I don't know if this is a problem for all plurals or what, but if you search for "birds" (tags only), the results you get are identical to what you would have gotten had you searched for "bird". Most of the results that are tagged "bird" aren't necessarily tagged "birds" - so you actually end up getting very few results tagged "birds".
The only way to get accurate results is by viewing the page for the "birds" tag. While this tag does happen to be in the top 150 on the tags page, if a tag is not there, there's no way of finding the tag page by the name of the tag (because the ID, not the name, is what's required), and if the tag was similar to (a plural of?) another one, it would be impossible to find sounds tagged with it, without manually going through every sound until a sound with the tag was found.