  We assess the global properties of associated C IV 1548,1550 absorption lines
measured in the spectra of radio-loud quasars drawn from a near-complete,
low-frequency selected sample. The observations span restframe C IV in two
redshift ranges -- 0.7<z<1.0 and 1.5<z<3.0 -- which were targetted in the UV
with HST/STIS and in the optical with ground-based telescopes, respectively.
First, we corroborate trends for C IV associated absorption to be found
preferentially in steep-spectrum and lobe-dominated quasars, implying the
absorbing material tends to lie away from the radio-jet axis. Furthermore, we
find a clear anticorrelation between C IV absorption strength and the projected
linear size of steep-spectrum quasars, indicative of an evolutionary sequence.
We also find that heavily-absorbed quasars are systematically redder, implying
dust is mixed in with the C IV-absorbing gas. No redshift dependence was found
in any of the trends considered. These new results show that radio sources are
triggered in galaxies which are exceptionally rich in gas and dust, which then
dissipates on a timescale comparable with, but less than, that of the radio
source. This observational sequence, together with the lack of
redshift-dependence, points to a direct causal link between the event which
triggered the radio source and the build-up of absorbing gas and dust, whose
make-up is tantalisingly similar to the products of a post-merger starburst.
Thus, these new results provide direct evidence for the clearing of absorbing
material around quasars with time, as well as the probable association of
starburst activity with the onset of the radio activity in AGN.
