Biologically active substances of bird skin: a review

Bird skin has a number of specific properties. The uropygial gland is a significant skin gland in many species. The secretion of this gland is particularly necessary for maintaining physical characteristics, including feather waterproofing. In some bird species this gland secretion has a repellent effect against potential mammalian predators; in other species it affects the final colour of feathers. In the investigated species of storks (genus Ciconia), secretions of the uropygial gland have been found to be mixtures of monoester waxes, diester waxes, triester waxes and triglycerides. Wax diesters were also found in the red knot Calidris canutus (order Charadriiformes). Lipid substances in the secretions of the rock dove (Columba livia) consist mainly of unsaturated fatty acids (59% secretion; mostly oleic acid – 37%, linoleic acid – 6% and arachidonic acid – 7%). Free fatty acids, which are decomposition products of epidermal lipids, can regulate microbial colonization of skin (e.g., by modification of pH); a shift of these values was detected in poultry in battery husbandry. Analysis of fatty acids from lipids shows the influence of age, diet, and also the relationship to feather pecking – the individual composition affects the smell and taste, and thus the attractiveness to other individuals. The antibacterial activity of skin secretions has been demonstrated. Secretions of the hoopoe (Upupa epops) have besides the function of maintenance of physical properties of feathers also a repellent effect on parasites and predators. Its active substance is a peptide bacteriocin, produced by strains of Enterococcus faecalis. This substance is active against a number of both G + and G-bacteria and helps to sustain the nest hygiene, it is also effective against Bacillus licheniformis that produces keratin-decompositing enzymes. A similar antimicrobial activity of uropygial secretion against bacteria which degrade feathers was demonstrated in the wild house finch Carpodacus mexicanus. Changes in skin microflora have been demonstrated in parrots kept for breeding in comparison with those living in the wild, which may have significance for husbandry practices and veterinary care. Passerines of the genus Pitohui and Ifrita living in New Guinea store in their skin and feather batrachotoxins, which they receive from food beetles of the genus Choresine. These toxins are active against parasites (e.g. lice – Phthiraptera). In contrast, substances that act as potential attractants for hematophagous insects (e.g. mosquitoes of genus Culex) were found in the skin of chickens. Alcohols, ketones and diones were detected in these substances. The composition of uropygial gland secretions may be a guide in assessing the relatedness of bird species. Feather waxes can be analyzed also from old museum specimens. Lipid-enriched organelles, multigranular bodies in the epidermis mean that zebra finches (Taeniopygia castanotis) are facultatively waterproof, which appears to have a function in protecting the organism against dehydrating when water is unavailable.