The Hamilton-Zuk hypothesis of sosigonic selection argues that intensi
ty of mate choice and corresponding ornamentation is positively relate
d to pressure from parasites that are competent to engender positive h
eritability of fitness. Research into this idea has been controversial
, not least for the original group of species studied: nearctic passer
iforms. These birds are examined yet again, addressing certain publish
ed reservations about previous results: (1) using phylogenetic regress
ion to cater for similarity linked to shared ancestry, (2) investigati
ng only independently scored brightness data, (3) excluding species re
presented by small samples of birds examined for parasites, as well as
(4) those represented by zero haematozoa prevalences, and (5) control
ling for sampling effort, body weight and ecological variables. For th
e data as a whole the association between total brightness and the num
ber of birds parasitised by haematozoa relative to the number of indiv
iduals examined (haematozoa relative presence) was not consistently si
gnificant. Likewise when front and back brightnesses were considered s
eparately nonsignificance resulted. On controlling for ecological vari
ables, significant positive regressions were found more often with fro
nt brightness than with back brightness. Furthermore, brightness and m
ating system interact significantly with each other in their relations
with the parasitism index. Following from this, omission of overt pol
ygynists produced a robustly significant, positive, relationship betwe
en front brightness and relative parasite presence. This regression, i
nvolving 82 apparently monogamous species, explained only ca 10% of th
e total variation. The findings support parasite-mediated sexual selec
tion, while at the same time emphasising additional factors in the det
ermination of bird species' brightness and sexual selection intensity.