Go to http://birdfeedapp.com/bookmarklet in Safari on your iPhone.
Tap the plus button in the toolbar at the bottom of the screen.
Choose "Add Bookmark" from the menu.
Tap the "Save" button in the upper right corner of the screen.
Tap the Bookmarks button in the toolbar at the bottom of the screen.
Tap the Edit button in the lower left corner of the screen.
Tap the "Post with Birdfeed" bookmark you just saved to edit it.
Edit the URL field so that everything up to the word "javascript" is removed.
Now when you want to post a URL to Birdfeed, simply visit the page in Safari and click on the "Post with Birdfeed" bookmark. Birdfeed will open in the posting interface and insert a shortened version of the URL you want to share.