Yahoo is closing the Yahoo Publisher Network, its rival to Google's AdSense program for delivering ads to third-party websites, according to an email sent to publishers today. As TechCrunch notes, Yahoo is referring those publishers to an alternative service, Chitika -- which seems strange on the surface given that Yahoo's new partner, Microsoft, has its own AdSense rival, dubbed Content Ads, that could certainly use the help.

[Follow-up, Thursday: Microsoft considered but declined Yahoo's self-serve ad biz]

There's no official comment yet from Yahoo, but the move to shutter the Yahoo Publisher Network as of April 30 appear to be the latest step in the company's effort to narrow its focus under CEO Carol Bartz. The move means that the company will focus more on selling ads for its own online properties.

That narrower focus is also one of the main objective of the Yahoo search partnership with Microsoft, in which the Redmond company will handle the underlying search technology for Yahoo, combining the Yahoo queries with Bing on the back end to achieve more critical mass in the battle against Google in the search market.

It's possible that Yahoo was wary of channeling too much of its business over to Microsoft by also referring Publisher Network sites to the company. We've put messages into Microsoft to see if we can find out more.

Previously: Microsoft partners with OpenX in new move vs. Google AdSense