I use 'Cydectin' pour on to worm my chickens at the rate of 0.5 ml per kg or 1ml per (standard) chicken. You can't overdose chicken with this stuff and has the plus side effect of getting rid of fleas, ticks and lice.
The active ingredient is moxidectin. But I believe (from research) that any of the '...ectin' family are good for this. The below address to cut/paste into Google is pretty good info; http://www.shellspoultry.com/poultry-health.php
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there is an oral form of Cydectin which is a lot lighter. Problem is you have to get it into the chicken...
There's some info for this stuff at the below address; http://www.poultrycentral.co.nz/viewtopic.php?f=19&t=885