  Transport measurements through a few-electron circular quantum dot in bilayer
graphene display bunching of the conductance resonances in groups of four,
eight and twelve. This is in accordance with the spin and valley degeneracies
in bilayer graphene and an additional threefold 'minivalley degeneracy' caused
by trigonal warping. For small electron numbers, implying a small dot size and
a small displacement field, a two-dimensional s- and then a p-shell are
successively filled with four and eight electrons, respectively. For electron
numbers larger than twelve, as the dot size and the displacement field
increase, the single-particle ground state evolves into a three-fold degenerate
minivalley ground state. A transition between these regimes is observed in our
measurements and can be described by band-structure calculations. Measurements
in magnetic field confirm Hund's second rule for spin filling of the quantum
dot levels, emphasizing the importance of exchange interaction effects.
