Use a .22lr rifle of your choice and a .22short round like the CCI CB. At 25 ft it's lethal to a bird and if you miss the bullet loses energy quickly and after 100 yards only has about 24 ft/lb of energy. If someone got hit by it it'd be less injurious than some sling shot hits at 100 yards. Further than 100 yards and well... I'm sure you get the idea.
Just be careful and aim well - even a .22 short can damage a tin roof if you hit it within 25 yards... albeit probably just dent it and not shoot through it. Oh speaking of range... ummm 35 yards will probably be as far as you want to be from your target.
Just a thought too, I remember my son shooting starlings and house sparrows in field with a pellet rifle, you know the run of the mill Daisy type with multiple pumps and max mv of around 500fps. At 10 yards the shots were through and through and the birds dropped dead where they were hit.
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