You mean pesticide drift? Yes that is an issue. The certifiers require us to keep so many feet buffer between our fields and conventional farmers fields. And each neighbor signs a contract agreeing not to spray on windy days. But pesticide drift is very hard to keep away from.
We actually have bees (or rather rent space to a beekeeper) that is why I used them as an example. He puts the bees on our clover fields, so most of the pollen is collected at a organic source, by he cannot guarantee all if it.
I mean invertebrate drift. Insects move, usually periodically (ie time of day or seasonally) with prevailing winds. Movement of insect biomass can be measurable and impactful on chicken foraging effort. I keep bees as well and do not even consider organic option for reasons you indicate.
But you work with what you have. And as I said I do not certify my birds...would not even if I sold them. It is way to much paperwork and money to justify chickens.