Sign Up for Puzzlehunt CMU's Bird Hunt 3!
Answer: PIGEON ROOSTS
Eric Wei

Okay, so this is a throwback puzzle to this previous PHCMU hunt. Each section will reference a puzzle from that hunt and probably extract a letter or something, and together they'll spell the answer. So let's go to the hunt archive and-
Oh. Bird Hunt 2 doesn't exist.

Guess there's nothing to do but to solve these puzzles individually, guessing at the extraction for each section:

These letters are in alphabetical order, which implies that we should find another way to sort them, in addition to helping with confirmation of the subpuzzle letters.
Since the puzzle theme references PHCMU, we should look at the PHCMU website. Bird Hunt (1), also known as Ornithology Hunt, was run in Fall 2014, and is not available on the website. However, since then, there have been 15 hunts, with names that have many similarities to the subpuzzles in this puzzle. Each subpuzzle's theme corresponds to the theme of one of the first 12 hunts that occurred after Bird Hunt 1, so this provides a chronological sorting order.
Subpuzzle Hunt Letter
Bacon on spaces Space Hunt P
Text adventure Adventure hunt I
Star-Spangled Banner Puzzlehunt: The Musical! G
Harry Potter potion Harry Potter and The Hunt of Puzzles E
Rebus to RINGOTOSS CarnivalHunt O
Portal chamber Aperture Science Student Testing Initiative N
Anaquote to Four Horsemen Apocalypse Hunt R
History, years off Time Travel Hunt O
Brainf*ck program CS Hunt O
"Gold" word search The Quest for the Dragon's Riches S
Odd cryptic clue out Anomaly Hunt T
GETTING -> STING Getaway Hunt S
Therefore, the final answer is PIGEON ROOSTS.

Author's Notes

Bird Hunt 3 has been an internal meme within PHCMU for many years, and I felt this was a fun way to make it into a puzzle. The other inspiration was seeing some throwback puzzles have minipuzzles that were pretty much solvable by just guessing the mechanic of the initial puzzle, so I wanted to try this solution to the problem that throwback puzzles tend to require you to spoil yourself on existing puzzles: The original puzzles never actually existed in the first place.
During testing, every solving team got the Musical subpuzzle's letter by noting that the missing note was a G and ignoring the line breaks. This was entirely a lucky coincidence, but I'm not complaining.