The Vatican has started construction on its annual nativity set in St. Peter's Square, and the display is expected to include a few animals that may not have been at Jesus' birth.

The Pope said in his third book on the life of Christ, "Jesus of Nazareth: The Infancy Narratives," released Nov. 21, that the ox and the donkey, which are regularly included in nativity scenes, are not mentioned in the gospels.

But they are included in other parts of the Bible, which could have inspired Christians to use them in representations of the birth of Jesus.

"No representation of the manger is complete without the ox and donkey," Pope Benedict says in his new book, explaining that "Christian iconography adopted this motif at an early stage" because it conveys the humble circumstances of Jesus' birth.

The square was first decorated in 1986 under Pope John Paul II and the 19th-century images usually come from the parish of San Andrés del Valle.

The construction of the nativity scene began on Nov. 19 and is expected to finish by Dec. 24, just in time for Christmas Eve.

The Governorate of Vatican City's technical services office designs a new nativity set every year, inspired by different scenes of the life of Jesus.

Although the scenarios of where the statues are located vary each year, the essence is the same.

In 2010 the set included nine Filipino figures in honor of the 60th anniversary of diplomatic relations with the Holy See. Last year's scene focused on biblical events where Mary was a key figure, including the Annunciation, the Visitation, the presentation in the temple and, of course, the birth of Jesus.

As of Nov. 21, the area where the display will be in St. Peter's Square sports the pillars that will partially support the nativity scene. Construction workers have also placed a metal frame next to the obelisk and a tent to protect their work, with scaffolding alongside it.

The nativity will likely include life-size figures of Jesus, Joseph, Mary, shepherds, the Magi and some animals.

The display will be placed next to a 78-foot white fir, which comes from the town of Pescopennataro, Italy and will be donated to the Pope by the southern region of Basilicata.

It will be lit during a ceremony on Dec. 14, although the manger will be inaugurated on Christmas Eve.

Pope Benedict will celebrate a vigil Mass on Christmas Eve at 10:00 p.m. in St. Peter's Basilica.

Updated at 9:22 a.m. MDT on Nov. 28, 2012. Corrects quote from Pope Benedict's book about the presence of ox and donkey in paragraph four.