Water birds (<a href="//commons.wikimedia.org/wiki/Larus_argentatus" title="Larus argentatus">Larus argentatus</a> here) maintain and waterproof their feathers by smoothing with an oily substance produced by a gland called "uropygial" located near the anus. They may thus contribute to the spread of influenza virus. Author: F. Lamiot (Own Work)
<a href="//commons.wikimedia.org/wiki/User:Lamiot" title="User:Lamiot">Lamiot</a>
CC BY 2.5
<span class="int-own-work" lang="en">Own work</span>