How to add a default item to a custom taxonomy?

WordPress default taxonomy (Categories) has the item Uncategorized by default. How to add a default item to a new custom taxonomy?

functions.php:

// === CUSTOM TAXONOMIES === //
function my_custom_taxonomies() {
    register_taxonomy(
        'block',        // internal name = machine-readable taxonomy name
        'static_content',       // object type = post, page, link, or custom post-type
        array(
            'hierarchical' => true,
            'labels' => array(
                'name' => __( 'Blocks' ),
                'singular_name' => __( 'Block' ),
                'add_new_item' => 'Add New Block',
                'edit_item' => 'Edit Block',
                'new_item' => 'New Block',
                'search_items' => 'Search Block',
                'not_found' => 'No Block found',
                'not_found_in_trash' => 'No Block found in trash',
            ),
            'query_var' => true,    // enable taxonomy-specific querying
            'rewrite' => array( 'slug' => 'block' ),    // pretty permalinks for your taxonomy?
        )
    );
}
add_action('init', 'my_custom_taxonomies', 0);

EDIT: I just want to have the taxonomy item there when the theme is installed. It doesn’t have to automatically be added to any empty term.

Have a look here:

https://web.archive.org/web/20150403012347/http://wordpress.mfields.org/2010/set-default-terms-for-your-custom-taxonomies-in-wordpress-3-0/

Basically what you need to do is use the save_post hook to check the terms for the post and add the default term from your taxonomy if it’s empty.

If you just want to have an initial term set in your custom taxonomy, then you can use wp_insert_term(). Probably easiest to add it in the same function that you’re using to create your custom taxonomy. As t3ios adds in the comments, you should call get_term() first, and only insert the term if the return value is null (ie the term doesn’t exist).

This example code is from the Codex: http://codex.wordpress.org/Function_Reference/wp_insert_term

$parent_term = term_exists( 'fruits', 'product' ); // array is returned if taxonomy is given
$parent_term_id = $parent_term['term_id']; // get numeric term id
wp_insert_term(
  'Apple', // the term 
  'product', // the taxonomy
  array(
    'description'=> 'A yummy apple.', 
    'slug' => 'apple', 
    'parent'=> $parent_term_id
  )
);