Hide custom fields

You can’t hide a custom field unfortunately. Not from the UI.

Also, in the database, we’re not storing individual fields, we’re storing the ENTIRE metabox in a serialized array. So putting _ in front of it won’t do anything.

You could just hide it with CSS.

Target it like so:

.row-birth-day {display:none;}

where birth-day is the Field slug.

To add css to the admin see this: http://css-tricks.com/snippets/wordpress/apply-custom-css-to-admin-area/