Fallen Leaves and Being Neighborly

[x2ski]

My back yard seems to be the only one with trees whose leaves fall during this season. All of my surrounding neighbors planted evergreens.

My neighbor immediately behind me takes good care of his yard, but a good portion of the leaves from one of my trees is falling into his yard.

Do I cross his property line to rake them, or is it his responsibility? I've never spoken to him, and if we were rather chummy, I would most likely take care of it on my own, but feel weird doing it now... It seems like trespassing or something.

Thoughts?