Wasn't there a species of birds on or near one of the Galapagos islands noted for siblicide? I forget if it's one Darwin went to.

The reason was that if more than one sibling survived, even if the mother could provide food for both, the liklihood of her herself not surviving to mate the next year increased dramatically.

Is siblicide or other forms of own-species killing found anywhere in nature without a reason?

We like to say animals can't reason, but we kill with no meaningful reason ourselves.