  Above-barrier cross sections of $\alpha$-active heavy reaction products, as
well as fission, were measured for the reactions of $^{10,11}$B with
$^{209}$Bi. Detailed analysis showed that the heavy products include components
from incomplete fusion as well as complete fusion (CF), but fission originates
almost exclusively from CF. Compared with fusion calculations without breakup,
the CF cross sections are suppressed by 15% for $^{10}$B and 7% for $^{11}$B. A
consistent and systematic variation of the suppression of CF for reactions of
the weakly bound nuclei $^{6,7}$Li, $^{9}$Be, $^{10,11}$B on targets of
$^{208}$Pb and $^{209}$Bi is found as a function of the breakup threshold
energy.
