Seasons

The opening quatrain of Shakespeare's poem uses the concept of the seasons as a metaphor for the process of growing older. Any guesses which season he picks? Well that would depend on what time of life he wants to represent, right? As it happens, Shakespeare uses the traditional associations of fall with death (technically, it's more like late fall or early winter) to suggest a speaker who is in late middle age, just on the brink of becoming old.

• Lines 1-4: The first thing that probably strikes you about quatrain 1 of this poem is its incredibly vivid imagery. Just look at the different senses that Shakespeare brings into play here: sight ("yellow leaves," "boughs," and "birds" to name a few), touch ("cold"), hearing (the birds that "sang"). Of course, this being a 14-line poem, there really isn't room for the poet to stick in all sorts of details just because he finds them pretty. In a sonnet, just like in a tiny apartment, maximum efficiency is key. But how do you get things to do double duty when those things are words? With metaphor, of course. Because all of the seasonal imagery in quatrain 1 of the poem adds up to one picture of the speaker at a certain time in his life, we're actually going to call this an extended metaphor or conceit. Another reason we're calling it this is because, as we're about to see, there are some other, smaller metaphors going on even inside the bigger metaphor. Now that's what we call efficient.
• Line 3: At first glance, it looks like the speaker is talking about branches of a tree shaking in the wind. Now, here's the thing: that first glance is probably 100% correct, but, because this is poetry, we get to have other stuff going on here at the same time. That's because of the word "against," which makes sound like the boughs are shaking all by themselves, instead of being shaken by the wind. When you look at it this way, the line starts to look like (a) metaphor and (b) a case of personification. Think about it: if the boughs are doing the shaking, and the "cold" is a metaphor for old age, then don't you think the shaking boughs might be standing in for the trembling limbs of an old person? This explosion of meanings within a single line is one of the things Shakespeare is famous for. 
• Lines 3-4: But that isn't even all the end of quatrain 1's metaphor-frenzy. Just check out what happens in the shift from line 3 to line 4. When line 4 rolls around, we learn something else about those boughs—specifically, we learn that they aren't boughs at all, but instead are "bare ruined choirs." But how can "boughs" be "choirs"? Easy: through metaphor. It's easy to miss the metaphor here, of course, because in the sentence spilling over from line 3 to line 4, you never find a word or set of words that tells you explicitly that Shakespeare wants you to think of these two things as being the same. Instead, he just relies on punctuation and the instincts of his readers.