You have posted your question in the WP “Multisite” forum. Turning on multisite allows you to have multiple sites (not just the blog function, but separate sites) that share their WP core files, themes, plugins & users in one database. It can be a great tool, but does require more than basic skills with servers, etc. (Instructions are here: http://codex.wordpress.org/Create_A_Network)
If that’s not your skill-set, here are a few ideas for getting a second blog going.
One simple way to do this is to create all posts in the one blog, but put different category labels on them. Categorize all your posts as either “birds” or “fish”, then create the illusion of two blogs by making a menu custom link to the archive for birds and another for the archive for fish. When the user clicks on the menu item, they will only see posts for birds or only posts for fish. You can remove the blog index with all the posts mixed together from the menu, and users probably won’t see it.
If you really need to keep these posts more separated (for instance, you want to make sure a user searching for bird info only sees bird posts and not any fish posts in results), then you might want to create a second separate single install (not multisite) of WP. You can do this on a subdomain like topic.domain.com, when your main/first site is at domain.com, so you don’t have to get a second domain name.
Within the same site, you could also create a “custom post type” for the second topic blog. This gets a little more complicated. If you google around, you will find some plugins for helping create CPT’s.
