A few thoughts:
-First, what size/shape of feather do you need? A single bird likely possesses over one thousand feathers total on it's body, in the same way I possess over one thousand hairs on the top of my head. If the letter of the law is "provide 1000 feathers", hand them 1 sparrow, mission accomplished.
-Since you likely need feathers of a certain size and shape (I'm sure others know better than I about this), I'm assuming you need the longer wing feathers from duck-sized & larger birds. Can you get 50 useful feathers off of a single Chicken or Goose? If so, 200 chickens or geese should do the trick. At 1gp each, you could buy them someplace and deliver to the locals, they can pluck the birds themselves.
-If it is a matter of larger sized feathers, how about a single, much larger sized feathered creature? Would the DM permit there to be a
Roc, a
Griffon, or some other bigger creature that could provide thousands of feathers? You might be able to ask for them, than see if Healing magic can restore the lost feathers quickly, or otherwise be in the debt of the animal or magical beast.
-You're a 15th level Druid, spam spells. To accomplish this specific task, you could:
Commune With Nature: inquire to the presence of the birds you need, or perhaps their nests already filled with shed feathers.
Call Animal +
Speak With Animals + Charm Animal or
Dominate Animal: Repeat the process enough times to bring the appropriate birds to you and get them to be willing to have the necessary feathers plucked.
-Finally, YOU ARE A 15th LEVEL DRUID. Offer your services against a small army, you can have so much more effect than one thousand arrows. You're talents are wasted on feather gathering. How about you offer the following:
Before the battle, use
Rampart,
Animate Plants, Wall of Stone, Stone Shape, Control Water, Obsidian Flow and
Move Earth to create whatever sort of massive walled fortress, moated defense, tree-fortified, glassed landscape you feel like. They have you picking feathers, you could be creating an elevated, defensible metropolis in the span of 2 weeks. Once that's created, you offer to
Transport Via Plants to where the enemy is located, cast
Control Weather before revealing yourself to whip up some impressive environmental effects, Wild Shape into something awe-inspiring like a Huge Air Elemental and calmly explain that you are a high level Druid speaking on behalf of the locals you are defending, politely requesting they back down, throw a few powerful spells around to demonstrate your point or
Euphoric Tranquility the enemy commander into declaring the war ended, than
Word of Recall yourself back. If the enemy arrives and STILL feels like taking them on, you've got a bazillion possible area damaging spells. One casting of
Earthquake should finish an army.
...And you've got a party that I'm assuming is close to your level, any of which could contribute a variety of helpful skills better than gathering feathers.