"Some male birds possess a wiggling tongue-like knob on their genitals, probably to titillate their mates.
In typical bird copulation, males and females momentarily press together their cloacas - genital openings - in what biologists call a cloacal kiss. A muscled tongue-like projection called a cloacal tip, spotted for the first time in males of several species of Australian wrens, means this might be more like a French kiss.
Melissah Rowe of the University of Chicago and colleagues studied eight species of wrens. Based on the alignment of the muscles, the cloacal tips seem able to wiggle from side to side."
- 'Wiggling bird genitals give the ladies good vibrations', New Scientist magazine, 16 June 2008.