Tyranid hive fleets do not travel through the Warp like Imperial starships. Yet, the hive fleets' incredibly fast rate of advance into the galaxy belies the argument made by some that they do not possess a mode of faster-than-light travel.
Whilst it is true that the Tyranids are forced to remain at sublight speeds when within the gravitic boundaries of a planetary system, they are capable of superluminal velocities when travelling through interstellar space. This capability is the result of a small Tyranid bio-ship classified by the savants of the Imperium of Man as a Narvhal.
Unlike most Tyranid bio-ships, the Narvhal is almost completely defenceless, and possesses almost no bio-weaponry and only a very thin protective carapace. A cluster of monofilament spines on the Narvhal's bow allow it to take in a wide variety of sensory input, including a broad spectrum of gravimetric and electromagnetic signals.
Using these sensors, the Narvhal can detect new planetary systems at extreme interstellar distances. In some unknown manner it then makes use of the origin star system's own gravity and creates a compressed space-time transit corridor through which the Narvhal and other Tyranid bio-ships can traverse interstellar distances.
This form of space-warping travel cannot be used near strong gravitational sources as they overwhelm the Narvhal's hypersensitive navigational sensors. As a result, a Tyranid hive fleet must use more conventional forms of biologically-induced reaction-based propulsion during the final approach to a new star system, which can slow their arrival by Terran years or even entire solar decades.
Whist this form of propulsion is ultimately slower than a reliance upon the Warp-Drive, it is a great deal more reliable and allows the Tyranids to move implacably, if slowly, across the galaxy.
However, the Narvhal's manipulation of a star system's gravity does sometimes produce unintended side effects. A destination world can suffer from earthquakes, solar flares, tidal waves and other natural disasters produced during the interval during which the Hive Fleet is in transition through the space-time transit corridor.
This chaos only further benefits the Tyranids' invasion, however, as the defenders of the target world will have their resources bound up in dealing with these problems when the swarm finally arrives in orbit and the first Mycetic Spores begin to fall.
- Codex: Tyranids (5th Edition), pg. 19