A little is fine as a treat but it was never overly popular when I added a little to my feed. If the seed coating is broken it goes rancid quicker so I'd feed it whole. But I found this statement from a study:
"While feeding poultry a diet composed of 10% flax results in significantly higher levels of omega-3 fatty acids in hens laying brown and white eggs, long-term use of flaxseed has been shown to increase the level of liver hemorrhages in laying hens. The cause of the hemorrhages is unclear. Researchers hypothesize that the livers of flax-fed birds contain more long-chain unsaturated fatty acids, which are more prone to oxidative rancidity."