Why are some dogs extremely picky eaters of "dog food," but will eat practically anything they find laying on the ground?

My pup will only eat a blend of two different specialty “dog foods.” If it’s one or the other, he will take his time eating it, or won’t eat at all.

Yet, he will eat pebbles (gravel), snotty tissues, paper towels, dead birds (once I had to pull from his mouth a carcass of a dead bird found outside… gross!!!), metal objects (twisty ties, hairpins, screws – things I’ve had to pull from his mouth), fabric (blankets, towels, etc.), electrical cords (he’s killed one laptop power cord and almost destroyed another), earphones (he completely destroyed a pair of iPod earphones), and anything else he manages to find on the ground.

Why would a dog eat non-food items, when perfectly good (and expensive) “dog food” is available for him?

What crazy things has your dog consumed?