The young wizard’s domination of the North American box office surprisingly only lasted one week, thanks to some adventurous guinea pigs.

Warner’s Harry Potter and the Half-Blood Prince earned $30m in its second weekend, down a big 61.5% from opening weekend, putting it in second place behind Buena Vista’s animated guinea pigs in G-Force.

G-Force, a 3-D combination of animation and live action, opened with a larger-than-expected estimated $32.2m, an average of $8,697 from 3,697 screens.

G-Force, produced by blockbuster king Jerry Bruckheimer, features a voice cast including Nicolas Cage, Sam Rockwell, Tracy Morgan and Penelope Cruz. The plot is about government trained spies, who happen to be guinea pigs, trying to save the world from an evil CEO.

There were also two other openers diversifying the top 10 — Katherine Heigl/Gerard Butler romantic comedy The Ugly Truth earned $27m for Sony. And lower-budget horror/thriller Orphan earned $12.8m for Warner Bros.

Ice Age: Dawn Of The Dinosaurs fell 53.3% from its third to fourth weekends, earning $8.2m for a new cume of $171.3m.

Bruno, in its third week, showed even more slipping with mostly poor word-of-mouth, falling 67.3% from its second
weekend (which was already down an embarrassing 73% from the opening weekend.)

Univeral’s Johnny Depp-starring Public Enemies made a solid $4.2m this weekend for a new cume of $88.1m.

Although it just missed out on the Top 10, Fox Searchlight’s (500) Days Of Summer is gaingin momentum as this summer’s speciality breakout. The film earned a whopping $19,176 per-screen average in its second week, adding 58 new screens to a new total of 85. The film was up 95.3% over its opening weekend, for a new estimated total just over $3m. (500) Days Of Summer, starring Zooey Deschanel and Joseph Gordon Levitt (he’s soon to appear in G.I. Joe), is an offbeat comedy about young love, and was a hit at Sundance 2009. Another building speciality title, Summit’s The Hurt Locker was up 96.8% in its fifth week (adding 144 new screens for a new total of 238). The Hurt Locker made an estimated $1.46m this weekend, for a new cume of $4m.

Overall, it wasn’t a banner weekend, with ticket sales down 8% compared to last week. Next weeked’s openers will include Judd Apatow’s Funny People starring Adam Sandler for Universal and Fox’s family adventure Aliens In The Attic.

Estimated Top 10 North America July 24-26, 2009
Film (Dist)/Est wkd gross/Est total to date

1 (-) G-Force (Buena Vista) $32.2m

2 (1) Harry Potter And The Half-Blood Prince (Warner Bros) $20m $221.8m

3 (-) The Ugly Truth (Sony) $27m

4 (-) Orphan (Warner Bros) $12.8m

5 (2) Ice Age: Dawn Of The Dinosaurs (Fox) $8.2m $171.3m

6 (3) Transformers: Revenge Of The Fallen (Paramount/DreamWorks) $8m $379.1m

7 (6) The Hangover (Warner Bros/Legendary) $6.5m $247.1m

8 (5) The Proposal (Buena Vista) $6.4m $140.1m

9 (7) Public Enemies (Universal/Relativity) $4.2m $88.1m

10 (4) Bruno (MRC/Universal) $2.7m $56.5m