Blank line before XML declaration (WordPress)

If an XML declaration (<?xml ... ?>) appears in your feed, it must be the first thing in the feed, before any whitespace.

Unfortunately, with WordPress it seems all too easy for a plugin, a theme, or for your configuration file to contain a blank line. Further compounding this problem, some — but not all — feed readers compensate for this common error, allowing the error to go undetected for quite a while.

• PHP also uses a similar notation: <?php ... ?>. Note that the final php ?> should be omitted from all PHP code files—modules, includes, etc. The closing delimiter is optional in PHP (but not in XML), and removing it helps prevent unwanted white space at the end of files which can cause problems with your feeds.
• Check your wp-rss2.php and wp-atom.php files for blank lines outside of <? and ?> bracketed sections.
• Check your wp-config.php file for blank lines outside of <? and ?> bracketed sections.
• Check your theme's functions.php file for blank lines outside of <? and ?> bracketed sections.
• One by one, disable plugins and revalidate until you isolate the one causing the problem.