How can Shriekers be justly assigned a CR of 1?

Why on earth are shriekers CR 1? They can't move or attack. Their shrieking doesn't cause deafness or have any real adverse effects.

It does not seem justifyable to have them be CR 1 due to the potential to attract nearby monsters. It's not a summon ability, after all, like a demon's summon ability, and doesn't need to get factored into the CR since a monster is not guarenteed to actually arrive, and if it did, the shriekers would probably all be dead by the time it arrived and defeating the monster will be no more difficult than encountering it in its lair - perhaps easier.

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