Some fungal leaf spotting, not serious. Can't see much of the tree in the pictures given but River birch, and I'm assuming you're tree is a River birch and not white, are sensitive to heat and weather changes. It is normal to drop a percentage of leaves from mid summer on, especially in hot weather with little rainfall. Can see no insect damage but what I do see is a possible chlorotic condition, evidenced by leaf yellowing to pale green with veins showing more prominently . River birch need acidic soil, a lower pH, for optimum growth and will not do well in alkaline soils (high pH). If the pH is out of range, the tree can not take up the nutrients it needs even if plentiful in the soil. A soil test will tell you what you need to know. http://msusoiltest.com/ For a small fee they will culture your soil in the Lab and make recommendations as to amendments and/or nutrients needed.
If the tree proves chlorotic, it will affect growth and also makes the tree more vulnerable to insects and disease because of the stress. You can read more about chlorosis here: http://msue.anr.msu.edu/news/it_isnt_fall_yet_so_why_are_my_trees_leaves_yellow