Once We Were Four (1942)

The Secrets of Life series (1934-50) may not conform to modern expectations of nature filmmaking, inclined as it is towards giving cute fluffy creatures human names and characteristics. But it couldn't be accused of shielding kiddies from the harsher realities of the food chain, as this exercise in ruthless Darwinism demonstrates to unintentionally hilarious effect. A more than usually eccentric narrator introduces us to the newborn bunny quartet of Donald, James, Charles and Clifford, but as the film's title gives away, "the boys" aren't all long for this world as they face an assault course of hungry owls, predatory badgers, shotgun-happy gardeners and aerial bombardment (no harm in a little anti-Nazi detour, this is 1942 after all). (from http://player.bfi.org.uk/film/watch-once-we-were-four-1942/)

Unfortunately the movie Once We Were Four is not yet available on HBO Max.