Starting suddenly (mettersi a)

To describe an action that begins abruptly or unexpectedly, use the reflexive verb mettersi followed by the preposition a and an infinitive verb.

to start (doing something)
mettersi a

Because it's a reflexive verb, mettersi a takes the auxiliary essere in compound tenses like the passato prossimo. The past participle (messo) must also agree in gender and number with the subject.

The ostrich started to peck the glass.
Lo struzzo si è messo a beccare il vetro.
Why did that little boy start to shriek?
Perché quel bambino si è messo a strillare?
The herd started to chase the car.
Il branco si è messo a inseguire la macchina.

When the subject is plural, the participle changes to match (messi for masculine plural or messe for feminine plural):

Suddenly, the lions started to roar.
All'improvviso, i leoni si sono messi a ruggire.