I have come to appreciate the amazing versatility of the Atahualpa theme. I am hoping someone can advise me how to implement a 2 -tiered navigation structure. An example of this can be found at http://www.portsmouthhistory.org (note that the second tier does not display until you click one of the pages that is not the home page).

I am guessing that there may be a plugin for this, and if so, is it recommended (or if there are several, which are recommended). That said, I have googled around a bit and not seen anything that was quite like what I am looking for.

Or - is there a solution using jQuery or some other tools (which I am not familiar with, it could at least find someone to code up for me). A major goal is to implement it in such a way that updates to the theme don't require re-programming.

Thanks,

you could do that via widgets and the widget logic plugin or CSS
1) create a custom menu for each set of sub pages
2) add a new widget area to the 'Configure Header Area' option
3) add a 'custom menu' widget for each sub page and select the appropriate menu
4) add CSS to set each div for each menu to 'display: none;" ie

div#nav_menu-2 {display:none;}
div#nav_menu-3 {display:none;}

5) add more css to display the area when on the correct page for example

body.page-id-45 div#nav_menu-2 {display:block;}

You will have to style the menus to display across instead of down, but I will leavethat as an exercise for you to figure out.

Thank you.