Great Bustards (Otis tarda) are the world's heaviest flying birds, sharing the title with Kori Bustard (Ardeotis kori). Great Bustards occur in Europe and Asia and are known for their elaborate display. At the beginning of spring the males start their display by showing off their white feather "base layer". Females are able to spot these large white snowballs from miles away and are drawn to the leks to inspect the males before making their choice in picking a mate. In other times of the year, despite their large size, Great Bustards can be surprisingly difficult to find in the field, especially during the middle of the day when they often lie down taking a rest.