I found this post on our neighborhood blog:
Squirrels and birds need trees for food, shelter, and especially for nests to rear their young. In the Bay Area, there are babies in tree nests from January through early October. These infants cannot “run away” when the chainsaws arrive. Every year our local wildlife hospital receives hundreds of baby birds and squirrels that are killed, maimed or orphaned by tree trimming. Often they are brought in by saddened homeowners who just “didn’t realize” their decision to trim would cost little lives.
So if you care – please plan ahead and trim trees late in the year – late October through December! And please spread the word to your friends and neighbors too. If you MUST trim now due to an unsafe tree or branch, please check carefully for nests and trim around or remove them intact and look for babies in the downed branches. Many thanks for your kindness to our wild friends!
WORST TIME TO CUT/TRIM ~ JANUARY THROUGH mid OCTOBER
BEST TIME TO CUT/TRIM ~ late OCTOBER, NOVEMBER & DECEMBER