org-agenda-start-with-log-mode

org-agenda-start-with-log-mode is a variable defined in `org-agenda.el'.
Its value is

nil

Documentation:
The initial value of log-mode in a newly created agenda window.
See `org-agenda-log-mode' and `org-agenda-log-mode-items' for further
explanations on the possible values.

You can customize this variable.