gene for yellow skin was incorporated into domestic birds through hybridization with the grey junglefowl (G. sonneratii). Modern crosses are also favorable for meat production because they lack the typical "hair" which many breeds have that must be removed by singeing after plucking the carcass. Both male and female broilers are reared for their meat. Behaviour Broiler behaviour is modified by the environment, and alters as the broilers' age and bodyweight rapidly increase. For example, the activity of broilers reared outdoors is initially greater than broilers reared indoors, but from six weeks of age, decreases to comparable levels in all groups. The same study shows that in the outdoors group, surprisingly little use is made of the extra space and facilities such as perches – it was proposed that the main reason for this was leg weakness as 80 per cent of the birds had a detectable gait abnormality at seven weeks of age. There is no evidence of reduced motivation to extend the behavioural repertoire, as, for example, ground pecking remained at significantly higher levels in the outdoor groups because this behaviour could also be performed from a lying posture rather than standing. Examining the frequency of all sexual behaviour shows a large decrease with age, suggestive of a decline in libido. The decline in libido is not enough to account for reduced fertility in heavy cocks at 58 weeks and is probably a consequence of the large bulk or the conformation of the males at this age interfering in some way with the transfer of semen during copulations which otherwise look normal. Feeding and feed conversion Chickens are omnivores and modern broilers are given access to a special diet of high protein feed, usually delivered via an automated feeding system. This is combined with artificial lighting conditions to stimulate eating and growth and thus the desired body weight. In the U.S., the average feed conversion ratio (FCR) of a broiler was 1.91 kilograms of feed per kilograms of liveweight in 2011, an improvement from 4.70 in 1925. Canada has a typical FCR of 1.72. New Zealand commercial broiler farms have recorded the world's best broiler chicken FCR at 1.38. Welfare issues Meat birds Artificial selection has led to a great increase in the speed with which broilers develop and reach slaughter-weight. The time required to reach live-weight decreased from 120 days to 30 days between 1925 and 2005.