In a major study comparing the environmental burdens and resource use in production of Beef, Sheep Meat, Pig Meat, Poultry Meat, Eggs and Milk, poultry meat came up as the clear winner. The university study found that poultry meat production, in particular conventionally produced chicken which represents the overwhelming majority of chicken sold in Australia, is the most environmentally efficient, followed by pig meat and at some considerable distance sheep meat and beef.
Some summary data from the report is presented in the graphs on this page. The report itself is available form www.defra.gov.uk.
Even though the report is dated 2006, it is only in more recent times, namely in late 2007 and early 2008 that these interesting results became more broadly known and reported in the media.
Source: Williams, A.G., Audsley, E. and Sandars, D.L. (2006) Determining the environmental burdens and resource use in the production of agricultural and horicultural commodities. Main Report. DEFRA Research Project IS0205. Bedford: Cranfield University and DEFRA. Available on www.defra.gov.uk (or try this direct link)