Sibley also has a field guide, which is the same size as Peterson’s. You know that Peterson died and his latest edition is quite old. Sibley has just come out with a new edition of the field guide. This is important, not because birds change, but our names for them, and sometimes even the scientific name, changes. One obvious example, which you won’t encounter in Illinois is the tri-colored heron, which used to be called the Louisiana heron. The common morehen is now the common gallinule, another bird you won’t find in Illinois. Sometimes DNA studies will show that a bird that was thought to belong to one genus actually belongs to another. Sometimes birds that were thought to be different species actually are variations of the same species.
I have the large “the Sibley Guide to Birds” (not the field guide) and it shows that both the golden-crowned kinglet and the ruby-crowned kinglet are found where I live (in SC) only in the winter. The same is true for the golden-crowned where you live, and the ruby-crowned is found only in migration in Illinois. Although quite common here in the winter, they are rarely seen now.
I find quite useful is the iBird Pro app for iPhone. It plays the various variations of the songs and calls of the birds but also gives much useful additional info. AudubonBirds is another app. Sibley has an app too, but I downloaded Sibley Lite because it is free, and useless. It doesn’t even have many common birds.