More about driving and transport
Kota Kinabalu to Mountain Kinabalu (park entrance, 2 hrs drive, uphill); Mountain Kinabalu to Sepilok (200 km, 5 hrs including breaks)
Alternatively, you could catch a bus from Kota Kinabalu to Mountain Kinabalu (don’t know the details. But most day trip tourists get there by bus or van). Get back to Kota Kinabalu after birding, take a domestic flight to Sandakan. Ask the resort to pick u up at Sandakan airport.
Pls note it is better to arrange transport inside the Mt Kinabalu Park. Although already hundreds of birds along the park entrance, some birds are only found at the upper part (from park entrance to the end of main road, about 4.5 km but uphill. Expect at least 3 hrs one way if on foot).
In sepilok, transport to Rainforest Centre is simple (resorts can arrange it easily). There is a taxi calling counter at the centre as well.