Chickens

A chicken is a domestic bird kept for eggs and meat, and it is the bird rather than its house that does the work. Hens begin laying at between about 21 weeks and six months old, and a hen lays at most one egg in a day and some days none. Output rises quickly to a peak and then slowly falls, the eggs growing larger each year while their shells grow thinner, and household flocks commonly lay on and off for three to four years - a laying life, not a lifespan.

Day length drives laying: about 14 hours of light a day stimulates a hen, and carrying production through the short days means keeping that up and lifting it slowly toward 16 hours. After ten or more months a hen stops regardless, sheds and regrows her feathers in a pause called a moult, then starts again, leaving a predictable gap in supply. Space is the other governing figure - at least 0.14 square metres (1.5 sq ft) of indoor floor per bird, 0.19 square metres (2 sq ft) the usual allowance, and about as much again outside.

Moved daily onto fresh ground in a floorless pen, the flock pays twice: over 80 percent of the nutrients in bought feed pass through the bird, so it manures as it goes while eating the plants and insects there. Penned on a weedy patch it clears the ground completely and scratches the surface loose, preparing a bed without digging; vegetables did best planted 14 days later, and raw manure needs longer still. Left in one place too long, or crowded on wet ground, the same birds kill the forage and compact the soil. Poultry are widely said to clear fly larvae from cow dung behind cattle, but a controlled trial of meat birds found no effect and almost no interest in the pats. Unmanaged scavenging flocks lose over 60 percent of chicks and 45 to 100 percent of adults, so guarding against predators and disease, with daily feed, water and egg collection, is the real cost.