(written from a Production point of view)
Birds & Animals Unlimited is a company which provided film and television productions with trained animals. It was founded by animal handler Gary Gero. Gero and his company worked on Star Trek: The Next Generation and Star Trek: Deep Space Nine and trained and handled the Somali cat who played Spot for the fourth season episodes "Data's Day" and "In Theory" and the guinea fowls for the fifth season episode "Ensign Ro" in 1991. One of the cat trainers was Scott Hart.
Gero and his company are probably best known for the animal supervision on the Harry Potter films Harry Potter and the Sorcerer's Stone (2001), Harry Potter and the Chamber of Secrets (2002), Harry Potter and the Prisoner of Azkaban (2004), Harry Potter and the Goblet of Fire (2005), Harry Potter and the Order of the Phoenix (2007), and Harry Potter and the Half-Blood Prince (2009).  
Gero and his company provided and trained birds, hawks, wolves, bugs, dogs, cows, and Orang Utans for films such as the drama Never Cry Wolf (1983), the war drama Birdy (1984), the drama Ladyhawke (1985), the romance Pretty in Pink (1986), John Carpenter's horror thriller Prince of Darkness (1987), the science fiction sequel Back to the Future Part II (1989, starring Christopher Lloyd), the television series Dark Shadows (1991, starring Jean Simmons and Ben Cross), the action sequel Lethal Weapon 3 (1992), the comic adaptation Batman Returns (1992), the fantasy film Death Becomes Her (1992), the fantasy film The Crow (1994), the adventure The Jungle Book (1994), the family movie 101 Dalmatians (1996) and its sequel 102 Dalmatians (2000), the comedy Doctor Doolittle (1998), Joe Dante's Small Soldiers (1998, with Kirsten Dunst), the comic adaptation Inspector Gadget (1999), the action film Casino Royale (2006), the comic adaptation Thor (2011, starring Chris Hemsworth), and J.J. Abrams' science fiction thriller Super 8 (2011).