Wouldn't this happen as they are flying around anyway? I've only seen them do this at our farm in the morning after they roosted for the night on our fences before they take off.

https://turkeyvulturesunning.weebly.com/scientific-background.html

>Turkey vultures' main reason for sunning is to raise their body temperature so they can have enough energy to go about their daily activities. 

>It is important to note that turkey vultures are one of few birds that intentionally put themselves into a daily hypothermic state each night, lowering their body temperature a few degrees.