Proximate and ultimate causes of natal dispersal in the great bustard Otis tarda

Abstract

Between 1991 and 1997 we studied the offspring independence and juvenile dispersal in a wild population of great bustards (Otis tarda). Young males were independent and began their juvenile dispersal at an earlier age (6-11 months) than young females (8-15 months). The juvenile dispersal period was longer and the distances reached farther in males than in females. Natal dispersal distances were also longer in males, all of which dispersed from their natal areas and established as adults at 565 km from their natal nests. In contrast, most females were strongly philopatric, settling at 0.5-5 km from their natal nests. These marked sex differences in offspring independence and dispersal may have evolved originally to maintain genetic diversity and are probably reinforced through male competition for mates. Young males that had fed at higher rates and received more feedings from their mothers during the early maternal dependence period became independent and tended to disperse earlier. They also integrated earlier into adult male flocks and settled earlier at their definitive leks, which were closer to their natal sites, in areas of higher adult male density. None of these correlations was found among young females. These results suggest that enhanced food intake and maternal care of male offspring are vitally important in increasing their competitive ability during the immature period and probably also in their fitness as breeding adults. These results are in accordance with the selective value of large size in males and suggest how this species might have reached such a marked sexual dimorphism in size.