Sex differences of Paraus caeruleus (Blue tit). Adapted from: Manfred Föger, Karin Pegoraro: <i>Die Blaumeise</i>. Neue Brehm Bücherei, Hohenwarleben 2004, <a href="//commons.wikimedia.org/wiki/Special:BookSources/3-89432-862-2" title="Special:BookSources/3-89432-862-2">ISBN 3-89432-862-2</a>
<a href="//commons.wikimedia.org/wiki/User:Cactus26" title="User:Cactus26">Cactus26</a>
CC BY 3.0
<span class="int-own-work" lang="en">Own work</span>