Game biologist in AZ was experimenting with infra red sensors for the purpose of making field counts of wild turkey. Said he found that the turkeys could manipulate their down/feathers such that their infra red signature was not detectable with the very good equipment he was using. From this he surmised that a bird capable of locking in heat could probably limit scent with the same control. So I suppose a nesting bird might be doing just that to protect the nests.