If you look at the link, you will see that it is only the lower leaf that is not serrated at the end section (not the foreground leaf), demonstrating the variation that can occur even on the same plant. A close up of the twig stem can easily differentiate birch from popular. Birches can be any shade of brown with with thousands of slightly raised white spots, specks, or tiny horizontal dashes. Popular twigs will be shades of gray or green/brown, generally smooth, with sparse raised spots. Birch twigs are quite pliable compared to poplar.
