David M. Larson
There is widespread worry throughout the climate, ecology, and conservation fields about the species-level and ecosystem-level effects of anthropogenic climate change. For birds, two obvious problems are exposure to altered, higher temperatures at critical points in the life cycle and loss of the linkages between resource availability and breeding. Most importantly, strong selection pressure effects on breeding time, location, and success are expected during global and regional shifts in temperature and other habitat-critical climatic changes. Global temperatures have risen approximately 1oC during the last century. Long-term studies on bird populations seem best suited to sort out avian adaptations to such climatic shifts.
Birds can attempt to maintain their current maximum daily temperature in the face of climate warming by changing location to cooler places—moving their breeding territory to the north or to higher elevations—if they are trying to maintain optimal temperatures at the same breeding date. Alternatively, they can shift to earlier breeding times if they are trying to maintain optimal nesting temperature at the same location. Of course, they can do both to varying extent. If they cannot accommodate, they risk extinction.
Socolar et al. (2017) have taken advantage of a comprehensive survey of California terrestrial vertebrates, directed by Joseph Grinnell, the founding Director of the Museum of Comparative Zoology at the University of California, Berkeley, between 1904 and 1940. One century later, the Grinnell Resurvey Project (mvz.berkeley.edu/Grinnell/) is generating data from the same areas as the original survey to assess changes in habitats, distributions of vertebrates, and phenology. Socolar and colleagues focused on the full spectrum of avifauna from two areas of these projects, California's Southern Sierra Nevada (SN = 160 species) and Coast Range (CR = 150 species). The two study areas contain a total of 224 species over 6.5° latitude and 6.2° longitude. For current data, they used a database of more than 47,000 monitored nests from Cornell Lab of Ornithology's Project NestWatch (nestwatch.org) to assess nest success versus ambient temperature.
Data sets were far too small to study changes in individual species, but using the entire avifauna provided an unparalleled view of breeding-season phenology for the suite of species. Nesting dates have shifted 5–12 days (mean of 8.6) earlier between the 20th and 21st century surveys. Overall, this temporal shift in breeding dates compensates for the 1oC increase in maximum daily temperature over the last century. The observed shifts are most significant in June and become less consistent in July.
Without any compensation for rising temperature, nesting success can suffer. Analyses of Project NestWatch data on 110 species suggest that temperature at the nest site is a complicated determinant of nest success. At the edges of the species' range, a 1°C increase in temperature changes the probability of fledging young birds by 0.03, both an increase at the cold end and a decrease at the warm end.
These analyses suggest that these birds adapt to increasing temperatures due to climate change by moving to earlier, cooler breeding dates, a phenological adaptation. This response could help to explain the lack of altitudinal changes in 16% of Sierra Nevada bird species and 37% of individual range margins found in previous studies by this research group.
While it is clear that birds would be most successful in evolutionarily determined temperature ranges, especially considering the low thermoregulatory capabilities of very young birds, the connection between phenological adaptation and resource tracking remains unclear. Of course, resources such as insect emergence probably track temperature but may be more dependent on winter conditions than spring warmth. If temperature tracking in birds diverges from resource availability, then geographical range shifts could compensate.
The strengths of this ongoing series of investigations lie in the long timeline and the large base of species included. Analyses from these studies are, of course, providing new and refined questions for future studies as the march of anthropogenic climate change proceeds.
- Socolar, J. B., P. N. Epanchin, S. R. Beissinger, and M. W. Tingley. 2017. Phenological shifts conserve thermal niches in North American birds and reshape expectations for climate-driven range shifts. PNAS 114 (49): 12976-81.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon's Joppa Flats Education Center in Newburyport, the Director of Mass Audubon's Birder's Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.