Methane is largely produced through anerobic (oxygen-free) digestion of grasses, by methanogenic (methane-producing) bacteria in the guts of cows.
Chickens do not have a lot of methanogenic bacteria in their guts - those kinds of bacteria are mostly seen in animals which eat high-cellulose foods (grasses, trees, etc).
I don't recall where I saw the numbers, but GHG emissions from poultry is quite low - on par with vegetable/fruit production.
The worst agricultural polluter is rice farming. Rice farming releases a large amount of methane - ironically, the methane is a product of "organic" farming techs. Rice is gown in flooded fields, and are usually fertilized by adding plant matter to those fields - methanogenic bacteria break down the plants, releasing nutrients for the rice, along with a bunch-o-methane. Chemical fertilizers can be used to greatly reduce that methane production.
Methane from cows, unfortunately, is harder to reduce. There has been some suggestion that changing their diet, and possibly feeding them alternate bacteria may reduce emission. But AFAIK, none of these have been shown to actually work.
Bryan
