I doubt it was just a prank- close to 1000 chickens seems a lot more like work to me.
As for damages - lost profits would be most appropriate. The word chicken is probably misused - so I have no clue what was actually killed (chick, pullet, fryer, laying hen, etc...). Killing 1000 chicks would not be a whole lot lost profit, only the margin lost due to not being able to replace these for this batch, plus a small amount to cover the cost of the chick. If they were closer to sale, then the cost moves closer and closer to the farms normal selling price (as feed and time were invested). If they were hens - then the cost is whatever the producer must pay to replace them (plus the cost of lost production while trying to replace them). By this time it is going to be somewhat more expensive - as most hens do not start producing until after 6 months (24 weeks - many commercial chickens are marketable closer to 12 weeks) - more sunk feed cost.