I recall reading about a tribe of maori on outlying islands that preferred plainly carved taiaha like that. The holes could be for feathers, flax fibre, or dog hair bundles which were normally near the tiki face. as colin says, this was not usual for taiaha, tho a feather bundle was common on the 'blade' of a tewhatewha, tied thru a hole. some patu also show holes for bundles. The tiaha generally had theirs wrapped around and cord wrapped to hold them in place. If the holes are on the striking blade, i doubt they were for feathers or hair bundles.
or added later by some nasty museum 'curator' to tie it in a display. do you have a photo of the whole weapon? The actual striking end of the taiaha (the blade part - the carved 'spear' point was not normally used to stab with). i also attach A copy of a photo of mine for further illustration (both are around 1.5 metres).
aha! found it. try reading thru
the chatham islands are mentioned for plainer taiaha.