  On the Sun, the rotation periods of individual sunspots not only trace the
latitude dependence of the surface rotation rate, but also provide clues as to
the amount of subsurface fluid shear. In this paper we present the first
measurements of stellar differential rotation made by tracking the rotation of
individual starspots with sizes comparable to the largest sunspots. To achieve
this we re-analyse four sequences of densely-sampled, high signal-to-noise
echelle spectra of AB Doradus spanning several stellar rotations in 1996
December. Using spectral subtraction, least-squares deconvolution and
matched-filter analysis, we demonstrate that it is possible to measure directly
the velocity amplitudes and rotation periods of large numbers of individual
starspots at low to intermediate latitude. We derive values for the equatorial
rotation rate and the magnitude of the surface differential rotation, both of
which are in excellent agreement with those obtained by Donati & Collier
Cameron (1997) from cross-correlation of Doppler images derived a year earlier
in 1995 December, and with a re-analysis of the 1996 data by the method of
Donati et al (2000). The differences between the rotation rates of individual
spots and the fitted differential rotation law are substantially greater than
the observational errors. The smaller spots show a greater scatter about the
mean relation than the larger ones, which suggests that buffeting by turbulent
supergranular flows could be responsible.
