Climate change affects bird nesting phenology: Comparing contemporary field and historical museum nesting records

Global climate change impacts species and ecosystems in potentially harmful ways. For migratory bird species, earlier spring warm-up could lead to a mismatch between nesting activities and food availability. CO₂ provides a useful proxy for temperature and an environmental indicator of climate change when temperature data are not available for an entire time series. Our objectives were to (a) examine nesting phenology over time; (b) determine how nesting phenology relates to changes in atmospheric CO₂ concentration; and (c) demonstrate the usefulness of historical museum collections combined with modern observations for trend analyses. We assessed changes in nesting dates of 72 bird species in the Upper Midwest of the United States by comparing contemporary lay dates with those obtained from archived, historical museum nest records over a 143-year period (1872-2015). Species-specific changes in lay date per one unit change in the CO₂ residual ranged from -0.75 (95% CI: -1.57 to -0.10) to 0.45 (95% CI: -0.29 to 1.43). Overall, lay dates advanced ~10 days over the 143-year period. Resident, short-distance migrants and long-distance migrants lay dates advanced by ~15, 18 and 16 days on average respectively. Twenty-four species (33.3%) significantly advanced, one (1.4%) significantly delayed and we failed to detect an advance or delay in lay date for 47 species (65.3%). Overall mean advance in first lay date (for the species that have significantly advanced laying date) was 25.1 days (min: 10.7, max: 49.9). Our study highlights the scientific importance of both data gathering and archiving through time to understand phenological change. The detailed archived information reported by egg collectors provide the early data of our study. As with studies of egg-shell thinning and pesticide exposure, our use of these data illustrates another scientific utility of egg collections that these pioneer naturalists never imagined. As museums archive historical data, these locations are also ideal candidates to store contemporary field data as it is collected. Together, such information will provide the ability to track, understand and perhaps predict responses to human-driven environmental change.