What specific geological feature is targeted for the kingfisher's underground nesting tunnel?

Stream banks, road cuttings, or soil near tree roots.

The Oriental Dwarf Kingfisher exhibits unique underground nesting habits, deviating from tree-nesting behavior. It excavates a horizontal tunnel, which can extend up to a meter in length, typically into areas like stream banks, road cuttings, or soil located near tree roots. These locations provide the necessary substrate for burrowing. A vital adaptation of this structure is that the tunnel is deliberately inclined upwards from the entrance. This specific upward slope serves a crucial protective function: it prevents water, especially during monsoon periods, from flooding the terminal egg chamber and also assists in draining waste materials, thus safeguarding the eggs and young chicks.