S is for substrate.
Some waterfowl pull nesting material down from surrounding vegetation, crafting a beautiful nest. They might pull in straw from nearby if it takes their fancy.
Some prefer a cavity; we usually line a box with nesting material to protect the eggs if it has a solid base. Good substrate materials are fresh wood shavings, straw, an upturned turf or chopped hemp. Some customisation from the householder is expected!
