非洲灰鸚鵡是社會關係最複雜的鳥類之一，也由於「能言善道」成為熱門寵物。PHOTOGRAPH BY JOEL SARTORE, NATIONAL GEOGRAPHIC PHOTO ARK

非洲灰鸚鵡尼基（Nikki）與傑克（Jack）在實驗期間交換代幣。PHOTOGRAPH BY ANASTASIA KRASHENINNIKOVA