erc-header-line-format

erc-header-line-format is a variable defined in `erc.el'.
Its value is

"%n on %t (%m,%l) %o"

Documentation:
A string to be formatted and shown in the header-line in `erc-mode'.
Only used starting in Emacs 21.

Set this to nil if you do not want the header line to be
displayed.

See `erc-mode-line-format' for which characters are can be used.

You can customize this variable.