Eminence of sexually selected characters is believed to be mediated through parasites and costly immune system. Gastrointestinal parasites are responsible for impairment of digestion and absorbing nutrients. In this study we investigated how experimental manipulation of coccidian parasite load affects the faecal fat content of greenfinches (Carduelis chloris). The birds were divided in three experimental groups: control group, experimentally infected group and a group cured with anticoccicidian medicine sulfadimethoxine. Faecal fat was measured with acid steatocrit method which to our knowledge has never been used in birds before. We found the acid steatocrit method to be suitable for using in bird study systems. The treatment had significant effect on the change of percentage of steatocrit. Faecal fat content increased in all birds in the infected group, except for one bird. The results indicate that parasites impaired the organism's ability to absorb fat from the dietary intake. The underlying mechanisms might be tissue damage caused by parasites or host's immune response. Previously it has been shown that parasites infections decreases carotenoid levels in plasma and feathers which suggest carotenoid malabsorption. As carotenoids are fat soluble, the fat malabsorption might also be responsible for less intense carotenoid plumage coloration.