Preen gland secretion is known to serve several important functions essential for the survival of birds. When applied to the plumage, the waxes of the uropygial gland confer flexibility and water-repellent properties _. However, there has been considerable controversy concerning other possible roles of the gland’s secretions including the defence against micro-organisms.
Birds are the hosts of a battery of such organisms, some of which are feather-degrading or otherwise highly pathogenic. Therefore birds have been exposed to an extreme selective pressure to cope with this threat and have evolved effective strategies to protect themselves. This is the case especially for waterfowl, which live in wet environments, where hazardous bacteria and fungi have optimal growth conditions and the risk for birds is potentially high. For example, birds maintain mutualistic relationships with bacteria producing antibiotics and thereby reducing detrimental effects on their hosts. To date, little is known about the antimicrobial substances in uropygial oil and research using bioactivity guided fractioning will be needed in order to identify them. _ This is the purpose of this master's thesis. In the light of the rise of multi-resistant micro-organisms, this constitutes a first step towards the treatment of such strains and could be of major importance to medicine.
_ The duration of preening in captive birds will also be studied, providing hints on the importance of this behaviour to the species in question.
 Removed 'on the coat' from the 2nd sentence, since I would say coat is wrong for a bird's feathers. It could be plumage, but then you're just repeating the beginning of the sentence.
 Surely it's the gland's secretions which possibly have an anti-microbial effect, not the gland itself.
 'They' here would refer to the micro-organisms, not the birds.
 Whether or not you need a comma after waterfowl (no 's') depends on whether there are waterfowl which do not live in wet environments. If there are, then leave the comma out, since you want to refer only to 'waterfowl which live in a wet environment'. If there are not, then the phrase 'which live in a wet environment' is only explaining the habits of waterfowl, and the comma stands.
 'wet' not 'humid'. We all live in a more or less humid environment if we're not living in the desert.
 This sentence was trying to do two things at once, so I moved the phrase 'using bioactivity guided fractioning' into the previous sentence.