Zazu has 2 conflicting histories as to how he became the King's majordomo:
Friends In Need
Young Zazu is about to be cooked and eaten by the young 3 hyenas (which explains his cry of "Oh, no! Not the birdie boiler!" in the first movie) Shenzi, Banzai and Ed, when a young Mufasa comes along, frightens off the hyenas and saves him. Zazu is thankful and hopes that he can be of service to the Lion King someday. Mufasa is doubtful of this. How could a small bird be of any use to a lion?
Zazu follows Mufasa around and watches over him. Mufasa tires of this quickly as Zazu disturbs his hunts and his private time with Sarabi.
One day Zazu notices some vultures hovering. He asks Mufasa if he wants him to fly over and see what's happening, but Mufasa is not concerned and lies down for a nap. Zazu flies over to where the vultures are anyway and sees that Sarabi has fallen into a pit and can't get out. Zazu flies back to tell Mufasa who leaps up and runs to the gorge. But Mufasa can't find a way to rescue Sarabi. Zazu finds a tree trunk that Mufasa drags over to the pit, allowing Sarabi to climb out.
Mufasa realizes that the little hornbill is useful after all and appoints him as royal advisor.
How True, Zazu? from The Lion King: Six New Adventures
Zazu's mother, Zuzu, was a steward to Kopa's great-grandfather Ahadi. When Zazu was young, Zuzu trained him to take her place when Mufasa's rule began.
In the book, Zazu reports everything he sees and tends to jump to conclusions. He ends up reporting false stories to Mufasa as three young animals make up stories to trick Zazu. Zazu redeems himself though as he spies an army ant attack and alerts Mufasa to the problem.
As majordomo to King Mufasa, his proper, dignified, nervous, and easily-rattled personality is the polar opposite in temperament of Simba and Nala. He tends to panic during emergencies, such as the wildebeest stampede.
Zazu's role in the film is primarily comic relief. As King Mufasa's majordomo and close friend (he is one of the few who call Mufasa by his name), he is very serious and always sticks to the rules, which contrasts with Simba's playful nature.
He is first seen in the film during the opening sequence in which Simba is presented. He later confronts Scar on his absence from the ceremony and is almost eaten by him.
When Simba has grown into a cub, Zazu is often ridiculed by him, being the target in Simba's pouncing lessons. He is ordered to watch over the cubs by Sarabi when Simba claims he wants to go to the waterhole with Nala. In fact, Simba had been lured by Scar to go to the Elephant Graveyard, and after the song "I Just Can't Wait to Be King", Simba and Nala successfully lose Zazu and go to the graveyard. Zazu later catches up with them, but they encounter the three hyenas Shenzi, Banzai, and Ed and the hornbill is put into the hyenas' "birdie boiler" geyser which rockets him up to the sky. Zazu manages to find Mufasa and lead him to the cubs, just in time to save them from the hyenas.
During the wildebeest stampede engineered by Scar to kill Mufasa and Simba, it is Zazu who leads Mufasa to Simba. As he watches the stampede, he wants to go back for help, but is struck by Scar into a wall and rendered unconscious.
After Simba runs away, Zazu is shown comforting Sarabi after Scar tells the pride of Mufasa's and Simba's deaths. He also briefly appears in a scene after Scar has assumed the kingship of the Pride Lands. In this scene, Zazu is imprisoned and is being forced to entertain Scar by singing songs like "I've Got a Lovely Bunch of Coconuts" (but not before briefly singing the lyrics of It's a Small World). When the hyenas come in to complain about the lack of food and water, Scar casually tells them to eat Zazu, and he frantically insists that he'd be "so tough and gamey and eyugh!"
Later on, Simba returns to reclaim his throne. Simba, the lionesses, Timon, Pumbaa, and Rafiki battle the hyenas. Zazu pleads Timon to let him out of the ribcage he is imprisoned in, but Timon goes inside the ribcage instead as he is pursued by the hyenas. Pumbaa comes to the rescue and chases away the hyenas after Banzai accidentally insults him. Zazu is freed along with Timon, and he joins the battle. Later, Zazu is seen at Simba's ascension to the throne and during the presentation of Simba and Nala's newborn cub.
Zazu plays a minor role in the film, functioning mostly as a messenger. He is first seen at Kiara's presentation, signaling that everyone has gathered. He is next seen when Simba is lecturing Kiara before she goes out to play, and Zazu mentions the dangers of the Outsiders, calling them backstabbing and murderous. Some time later when a wildfire starts, Zazu is sent ahead by Simba to find Kiara, who had been out on her first solo hunt. When Zazu finds that Kiara is being rescued by Kovu, he reports the information back to Simba. After Kovu asks to join the pride, Zazu notes that royal protocol demands that debts (such as the one Simba owes Kovu for rescuing Kiara) must be repaid, though Zazu would be quite happy to make an exception. Later, after Simba is injured in an ambush by Zira and the Outsiders, he is sent by Kiara to find help. Finally,near the end of the film, he appears in a panic, reporting that the Outsiders are preparing to attack. In the end, Zazu is seen standing next to Timon and Pumbaa at the presentation of Kiara and Kovu's wedding to which Timon and Pumbaa start crying with happiness and Zazu covers himself from the wetness of their tears.
Zazu's appearances are extremely brief in The Lion King 1½. He is first seen at the presentation of Simba, talking to Mufasa. He is then seen going to rescue Simba from the hyenas with Mufasa. In the credits for this film, he is incorrectly listed as "Zasu".
Zazu appears as a main character in the series starring Timon and Pumbaa. Zazu appears in the episode "Once Upon a Timon" where he is researching the reasons that Timon became an outcast. He eventually learns the reasons from Rakifi, and the story becomes so engrossing, that Zazu forgets about a meeting with Simba. "Zazu's Off Day Off" has Zazu being forced to work on his day off when an elephant is blocking flow of the river.
"Zazu's Off-by-One Day" shows that Zazu is responsible for taking a census of the animals. However, he is fired by the jungle inspector when it's discovered that he missed one animal. As a result, his job is given to his gopher assistant. In an attempt to regain his job, Zazu searches for the animal he missed, only to discover that it was himself. Zazu along with Timon, Pumbaa, Simba, Rafiki, Shenzi, Banzai, and Ed are the only characters from The Lion King to appear in the series.
Role in the series
Zazu reprises his role from the films, where it is revealed that he is Iago's different species cousin.