I can't think why you'd want to use hackle pliers for winding Peacock herl

I don't use pliers for winding my PT either (I use my fingers) but I can see why, with PT fibres being relatively short, that one would want to wind with pliers. The problem of slippage with PT is due to trying to grip several relatively bulky fibres at once - there's always one or two fibres that slip out... then the grip loosens on the 'bunch'... then a few more slip out. Not an awful ot you can do. 'Soft' pliers can be bad enough, but I found that the problem is exacerbated even more by pliers with hard, bare metal surfaces at the business end.
Try
these for size.