Differential changes in the onset of spring across US National Wildlife Refuges and North American migratory bird flyways

Abstract

Warming temperatures associated with climate change can have indirect effects on migratory birds that rely on seasonally available food resources and habitats that vary across spatial and temporal scales. We used two heat-based indices of spring onset, the First Leaf Index (FLI) and the First Bloom Index (FBI), as proxies of habitat change for the period 1901 to 2012 at three spatial scales: the US National Wildlife Refuge System; the four major bird migratory flyways in North America; and the seasonal ranges (i.e., breeding and non-breeding grounds) of two migratory bird species, Blue-winged Warbler (Vermivora cyanoptera) and Whooping Crane (Grus americana). Our results show that relative to the historical range of variability, the onset of spring is now earlier in 76% of all wildlife refuges and extremely early (i.e., exceeding 95% of historical conditions) in 49% of refuges. In all flyways but the Pacific, the rate of spring advance is generally greater at higher latitudes than at lower latitudes. This differential rate of advance in spring onset is most pronounced in the Atlantic flyway, presumably because of a “warming hole” in the southeastern US. Both FLI and FBI have advanced markedly in the breeding ranges–but not the non-breeding ranges–of the two selected bird species, albeit with considerable intra-range variation. Differences among species in terms of migratory patterns and the location and extent of seasonal habitats, as well as shifts in habitat conditions over time, may complicate predictions of the vulnerability of migratory birds to climate change effects. This study provides insight into how differential shifts in the phenology of disparate but linked habitats could inform local- to landscape-scale management strategies for the conservation of migratory bird populations.

Biologically‐Relevant Trends in Springtime Temperatures Across the United States

“…In the East, the pattern of advancing thresholds occurring at higher latitudes and not at lower latitudes is consistent with other studies reporting an increasing "speed of spring," where the span of time between when a single threshold is met at lower latitudes or elevations and when it is met at higher latitudes or elevations is decreasing (Asse et al, 2018;Post et al, 2018;Vitasse et al, 2018). Attributed primarily to a slower rate of warming in the Southeastern United States (Ault et al, 2015;Meehl et al, 2012;Pan et al, 2004;Schwartz et al, 2006;Waller et al, 2018), the pattern observed in this study is anticipated to persist into the future (Jeong et al, 2013;Wuebbles et al, 2017).…”

“…However, even excluding the strongly advancing thresholds in the Southwest, this pattern of a decrease in the "speed of spring" in the West across latitude and elevation in the thresholds evaluated persists. In a similar evaluation of changes in the start of the spring season, Waller et al (2018) reported no trends across latitude in the Western United States; however, the temporal extent of the study encompassed the entire 20th century, and the metric used, the Spring Leaf Index (Schwartz et al, 2006), represented the leading edge of the spring season. The varying rates of change observed across latitudes have implications for migratory birds and insects seeking resources across the gradient (Armstrong et al, 2016;Waller et al, 2018).…”

“…In a similar evaluation of changes in the start of the spring season, Waller et al (2018) reported no trends across latitude in the Western United States; however, the temporal extent of the study encompassed the entire 20th century, and the metric used, the Spring Leaf Index (Schwartz et al, 2006), represented the leading edge of the spring season. The varying rates of change observed across latitudes have implications for migratory birds and insects seeking resources across the gradient (Armstrong et al, 2016;Waller et al, 2018). These patterns suggest that species migrating north in the spring in the East may need to speed up their rates of migration to keep pace with food resources, whereas species in the West may need to slow down their migrations to keep pace with resource availability.…”

“…First, advancements have been greater for species undergoing phenological transitions earlier in the season than those active later in the season (e.g., Fitter & Fitter, 2002;Hepper, 2003;Mo et al, 2017;Park et al, 2018). Species also show varying rates of change in different locations, with a general pattern of more rapid rates of advancement occurring at higher latitudes and elevations (Asse et al, 2018;Post et al, 2018;Vitasse et al, 2018;Waller et al, 2018;Zohner & Renner, 2014). Finally, locations with higher mean temperatures tend to harbor fewer species demonstrating advancements (Cook, Wolkovich, Davies, et al, 2012).…”

Trails‐as‐transects: phenology monitoring across heterogeneous microclimates in Acadia National Park, Maine

“…, Waller et al. ). However, long‐term observational records of plant phenology are geographically limited, and new monitoring programs require years of records before researchers can track trends in phenological responses to variations in spring temperatures.…”