They can be pretty much anything. It's just an embedded jutsu that can trigger like a trap.
To keep it non-lethal, it could be a genjutsu -- an illusionary art -- that shows the birds a larger predator keeping watch over the fields.
Other options:
no subject
They can be pretty much anything. It's just an embedded jutsu that can trigger like a trap.
To keep it non-lethal, it could be a genjutsu -- an illusionary art -- that shows the birds a larger predator keeping watch over the fields.