Written by Randall English, NBC Naturalist 2019
Photos courtesy of Denise Chilcote
First let me say congratulations on providing suitable habitat for birds to nest. Habitat loss puts pressure on many species and we can help by how we choose to landscape and maintain our yards. The short answer to your question is no, nesting season is not over. In general nesting season in Southern California extends from February 1 to August 31. Different species have different nesting behaviors and timing may be dependent on temperature or specific food availability so it can vary from year to year. Some species may have multiple clutches in the same nest, some may build a new nest in a new location. Unless the tree is presenting a safety issue the best practice is to hold off on pruning and schedule tree maintenance between September 1 to January 31. As birds do not just nest in trees the same schedule should be applied to trimming large bushes and shrubs. Some species like Hummingbirds may nest any time of year so it is always best to observe the plant beforehand watching and listening for nesting activity. On the plus side this means you can count your time spent backyard birdwatching as doing yard work.
The folks over at Sea Sage and Audubon created a wonderful fact sheet about tree maintenance and nesting birds. Follow this link https://www.seaandsageaudubon.org/Conservation/TreeTrimming/TreeTrimmingBrochure2013.pdf for more information.
If you would like more information on providing native habitat for birds follow this link https://www.cnps.org/gardening/choosing-your-plants/native-planting-guides to the California Native Plant Society.