That's not 'wild foraging' as humans brought the fish to the chickens.Well in the spirit of natural foraging I left a wild caught brim in the chicken pen to see what they do when they wake up hungry in the morning. My dad is an avid fisherman and brings home 20 to 30 brim every week. He's given them to everyone who will take them, so now I'm going to try feeding them to the chickens and pigs. My investigoogling suggests it will be safe and not affect egg taste. Although the reports on pigs were that you had to get the fish out of their diet a while before butchering or you taste fish in the meat.
Just a byproduct of someone's wasteful over-gathering of 'food'.
Nor were grandma chickens living only on 'forage' as they most likely were eating spilled feed of other livestock animals and/or gleaning from what came out the other end of said livestock and/or grains grown by humans on the property as well as the kitchen refuse of farmhouse.
The whole 'that's how grandpa did it' thing rarely takes into account the losses, lack of production and constant and/or annual turnover of stock.