Path validation has already been incrementally deployed in the Internet architecture. It secures packet forwarding by enabling end hosts to negotiate specific forwarding paths and enforcing on-path routers to prove their forwarding behaviors along these paths. Most existing path validation solutions target static paths, paying less attention to fully dynamic paths that support flexible routing. In this paper, we present Hummingbird as the first validation solution over fully dynamic paths. It features a hidden equal-probability sampling technique. Gaining efficiency via routers probabilistically sampling packets to validate, we craft the sampling probability such that each router validates a similar amount of packets given an unknown path length. We further hide the state of whether a packet has been sampled and validated using a lightweight, non-cryptographic scheme. This prevents attackers from differentiating and selectively mis-forwarding packets. We validate security and efficiency of Hummingbird through both theoretical proof and experimental evaluation.