HAEMATOZOAN PARASITES, MATING SYSTEMS AND COLORFUL PLUMAGES IN SONGBIRDS

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.