Why don't my doves roost in the warm?

I have a pair of diamond doves in my outside aviary, and they share the space with finches and canaries. I built a large night-box for them all, which has plenty of space for roosting on bars and baskets. There is also a heater with a thermostat. The box has a Perspex front for light, and two holes with perches on both sides to aid entry and exit.

I'm concerned that the doves don't roost in the box, whereas all the other birds do. The doves don't go in the box at any time, day or night. The holes are plenty big enough, so that can't be the problem. On one occasion I caught them both and put them in the box, but I can't do that every night when a frost is forecasted. They just don't seem inquisitive enough to explore the box for a safe haven.
S.L., via email.

Bob Baggs, foreign bird expert, replies: The best way to encourage birds to use a house or shelter is to initially place their food and water containers inside, and keep them in for a few days. Your diamond doves should also prefer to roost on the highest perches available, so these ought to be in the shelter. Getting them used to feeding and roosting inside, they should continue to do so when allowed out into the aviary.

Although diamond doves are reasonably hardy, they should not be exposed to the weather we have experienced in recent winters. If they still refuse to spend time in the box, perhaps you could protect them during the coldest months by covering the roof and any open sides of the aviary with PVC, or something similar. 

Are there too many birds occupying the box? A constant jostling for perches may have discouraged your doves from entering.