I like to think some boy's been swinging them (Line 3)
The image of the birch trees under snow and ice conjures up some vivid memories for the speaker. When he sees birches he immediately thinks back to the days when he would swing on their trunks and wishes that some other boy were enjoying the trees as he had.
Such heaps of broken glass to sweep away (Line 12)
It's interesting that the speaker moves quickly from the realm of the natural world to that of human beings. There seems to be a tension between Nature and Man in "Birches," and here we see that he can negotiate easily between the two. Do you think that the speaker prefers one sphere over the over?
Like girls on hands and knees that throw their hair (Line 19)
It almost seems as if the speaker is noting that boys and girls appreciate nature in different ways. Does he imply that boys are more active, or is he merely stating that there is a difference? Do you think that there is a gender difference, and if so, what is it?