The present experiment investigated the effects of a calcium deficient diet on the use of outdoor areas, foraging activity and production parameters in two strains of slow-growing broilers (i657 and a Labresse cross). The birds were housed as day-old in an indoor system of 2×8 groups of each 111 birds (11.8 birds/m2). After 6 weeks, the groups were moved to an outdoor housing system of 2×8 houses (3 m×5 m) with free access to outdoor grass-covered areas (9 m×22 m). The outdoor areas were equipped with two supported sloping roofs and a sandpit. In the outdoor system, the birds were fed one of the two feeds; high (1.1%) or low (0.3%) calcium, ad libitum in a 2×2 factorial design with four replicates. In each group, number and behaviour of birds outside were recorded 1 day per week between sunrise and sunset (15 scan samples per group per day) during a 6-week period until slaughter at 84 days of age. The number of birds in the outdoor area and the proportion of these being inactive was unaffected by the treatment. Surprisingly, birds on the high calcium diet showed more foraging activity when outside (P=0.04), and these birds also consumed more pelleted feed (P=0.01) and consequently had a larger body weight at slaughter (P=0.01). Birds on both treatments showed a diurnal activity rhythm and stayed more in the zones closest to and furthest from the house. In conclusion, the deprivation of calcium diminished foraging activity, feed intake and growth rate of the broilers. [Copyright &y& Elsevier]