• by Gavin O'Malley  @mp_gavin, February 22, 2011

Editor's note: The headline of this story has been updated from an earlier version, which inaccurately implied that some of Yahoo's home page ads had fallen 21%.

Compared to other top portals, display advertising sales on Yahoo's home page are showing significant signs of weakness, according to a new report from Macquarie Securities.

Sales of oversized and custom ad units on the home page were down 21% during the first half of the first quarter, compared to 34% during the fourth quarter of last year.

While large brand advertisers continue to choose Yahoo as a "key partner," Macquarie estimates that Yahoo sold its log-in page only 20% of days during the first period -- down from 70% during the second half of the fourth quarter.

Also in trouble, AOL saw a slightly steeper step-down in oversized and custom ad units compared to the other portals -- 17% vs. 21%. Still, Macquarie expressed confidence that the company's Project Devil ad initiative would help it gain momentum going forward.

Reflecting a healthier business, YouTube continued to see momentum on its home page, with high-quality, engaging "masthead" buys almost every day, Macquarie reports.

Contributing to its success, Google's video hub is credited with effectively diversifying its advertiser base in the fourth quarter. That said, media advertisers were far and away the most visible category in the first half of the first quarter -- accounting for 57% of YouTube's total home page ads, compared to 50% in the fourth quarter.

The home page of Microsoft's MSN also continues to show positive momentum, according to Macquarie. Indeed, the portal aggressively sold oversized and custom ad units in the period, accounting for 21% of its total ads. That said, MSN continues to have the highest percentage of non-rich media ads: 31%.

Taking seasonality into account, Macquarie's analysis of U.S. home page ads did not suggest a meaningful change in the growth trajectory from the fourth quarter. As it continues to see solid growth and positive momentum for the industry, Macquarie said it would maintain its estimate of low-double-digit display growth for the industry in 2011.