California Institute for Energy and Environment (CIEE)

Long-term data recorded before regional temperatures were greatly affected by human emissions of greenhouse gasesprovide an historical baseline with which to compare modernand future trends. Long-term ecological data needed to investigate the influence of temperature on the time of songbird breeding are rare. Significant amounts of data exist in collector descriptions of bird eggs taken in the wild from mid-1800s to mid-1900s. The researchers draw on these data from three museums, and finds that seven of 21 species examined had significant trends in laying date, with some species laying eggs earlier and others later. The largest change was in the Oak Titmouse with an average shift of 20 days later per decade and Phainopepla with the largest earlier shift of 8 daysearlier per decade. The associations between the average California temperatures a month or two before laying and times of breeding were strong for six out of the nine resident California songbirds. For migrating species, only two of nine had a strong association between laying date and mean California temperature. These findings provide a baseline to which modern and future phenological shifts may be compared and calibrated. The researcher team offers suggestions for future studies and recommendations for the state of California.