Eclectus parrots make their nests in parts of Australia where floods are extremely common. When their nests start to flood, the parrots make a grim decision. They kill their chicks...but only the male ones. What drives them to commit gendercide?
It's rare for birds to kill their own children, and it's really unusual for these parrots to choose along gender lines. Indeed, it seems like the sort of behavior that evolution would select against, since artificially creating a gender imbalance leads to nothing except a lot of females one day struggling to find mates.
To figure out what's going on, Robert Heinsohn of the Australian National University carried out an eight-year survey of 42 nest hollows belonging to Eclectus roratus. He discovered the likely answer: female chicks fledge a week earlier than males. The fact that the females can fly earlier means they have a better shot of surviving the flooding than males.
It appears that the parents make the rather grim decision to kill off the males early and focus on keeping the females alive long enough so that they can escape the rising waters. There's a certain grim logic to all this, and the species has evolved to make the choice easier - males and females chicks are born as slightly different shades of gray, making it easier for the Eclectus parrots to identify which ones are worth saving.