Eating!

Im a FTM so I need some help, Do babies eat whenever they are hungry? Like for example my baby is 12 weeks and the only time she eats a lot is when she wakes up at 6am after a 7hr sleep she drinks 5oz and after that she only drinks 1-3 oz every 4-6 hrs. Shes was breastfeed and slowly moved to formula beginning of May and this week I feel shes been eating less, she sleeps fine, whenever she naps is 2-4 hrs and when she only naps for 2 hrs she doesn’t really wanna eat, when shes fussy, i offer her milk and doesnt want it so i just put her to sleep and then try again and only drinks 2 oz if im lucky she drinks 3oz. Can they really starve themselves? Or if they are hungry they will eat? Her weight is fine