gray and black bird during day

In Etosha National Park (Namibia) are some picknick/toilet places with a big fence around it. The big fence is because you can't get out your car because of the lions, elephants, leopards, cheetas and other dangerous animals. On one of those picknick places we saw this bird and it was really close. When we looked it up, we saw that it is a yellow-billed hornbill.