Chickens in motion: Effects of egg production level and pen size on the motor abilities and bone stability of laying hens (Gallus gallus forma domestica)

Laying hens have undergone strong selection pressure for traits related to productivity in recent decades. This has led to an impressive increase in egg production but is also assumed to cause undesirable side effects, such as behavioural changes or increased bone damage. Resource trade-offs resulting from egg production might trigger such changes. Our hypothesis was that hens from lines selected for high levels of egg production need to conserve energy to maintain productivity by reducing locomotor activity. We further assumed that the reduced locomotor activity is accompanied by poorer motor coordination abilities. In addition to the performance level, phylogenetic origin has potential implications for behavioural and physiological differences. Thus, we compared hens from four purebred layer lines in a 2 × 2 model that differed in productivity levels (a line with high egg production (320 eggs/year) and a line with moderate egg production (200 eggs/year)) and phylogenetic origins (white and brown egg layer lines). Furthermore, hens of each line were kept in compartments of two different sizes (large, with stocking densities of 3.19 hens/m2; and small, with stocking densities of 5.94 hens/m2), to examine the effect of space and stocking density on the behaviour and bone strength of the hens. In total, we tested 742 chickens from the four different layer lines. We recorded their general locomotor activity in their home compartments and tested their motor coordination using a rotarod test. After the laying period, we examined the bone breaking strength of the tibiotarsi and the humeri from chickens of all four lines. The hens from both highly productive lines possessed poorer motor coordination than the hens from the moderately productive lines of corresponding phylogenetic origin. Furthermore, the hens from the brown egg-laying lines performed better than the hens from the white egg-laying lines. Compartment size had a positive effect on the motor coordination of the hens from the moderately productive lines and interacted with productivity. However, locomotor activities were not different between the hens from lines with different productivities or of different phylogenetic origins. The hens from the moderately productive lines possessed higher bone breaking strengths than the hens from the highly productive lines within the phylogenetic groups. Our results show that the hens from the lines with high productivity have poorer motor coordination and lower bone breaking strength than the hens with moderate productivity. These results can help to gain further understanding of practical problems, such as the high prevalence of bone damage in laying hens.