  Collective migration of cells and animals often relies on a specialised set
of "leaders", whose role is to steer a population of naive followers towards
some target. We formulate a continuous model to understand the dynamics and
structure of such groups, splitting a population into separate follower and
leader types with distinct orientation responses. We incorporate "leader
influence" via three principal mechanisms: a bias in the orientation of leaders
according to the destination, distinct speeds of movement and distinct levels
of conspicuousness. Using a combination of analysis and numerical computation
on a sequence of models of increasing complexity, we assess the extent to which
leaders successfully shepherd the swarm. While all three mechanisms can lead to
a successfully steered swarm, parameter regime is crucial with non successful
choices generating a variety of unsuccessful attempts, including movement away
from the target, swarm splitting or swarm dispersal.
