Omar often saw woodpeckers perched in trees in his backyard, but he never saw them at his bird feeders. Omar usually filled his feeders with sunflower seeds. He wondered if he could attract woodpeckers by adding a different type of food to the feeders.
To test this idea, Omar gathered eight identical bird feeders. He filled four with sunflower seeds. He filled the remaining four feeders with crushed peanuts. Omar then placed the bird feeders in his backyard. Over the next five days, he counted the number of woodpeckers that visited each bird feeder each morning.
Which of the following was an independent variable in this experiment?