They still would be free to do anything other than move if it was a grapple.

"Stirges are the BEST animal for the druid spell "Conjure Animals". Don't believe me? Try it out yourself, you won't be disappointed."

 Wolves, Giant Poisonous Snakes, Giant Owls, Black Bears, Apes

Quote from Enesta_Anistari >>

So I saw stirges on the list of 1/8 cr woodland creatures on the spell conjure woodland creatures, and since the spell only limits things of 1/4 cr and higher, does this mean you can summon a swarm of stirges, either 16 of them to follow the pattern of doubling the amount as one goes down in cr, or just an nigh on unlimited... 

 No. You have 4 options. 1 CR2, 2 CR1, 4 CR1/2, or 8 CR1/4 or lower. If you want to summon CR1/8 you have to deal with having the same amount as CR1/4. Some of them, like stirges and giant crabs, could be worth it sometimes, but for the most part you don't want them. Especially if your DM lets you choose your monsters without limiting any of the weirdly strong ones. Wolves are a deadly use of this spell, but giant poisonous snakes and giant owls shouldn't even be CR1/4 in the first place so summoning 8 of them is pure madness. 

I think stirges are disgusting (especially in the art) but the first thing I ever killed was a stirge so they hold a special place in my heart

I just shoot magic missiles at them, each missile is guaranteed to kill it, and that means you can kill up to 3 storages in 1 turn, or 11 if you use a 9th level spell slot, (but I don’t know anyone who would use a 9th level magic missile)

Does the stirge's blood drain ability cause failed death saves to an unconscious target? If it attached to someone, and then that target got downed by external means, but not killed, does the stirge stay attached and drain blood, or does it automatically unattach?

They are most fun if they carry diseases like mousquito's and fleas can :D

Sleep is pretty much a hard counter to these. 2 hp means you can knock out the entire flock with minimal friendly fire worries, and if they fall far enough to take damage you've got really good odds of killing them outright instead of just waking them up.

thay have to role depending on their strength

our fighter ate stirges three and then i showed him the pic of it he was grossed out

I have a homebrew rule for Stirges and the like - if a character attacks a stirge (rather than using their action to detach it) that is attached to a creature, if the attack hits, any hit points over the stirge's current hit points is applied to the creature the stirge is attached to.