as if he were right beside me on this clear day in March,
The sentence at the end of the last stanza carries over into this new stanza. That poetic technique (ending a line of poetry mid-sentence and picking up on a new line) is called enjambment.
Our speaker is still talking about Sonny Rollins here. Remember that he starts off the poem saying that he's "ambling along 44th Street/ with Sonny Rollins for company."
When we read those lines, we weren't immediately sure if the speaker is actually walking along with Sonny Rollins beside him. Line 6 makes it clear, though, that Rollins isn't walking next to the speaker, but that the speaker feels that Rollins is right beside him because he's listening to his music. (Who doesn't feel like Kurt Cobain is right there screaming in their ear when they listen to "Smells Like Teen Spirit"? We do, for sure.)
We also get more of a sense of setting in these lines: it's March, it's the spring, and it's a beautiful clear day. Ahh—how can this speaker not be enjoying himself?
the pavement sparkling with sunlight, pigeons fluttering off the curb, nodding over a profusion of bread crumbs.
These lines are all about setting. The pavement that the speaker is walking along is all shiny and pretty with sunlight and there are pigeons fluttering about and eating.
The description of the setting here is evocative, because it gives us a sense of the speaker's mood. He's listening to some cool jazz, he's happy, and the sunlight and the fluttering pigeons reflect his happiness. There's lots of light and movement all around him. And this reflects his high spirits. (See "Setting" for more on this kind of stuff.)
We can see, again, quite a bit of sound play going on in these lines. In line 8, the words, "sparkling with sunlight" offer alliteration with those S sounds consonance with the L sounds.
In lines 9-10, we also find a lot of repetition of the F sound in words like "fluttering," "off," and "profusion." By using consonance in this way, the poem creates a rhythm even though it doesn't stick to a regular form or meter. (See "Sound Check" for more details.)