So, not a whole lot to say about this one.
A Bird in the Hoof isn't really the most clever or the most funny episode, but it doesn't lack humour. There's still plenty of fun to be had watching it.
Most of the humour comes through watching how the characters react to Philomena, not knowing that she's a phoenix. Fluttershy's cute as always, and her attempts to make Philomena feel better are charming as always. Meanwhile, Twilight's much more forced approach is a bit less fun, though it's still reasonably entertaining.
A lot more of the comedy is based on Philomena, and while there's some pretty good comedic timing in there a lot of it is more or less gross-out gags. MLP doesn't regularly stoop to such low-brow humour, and here it's only partly effective. As said, especially compared to other episodes, much of this episode isn't really as funny. In fact, a large part of the episode's entertainment value is based upon Fluttershy's cute actions. Her choo-choo train noise, for example, is way too cute.
The best parts of the episode are the ones with Celestia. They're the strongest confirmation of Celestia's personality yet, and really, she needs an episode of her own. Almost every moment Celestia's on screen is hilarious. This comes down to her fondness for messing with the ponies around her, shown best by her prank at the beginning where she tricked the Cakes into over-filling her teacup. It seems only fitting that Philomena would also be a bit of a prankster, being Celestia's pet and all.
Of course, there's more I like about these scenes than just Celestia. I find the other ponies' reactions to her intriguing. They all seem to be almost afraid of her, and very wrongly so. Either Twilight doesn't actually know Celestia that well, or she's just uptight. The latter is more likely. Of course, it makes sense that ponies would be paranoid about the right way to act around royalty, which most likely bores Celestia to tears.
Aside from that, though, it's simply a decent episode. Not one of the best, and not one of the worst, but it's entertaining enough.