicks hatched from my incubator about a week ago and are about 15 cm+ high when standing! They are kept in large vivarium type unit, it has plenty of ventilation and i use a 150w ceramic heat bulb + stat set to 28degrees C but ducklings sit away from heat source. Where they sleep is about 21 degrees C.
I have moved lamp to opposite side but ducklings move away from heat again, should i take away the heat source??