Broilers are chickens bred and raised specifically for meat production. Chickens are one of the most common and widespread domestic animals, and although the global population has decreased from more than 24 billion in 2003 to 19 billion in 2011, there are more chickens in the world than any other species of bird. Typical broilers have white feathers and yellowish skin. Most commercial broilers bred for meat reach slaughter weight at between 5 to 7 weeks of age, although slower growing strains reach slaughter weight at approximately 14 weeks of age. Because of this young age, much of their behavior and physiology is that of an immature bird. Broilers and egg laying hen are the same species and share many characteristics, however, due to the rapid growth and selection for enlarged breast muscles, broilers are susceptible to different welfare concerns, particularly skeletal. Broilers are usually grown as mixed-sex flocks in large sheds under intensive conditions, but some strains can be grown as free-range flocks.