There was once a time and world where birds were considered mystical creatures. At times the deities of this world used birds to deliver messages, omens, prophecies, and other information to prophets or priests. The 'civilized' people quickly realized that the presence of a bird itself was an omen of different sorts. Some cultures of this old world had strict laws against those who would kill any bird with the usual penalty being death. Others only enforced such an extreme penalty against those who killed birds who could fly, as flight shows a close tie to the heavens. Other 'heathen' peoples ignored those rules completely as several deities did not use birds to deliver messages. Feathers themselves were considered a gift from the gods and were often treasured. A magic system was developed, based entirely on the latent magical energies of birds. One way of harnessing this power was through crafting crossbow quarrels (or arrows) and using it in a specially designed weapon.
Whether the legends of birds being messengers between the gods and mortals is true or not in the present world makes no difference, the weapon works the same.
This weapon was created in this old time with powerful avian magic. The power and magic was created to use the latent magical energies left in the feathers of whichever kind of birds would grant feathers to the priests and peoples of that world.
Variation 1 – The Original
The crossbow is much like any other only it has no string or cord. So on it's own it operates faster than a normal crossbow. The bolt only needs to be placed in position, aim the crossbow, and squeeze the trigger. A magical force launches the bolt. This improved efficiency may assist the user of the crossbow in a combat situation. (basically depending on the rules of your gaming system, the user may get a bonus action or not face penalties imposed on crossbow users compared to bow users, your call)
Variation 2 – The Bow
The weapon is a bow (with a string) but otherwise functions identically to the original only the weapon is more animus. It would appear that this ancient religion has been preserved through an order of druids or perhaps a group of some woodsman/ranger types.
Variation 3 – The Transforming Crossbow
This crossbow is fashioned after a generic bird in flight, with the wings being the bow, the tail being the handle and the feet being the trigger. As the bow is loaded with a quarrel/bolt of a bird type the crossbow itself changes to match the type of feather being used. This could give someone on the business end of the crossbow a clue as to what they are up against.
Variation 4 – The Crossbow with the Compartment
This crossbow variation requires the feather to be placed within the weapon itself. The compartment is in the shaft of the weapon. The feathers on the quarrel itself don't affect the magic.
Variation 5 – The Siege Arbalest
This crossbow variation is the bigger unit the the longer loading time. Birds with rather large feathers (such as ostrich, roc, stork, emu, crane, etc.) can be used to create bigger bolts.
Here is some examples of birds that you may or may not have in your fantasy world, and how a bolt made from their feathers would behave.
Chicken Feathers – When this bolt flies true, it's target is struck with a fear spell. Sometimes the fear is so strong the target becomes paralysed and cannot act for a time. Other times the target will flee, soiling their undergarments, and be incapable of rational thought until the spells duration ends.
Cuckoo Feathers – When this bolt strikes it's target. The target in filled with intense confusion. Sometimes this confusion is so intense that the target gains an insanity, usually a phobia, but any other type is also possible.
Woodpecker Feathers – This bolt really shines when used against things made of wood. When striking a wood item, such as a door or a shield, the usual result is that the item shatters. The bolt does extra damage to creatures made of wood, such examples would include wood golems, carnivorous plants, dryads, and treants. There is also a very small chance that the bolt would kill such a creature outright.
Vulture Feathers – This bolt is obviously most useful against the undead. Skeletons, liches, and mummies and other dry undead often shatter or crumble under the power of this bolt. Zombies usually explode in a disgusting shower of putrification. Vampires are not immediately killed upon being struck, unless hit in the heart. Most lesser vampires become paralysed by the vulture feather bolt lodged in their bodies. Only the most powerful vampires shrug it off and are able to carry on albeit seriously wounded. Ghosts flee as though hit with a chicken feather arrow (above). Vulture feather arrows also have a small chance of instantly killing anyone who has cheated death with either longevity potions, resurrection, breath of life, raise dead, reincarnation, or other death defying magic.
Owl Feathers – This bolt is an intelligence gathering weapon. When the bolt hits it's target, the target always survives the hit and remains conscious. The bolt quickly runs a mini telepathy, empathy, danger sense, on the target. In a quick wave of intelligence, the user of the bolt becomes aware of the target's general battle plan, including possibly an ambush, a secret weapon, and the target's strengths and weaknesses in combat. This can change as the bolt user reacts in anticipation of the targets original plan.
Phoenix Feathers – This bolt transforms into a fiery dart. The target takes additional fire damage and the bolt itself is consumed in it's own fire. There is a good chance, but not always, a new phoenix feather bolt is created in the same location as where the old one hit.
