Bird normally operates without Adj-RIB-In, but can be configured to operate with one:

import table switch

A BGP import table contains all received routes from given BGP neighbor, before application of import filters. It is also called Adj-RIB-In in BGP terminology. BIRD BGP by default operates without import tables, in which case received routes are just processed by import filters, accepted ones are stored in the master table, and the rest is forgotten. Enabling import table allows to store unprocessed routes, which can be examined later by show route, and can be used to reconfigure import filters without full route refresh. Default: off.

(I assume this contains routes discarded because of an incorrect OTC attribute but I have not verified this. Even then I'm not sure Bird can (currently) use it to give you information on why routes were filtered)