Light exposure during incubation affects competitive behaviour in domestic chicks

Abstract

Domestic chickens reared from eggs kept in darkness during the last 3 days of incubation developed a more flexible group structure than those which have received light exposure during this period. Position in the social hierarchy was scored in terms of competition for access to a food source on Days 8–16 of post-hatching life. Groups of chicks exposed to light during incubation formed a more rigid rank order with the lowest ranking birds rarely gaining access to the food. Groups of chicks hatched from eggs incubated in darkness showed more variability in the rank order from day to day and the lowest ranking birds competed for food more successfully. In addition, the mode of gaining access to the food dish differed between the groups; dark-incubated individuals usually approached from over the top of the other chicks, while those exposed to light pushed their way in from underneath. The difference in ability to compete was also reflected in an overall trend for dark-incubated chicks to show a greater weight gain than their light-incubated counterparts. It is suggested that the differences in behaviour resulting from light or dark exposure during the last days of incubation result from the known effects of this treatment on the organization of asymmetry in the brain. By maintaining eggs in darkness during the critical period, it may be possible to reduce the proportion of young chicks which die from starvation.