I was lounging about in my throne room when a man came to speak to me. He had one leg cut off (recently, too) and claimed he was an alchemist. He says that he wants to get revenge on the king of the neighboring kingdom, who cut off his leg with a spoon. I do not like this man so I happily agreed to the task. The alchemist said that he could reward me with an endless supply of gold, silver, and iron: by turning all of the pidgeons in my kingdom into spoons! (And good riddance, too.)
Our kingdoms are separated by a fast-flowing and impassible river which is 500m wide. No boat can survive more than a minute on those fast-flowing and dangerous waters. The alchemist, however, says that modifying the existing projectiles may solve my issue.
The counterweight trebuchet is the most far-flung of the bunch with a max range of 450 metres:
A larger trebuchet cannot be made in time, so instead the projectiles must be made to "float" the extra 50 metres. One of the knights suggested to use birds to give the stone a little extra lift. The problem is: I need the pigeons for spoons! Luckily, a swallow was spotted carrying a coconut across the realm, and it was decided to use swallows instead.
How could a medieval soceity catch a large number of birds within a day or two?