Demographic consequences of phenological asynchrony for North American songbirds

Changes in phenology in response to ongoing climate change have been observed in numerous taxa around the world. Differing rates of phenological shifts across trophic levels have led to concerns that ecological interactions may become increasingly decoupled in time, with potential negative consequences for populations. Despite widespread evidence of phenological change and a broad body of supporting theory, large-scale multi-taxa evidence for demographic consequences of phenological asynchrony remains elusive. Using data from a continental-scale bird banding program, we assess the impact of phenological dynamics on avian breeding productivity in 41 species of migratory and resident North American birds breeding in and around forested areas. We find strong evidence for a phenological optimum where breeding productivity decreases in years with both particularly early or late phenology and when breeding occurs early or late relative to local vegetation phenology. Moreover, we demonstrate that landbird breeding phenology did not keep pace with shifts in the timing of vegetation green-up over a recent 18-year period, even though avian breeding phenology has tracked green-up with greater sensitivity than arrival for migratory species. Species whose breeding phenology more closely tracked green-up tend to migrate shorter distances (or are resident over the entire year) and breed earlier in the season. These results showcase the broadest-scale evidence yet of the demographic impacts of phenological change. Future climate change-associated phenological shifts will likely result in a decrease in breeding productivity for most species, given that bird breeding phenology is failing to keep pace with climate change.

Methods
Bird capture data were collected as part of the Monitoring Avian Productivity and Survivorship (MAPS) program, a collaborative long-term bird-banding project operating across North America. Data were obtained from 179 banding stations. Each banding station consisted of 6–20 mist nets operated approximately every ten days beginning as early as May 1 (start date varying slightly by location) through August 8 (ordinal dates 121–220 in a non-leap year), which span the breeding season for most birds in North America. Only species/locations/years with at least 15 total captures, at least 5 of those being juveniles, species/locations with at least 5 years of data, and species with at least 15 locations/years of data were considered.

Bird breeding phenology was calculated using the capture dates of juvenile birds at MAPS stations. This measure of breeding phenology is indicative of the time of year at which young birds are fledging. For each species, at each location, in each year, our metric of breeding phenology was the mean date of first capture across all juveniles captured at that station that year. Following Saracco et al. 2019, we exclude subsequent captures of the same individual after its first capture. 

For each station, effort hours was calculated as the proportion of net-hours (total area of mist nets multiplied by the number of hours that these nets were deployed) during the period where juveniles were captured, excluding the first 2.5% of juvenile captures to remove outliers, following the procedure used by Saracco et al. 2019. 

Climate data was downloaded from climatena.ca.

Works cited

J. F. Saracco, R. B. Siegel, L. Helton, S. L. Stock, D. F. DeSante, Phenology and productivity in a montane bird assemblage: Trends and responses to elevation and climate variation. Glob. Change Biol. 25, 985–996 (2019).