[Mediawiki-l] Question on Search options

[Gregor Hagedorn]

Our own wikis search in Advanced mode (German:Erweitert) by default
(for signed-in and anonymous users):

http://species-id.net/openmedia/Special:Search
http://offene-naturfuehrer.de/w/index.php?title=Spezial%3ASuche&search=xxx

Compare http://en.wikipedia.org/w/index.php?title=Special%3ASearch&search=xzz
which searches in default mode.

I cannot find (or remember?) the setting responsible for this. Looked
through many pages, including
http://www.mediawiki.org/wiki/Category:Search_variables

who can give me a pointer?

Gregor