Ringy, the doves are not nesting for egg laying in the winter. They are just trying to stay warm, drink and eat. They will be fine in your garden, as thats why they chose that location. Their only problem in a hard freeze is access to water, but your temps get over freezing during the day.
Saw a water dish for birds that had an all weather light socket in a bowl underneath it, and a 40 watt bulb in there.
Would look in cutaway view something like this:
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Sorry Babe, but down south here the Mourning Doves are nesting. We have two of them trying to nest on our boat. All the pics I took last winter of the baby Doves was here at this time of year. I think they are like Sparrows and nest when they want if the weather is good enough. ;)