Continuity mistake: The Bird of Prey is the one captured by Kirk's crew in ST III That ship's bridge showed Klingon Cmdr Kruge in his elevated command chair with his helmsmen arrayed circularly below him, and nothing else. ST IV has this same ship, however the bridge now resembles The Enterprise layout with Kirk's command chair behind Sulu and Chekov at their rectangular helm, with Spock, Scotty and Uhura at their usual positions.
Kirk: Mr. Spock, have you accounted for the variable mass of whales and water in your time re-entry program?
Spock: Mr. Scott cannot give me exact figures, Admiral, so... I will make a guess.
Kirk: A guess? You, Spock? That's extraordinary.
Spock: [to Dr. McCoy] I don't think he understands.
McCoy: No, Spock. He means that he feels safer about your guesses than most other people's facts.
Spock: Then you're saying... It is a compliment?
McCoy: It is.
Spock: Ah. Then, I will try to make the best guess I can.
McCoy: Please do.
Spock: Your use of language has altered since our arrival. It is currently laced with, shall we say, more colorful metaphors, "double dumb-ass on you" and so forth.
Kirk: Oh, you mean the profanity?
Kirk: Well that's simply the way they talk here. Nobody pays any attention to you unless you swear every other word.
Question: Kirk and crew deliberately disclose crucial technological secrets, extend the life of a random stranger, deliver future technology to a primitive military power, abduct a cetacean biologist, and actually contribute to the extinction of a species during their brief stay in 20th Century San Francisco. Specifically: Scotty reveals the secret of Transparent Aluminum 150 years too early; McCoy arbitrarily uses 23rd Century medicine to cure a seriously ill 20th Century woman; and Kirk chooses to remove Gillian from the 20th Century. Perhaps most importantly, Chekov leaves behind a Starfleet Communicator and a Type 2 Phaser in the hands of the U.S. Navy (who would undoubtedly dissect the devices and try to exploit the technology a couple of centuries too soon). Beyond all that, Kirk and crew abduct two breeding humpback whales, one of which is pregnant, and that certainly contributes to humpback extinction in the 21st Century. Given what we think we know about disrupting linear time continuity (many instances are cited in Star Trek canon), how did Kirk and crew return to anything even resembling their own timeline after such blatant and deliberate interference in Earth history?Charles Austin Miller