Ostrich Feathers – This bolt strikes it's target like a catapult shot as opposed to a bolt. The target is hit with blunt force which also acts as a magical push. Targets that are normally immune to arrows and bolts (such as a stone or iron golem) take full damage from this bolt.
Hummingbird Feathers – This bolt hums as it flies. When the hummingbird arrow misses, it changes directions and takes another try at the target. This bolt continues to vibrate intensely when lodged in a target. This results in extra damage every round it is left in. (obviously very small damage)
Lovebird Feathers – This bolt deals no damage to the target and vanishes upon a successful hit. The target is then struck with a powerful love spell and will fall for the first person of their preferred gender that they see. Priests and priestesses of Aphrodite would really appreciate this one.
Crow Feathers – This bolt is a death omen. The bolt itself has a small chance of instantly killing the target. If the instant death ability does not take the life of the target and they survive the battle or skirmish they were in when hit, the target will be plagued with a death type curse. Within the next 24 hours, the target will have the worst luck, which will attempt to slay the target. Henchmen who were considering betrayal will choose this time, the bartender will tap the one tainted keg in the bar, accidents will occur around the target almost constantly until either the target is killed or the twenty four hours pass. If the time passes the target is now rid of the curse and back to life as normal.
Bonus feathers -
Albatross – the bolt gains the ability to pass through water as though it were air. Deals double damage to merfolk.
Bee Eater – the bolt deals double damage to insects & arachnids
Buzzard – the bolt contains a contagion. A successfully hit target has a good chance at getting a disease.
Duck – the bolt gains the ability to pass through water as though it were air. Deals double damage to sea monsters.
Eagle – the bolt deals bonus damage and casts a push spell upon a successfully hit target.
Falcon – large bonus to hit and damage when used to hunt wild animals.
Finch – small bonus to hit. Deals double damage to elves and faeries.
Fireback – the bolt catches fire in flight and deals additional fire damage
Fly catcher – large bonus to hit.
Goose – the bolt has a high chance of stunning a successful target.
Grouse – the bolt has a chance of causing a successful target to completely lose their cool and any rational thought. Spell casters and psionic users hit, have a 50% chance of screwing up their powers when trying to use it (100% upon 2 successful hits). If a successful target survives the fight, they will complain or 'grouse' about the incident for the next 24 hours.
Hawk - small bonus to hit and damage. Critical hits are 20% more likely.
Heron – the bolt deals double damage to giants
Ibis – this bolt is shot always at great proficiency. The shooter could be upside down, riding a galloping steed, walking a tightrope, or falling to their death, however the bolt will fly as though the archer were at total peace and made a carefully aimed shot. Barbarians, berserkers, and other creatures that 'flip out' take extra damage from this bolt.
Jay – the bolt's flight path bends and hits the target from the other side. Using this bolt counts as back stabbing. (I know there's a little Wanted in this one)
Kingfisher – this bolt has the power to successfully penetrate force fields, regular shields, walls of “”, and other obstacles. This bolt cannot be caught in flight with either extreme skill or telekinesis.
Kite – the bolt get a bonus to hit and damage if the target is airborne.
Macaw – the bolt casts a random (negative) enchantment upon a successfully hit target.
Nightingale – the bolt casts a powerful sorrow spell upon a successfully hit target.
Penguin – the bolt gains the ability to pass through water as though it were air. The target losses all magical protection to cold.
Pigeon – the bolt deals minimum damage. The bolt carries with it a magic piece of paper which has written the last words spoken by the user of the crossbow. The paper vanishes once the message is read.
Quail – the bolt deals double damage to elementals and has a small chance of casting a banishing spell upon successfully hit elementals.
Raven – bonuses to hit and damage when used against good creatures
Robin – if used as a called shot to knock something off of the target, the shot is made with large bonuses. Otherwise it's a regular shot.
Sea Gull - the bolt gains the ability to pass through water as though it were air. Deals double damage to sharks.
Swan – bonuses to hit and damage when used against evil creatures
Tern – the victim of this shot is 'turned'. The victim must avoid proximity with the shooter until the effect wears off.
Thanks to all the contributions given. I hope you enjoy.
Additional Ideas (4)
lighting bird feathers
releases a blast of thunder and lightning so powerful all target will die and persons close to him will lose most of there health
Pidgeon Feathers: Someone struck with arrows fletched with Pidgeon Feathers are suddenly swarmed by a flock of urbanized Pidgeons who are not afraid of people and extremely proficient at stealing food and other objects from the victim. The victim is harassed by Pidgeons for at least three days, sometimes up to a week.
Someone hit by a magpie feather arrow will, if they survive, be plagued by thieves for days or until all their stuff is stolen,whichever comes first.