Llleb on "Collapsible jump menu"

Hi,
I guess what I want to do is going to be a bit difficult.
Lets say I have a menu, sub menu structure like
Fauna / Birds / Sea Birds / Gulls / Black-backed Gull
.................../ Land Birds / Passerines / Fimches / Goldfinch
.................../ Waders Knots / Great Knot
With similar other menus, subs etc under Fauna, Insects for instance.
Is it possible to have the menu collapsed in it's base state.
You click on Fauna and it opens out to Birds and Insects, click on Birds and Sea Birds, Land Birds and Waders open out ?
I'm guessing the menus are html lists so there is probably no native support for this ?
Is there any plugin that has been written to do this ?
I'm guessing I'll have to do this myself using jQuery or similar.
While I know I shouldn't muck with the core code where is the code (function?) stored that creates the menu ?
I'll have a look and see where I go from there.
Thanks and any thought appreciated.