page 46 – the image at the top of the page (right under the heading “Misidentification”) should be labeled Glaucous-winged Gull (not Western Gull).
page 76 – the temperature conversion is wrong; 105 degrees F is about 41 degrees C (not 38 degrees C as written).
Page 88 – the diagram of the Northern Pintail incorrectly labels the dark feathers above the folded wing as wing coverts. These feathers are actually lower scapulars (see this link at BirdForum for a discussion and photos; thanks to Joe Morlan for pointing this out).
Since Birding Basics is all about the fundamental skills and approaches to birding challenges, virtually everything I’ve written on bird identification could be considered a supplement to the book. In particular a series of posts about the psychology of bird identification (listed along with others at the Bird Info page and also listed chronologically below) are directly applicable to general birding skills.