"Cet oiseau mange du riz."
Translation:This bird is eating rice.
Oiseau is a masculine word, so your line of thinking is correct. However, for euphonious reasons (it sounds better), when there is a determiner (ma, ta, sa, ce) or adjective (beau, nouveau) next to a noun beginning with a vowel, these words change to have a consonant at the end. It is like "an" in English. Instead of "a" it becomes "an" next to a vowel sound, same concept.
ce + oiseau → cet oiseau
ma + amie → mon amie
sa + idée → son idée
beau + appartement → bel appartement
nouveau + artiste → nouvel artiste