Great Slaty woodpecker
This is what I saw on my trip. It's the great slaty woodpecker (Mulleripicus pulverulentus). It's pretty big, about the size of a chicken (almost the size of a hornbill) but slimmer. You're more likely to hear it first before catching a glimpse of it because they're very noisy birds (their call is a loud, rapid wavering 2-5 note whinny: woi-kwoi-kwoi-kwoik...). A similar possibility is the white-bellied woodpecker (Dryocopus javensis) because of the size but the woodpeckers I saw were definitely grey, and had no red crown nor white plummage on the chest. Unfortunately, I couldn't identify the sex of the birds (the male great slaty has a red submoustachial patch) but I'm very pleased to have finally identified them. This sighting was definitely a lifer for me. And I saw three! How exciting!