There are many incredible designs around us that are unlikely to exist if everything is the result of mindless chance. Many birds eat insects through the summer. While such high protein diets like this are good in mating and reproductive season, they do not prepare the birds to survive the cold weather of winter.
What they need to do is build a layer of fat both for calorie storage as well as insulation. So, as winter nears, the berries that have been growing and ripening all summer on various shrubs become more numerous. The high sugar content of the berries’ juice helps the birds to quickly build up fat. The freeze on cold fall mornings even helps to increase the sugar concentration in the berries. Even more amazing is that the various types of berries eaten by the birds ripen in a staggered fashion so that berry season is long and the supplies are always available. For example, as the elderberry supply is just about consumed, highbush cranberries are in production. Often, the summer’s hatchlings’ plumage doesn’t reach full color until they eat the various pigments in the berries, and those pigments are incorporated into the new feathers. In exchange, the birds spread the seeds in the berries.
All of these complex, interrelated systems depend on each other in an elegant, fine-tuned design.