Warbler Guy, is warbler migration early this year? Are early sightings of returning warblers occurring this spring?

Good question, Molly (in Palo Alto).

My N. CA/Marin County location provides a limited answer, but I believe reported sightings suggest, "yes," early arrival of returning warblers has happened in 2015.

For example, I noticed two to four week early arrival dates for WILSON'S, BLACK-THROATED GRAY, and ORANGE-CROWNED WARBLER in February and March, 2015.
Note, over-wintering sightings of these three species occurs, but the vast majority are absent in my area during the non-breeding season (i.e., they are seasonal/nesting residents only).

Likewise, a scan of ebird records for warbler arrival times in my area supports my contention.

As for the East and Midwest where warbler species (such Yellow-rumped and Palm Warbler and Louisiana Waterthrush) are just beginning to return to breeding territory, I have no information to share, sorry.

Meanwhile, other early arriving songbirds this season where I live have included Pacific-slope Flycatcher, Grasshopper Sparrow, Warbling Vireo, and Cassin's Vireo.

In fact, this winter was so warm and dry, it felt like spring. And, now, with spring arriving on the calendar, it feels more like summer (with today's temperature reaching 83° F....!)

Warbler Guy, where can I read about multiple warblers' nesting habits, warbler migration patterns, warbler songs, etc.?

Syd, you cannot go wrong by visiting:

http://bna.birds.cornell.edu/bna/

This site features a comprehensive list of more than 720 North American species, with all of this area's Parulidae (warbler family) members present.

Yes, it costs money: $42 per year or $100 for three years.

Cannot recommend it enough.