I use the Smithsonian bird guide because I received it as a gift, which goes for
practically everything I own, what little there is. I imagine that part of your
need for repeated exploration into the treasures contained between the covers of your
precious guide is that it does not offer photos of juvenile birds. Without such
ancillary photos there is likely no description of juvenile birds either to help you manage
your curiosity. In future, try an internet "image" search for a good preliminary guess
and then also search "juvenile" so that some of the myriad of photos presented might
also contain shots of juvenile bird.
I now almost regret suggesting that approach, if you haven't tried it already, because
mining your reference text/guide is absolutely necessary to hone your skills at
identification. There is no quick and easy way of learning a field. You need to
stand in, and walk through that field-of-knowledge repeatedly, inspecting and
scrutinizing every detail. Only then can you walk-the-walk and talk-the-talk.
Ha, ... listen to me ... like, I had a clue about anything. Just do what you usual do.
Wait until mid-to-late August and into early September and there will be many
juvenile warblers, and other juvenile birds coming through the GTA. A great
opportunity to hone your ID skills. Juvenile birds will probably show more tolerance
for close approach because they don't know any better ... yet. Be prepared, some
of them are real punks.