One of Yahoo's prime pieces of ad real estate has been awfully ad-free of late, according to a report from financial-research firm Macquarie Equities Research.

Macquarie released a report this morning showing that Yahoo's log-in page -- which lets people sign in to their email account and other Yahoo services -- only had an ad on it 47% of the days in the first half of the fourth quarter. That's a sharp decline from the same period last year when the log-in page featured an ad 86% of the time.

An analyst at the firm said he checks the page once each day, around midday.

The sell-through rate doesn't necessarily mean that ad revenue tied to the log-in page has plummeted, since it's possible, though unlikely, that Yahoo has jacked up the price of those ad units. Still, it must be disconcerting that one of Yahoo's top spots for advertisers to reach the most users at one time did not generate ad money more than half of the time in the first month and a half of the quarter.

A Yahoo spokeswoman declined to comment on intraquarter performance of its log-in page ads.

Yahoo started selling ads on the log-in page in June 2010, adding another huge display unit to appeal to brand advertisers. At the time, ComScore said the page attracted 26 million visitors a day, making it one of the most-visited pages on the web.

In the third quarter, Yahoo's overall display revenue was flat at $452 million vs. $449 million in the same period last year, though display revenue in the Americas region registered modest growth of 4%.