Keep in mind I¡¦m talking eastern indigos here:
The habitat preference changes seasonally, with ¡§tortoise town¡¨ being the preferred habitat in fall and winter. All bets are off and they go everywhere in the spring and summer.
The coldest the burrows EVER get (judging from a cold winter day in GA) is about 50F.
If your snakes are not basking, there are several reasons to consider:
1- The warm part of the cage is too hot¡Kmid seventies this time of year may be a little warm for the hide. They will be fine¡Kbut just as happy as can be in the hide. No reason to come out. A hide of 65-68 at night and low 70¡¦s during the day will encourage them to come out.
2- They don¡¦t¡¦ feel secure enough to come out. Try putting a small ¡§secondary¡¨ hide in a warm part of the cage. When indigos come out of the primary retreat (the burrow) in the wild, they bask¡Kthen when warm enough they will (rather than go back in the burrow) often stay up, but take shade under a palmetto bush¡Kpalm fronds, etc. If you put in a small hide in the warm area, they will go in and out of both hides (encouraging more activity which is good). Also, they may stay out resting along the side of the secondary hide or wrapped around it¡Kthese snakes (especially youngsters) don¡¦t like to feel like they are out in plain view. Don¡¦t make them choose between feeling secure and thermoregulating.
Large adults (esp. males) are another story. They will bask out in the open for all the world to see with one of those bumper stickers that says: AINT SKEERT. ļ
Hope this helps