Bright male Great Tits have a low parasite intensity
Abstract from Functional Ecology
3rd May 1996
There is a theory published by Hamilton and Zuk in 1982 that male birds might develop very bright secondary sexual characteristics to signal to the females that they have lesser parasite loads than less bright birds. These would not be able to divert resources from fighting their infections to developing the bright (and costly) plumage.
Dufva and Allander from Uppsala University tested this theory by checking smears from 35 male Great Tits that were also scored for their brightness of yellow on the belly. The bright birds had a higher heterophil level probably indicating a better immune response to parasites than the duller birds.
The blood of the birds was also scanned for parasites present but no meaningful relationships were discovered. The value of the plumage feature acquired in the autumn for females deciding on who to mate with some six months later would still be a real signal if, as the authors suggest, it is connected to genetic resistance to parasites. This sort of study is beginning to unravel just what the she Great Tit might be looking for in her mate - and that beauty is much more than skin-deep!