It would be interesting to see any old photos. Would speculate the start would be after Chamberlain Rd.

I live nearby and drive Birdwood Rd at least a couple of times a week. There are aerial photos from 1959 on the Auckland Council GIS Viewer, that show the flat section alongside the stream to be pretty much as windy as it ever was, but the sections between the hairpin bends are much straighter (but the hairpin bends are all still the same).

Back in the '60s or '70s, this was planned to be the western end of the Henderson Motorway, connecting Don Buck Rd in Massey with Henderson, New Lynn and Avondale. Thankfully that plan bit the dust (although there are only two streams to be bridged to complete the route from Henderson to Swanson).