This has led to a correlation between the size of predators and their prey. The beak is typically heavier than that of most other birds of prey. Occasionally, these owls may prey on threatened species. Snakes do not ordinarily prey on humans. Among marine mammals, killer whales sometimes prey on adult swordfish. Falconidae is a family of diurnal birds of prey. They form the nucleus of the Birds of Prey organization. Bengal tigers occasionally prey on sloth bears. There have been incidents of tigers attacking domestic animals in times of shortage of prey. Centipedes and spiders may frequently prey on one another. Lemon sharks feed on prey that are intermediate in size compared to other available prey. The largest avian prey for barred owls are gamebirds. Eyesight is a particular characteristic of the owl that aids in nocturnal prey capture. Red foxes may on occasion prey on lambs. Given the opportunity, however, both species will prey on the other's cubs.