Chickens make a big mess. Killing chickens makes an even bigger mess. Fortunately, these messes are entirely marketable. Chicken feces and offal are cooked in hot oil and sold as 'chicken litter', to be used as cattle fodder in industrial feedlots. But there's an even cheaper feed for the cattle: chicken feathers.
About nine percent of a slaughtered chicken's weight is in its feathers. These feathers contain large amounts of the structural protein keratin (the same protein found in your hair and skin), but unfortunately the chemical bonds in keratin are much too strong to be broken down by a cow's digestive system. This is a lot of good protein that could go to waste, but we are a thrifty society, and we have found a way to make these feathers useful. By simply steaming the feathers at high temperatures we can break down the bonds keeping the keratin together, making it digestible (by ruminates; you should not try eating feather meal).
Even ruminates may have trouble eating feather meal, and it must be mixed with more natural foods (cattle evolved to eat grass, not meat and feathers). But it is a great source of protein, which is necessary if we want our beef to reach a size suitable for slaughter in under 16 months. In case you were wondering, we very much do want our cattle on our plate ASAP; a more natural age-of-slaughter, along the lines of four years, is just not profitable in the modern, highly-competitive meat market.
There has been some concern that the practice of feeding chicken offal to cattle and then feeding cattle offal back to the chickens (both common practices) might allow prions to pass through one species and back to the original species. Although there was some criticism of the use of feather meal after the outbreaks of mad cow disease, no apparent harm has yet come from these practices.
Feather meal is also a useful fertilizer. Feathers contain both ammonia and various nitrates. But because the proteins are not completely broken down, feather meal doesn't contain any free ammonia and nitrates, meaning that they can't be washed off to pollute local water supplies and streams. The nitrates in feather meal are released steadily over a period of weeks and months, giving a reasonably consistent supply of fertilizer to the crops. Many organic farmers are turning to feather meal to fertilize their fields (although most feather meal currently available is obviously not organic).
References:
The Omnivore's Dilemma by Michael Pollan
http://www.ingredients101.com/feather.htm
http://www.wipo.int/pctdb/en/wo.jsp?wo=1982000084&IA=NO1980000021&DISPLAY=DESC
http://www.vincentcorp.com/applications/issue35.html
http://www.pacificcalcium.com/products/feathermeal.html
http://www.edfa.eu/englisch/eumwelt.htm