That would be my impression, yet I noticed the "feeding" dialog is structured like you could choose something else to (otherwise it would be "how many rods?"). Any naturalist out there knows the answer?
Apparently they particularly like eating fresh-made rods - take your metal machine out to a sloth tree and see - make sure the music feature of your machine is enabled.
The boring answer is that the same "picker" menu format is using for anything, even if there's only one type of thing you can actually use in that context. See also spinning fiber - what else am I gonna spin, and yet it always asks.