f you’ve ever planned a trip, especially one with multiple stop-overs, you’ve considered departure times, arrival times, lodging, meals, and weather conditions. Migratory birds face similar decisions about when to depart for their summer breeding grounds: they must time their arrival to match the availability of food sources and suitable nesting habitat. The system of U.S. National Wildlife Refuges supports traveling birds by providing stopover sites and food along their migratory path. But, could changing habitat conditions affect the northward migration in spring?
Food resources and nesting conditions are closely linked to the start of spring activity in plants, which the USGS-led USA National Phenology Network predicts with indices of Spring Leaf and Bloom. The indices estimate when the emergence of new leaves, known as leaf out, and bloom of early season plants occur each year, with data going back to the early 1900s.
In a newly released study, a team of USGS, University of Arizona and USA National Phenology Network (USA-NPN) scientists puts the annual onset of spring over the last century in the context of National Wildlife Refuges and the four major migratory flyways of North America. They found that in recent decades, spring leaf out and bloom arrived extremely early at 49 percent of refuges. Also, the shift to earlier spring is not uniform across migratory flyways. In the Central, Mississippi, and Atlantic flyways, the rate of spring advance was greater at northern latitudes than southern latitudes.
Calculating the rate of changing spring
To calculate the changing timing of spring leaf out and bloom in wildlife refuges, the researchers compared the onset of spring in recent decades to the onset of spring over the last century. Shifts in spring leaf out were as high as three days earlier per decade.
This study confirms recent findings of similar changes in spring onset in the national parks. Combined, this shows that many of our national jewels – parks and wildlife refuges alike – are experiencing seasonal changes over time.
Extending beyond the scope of wildlife refuges, this study shows how local changes are embedded in continental-scale change. Along the migratory flyways, the researchers calculated rates of change in spring onset from south to north across the continent. Spring leaf out and bloom not only advanced between a third of a day and more than half a day per decade on average, within flyways, but also demonstrated different rates of change across latitude. Spring advance was significantly earlier in the north compared to the south in all flyways except for the Pacific.
What does this mean for migratory birds?
“Migrating birds depend on the right conditions in habitats that are spread out over really large distances” said co-author Jake Weltzin, an ecologist with USGS and the Director of USA-NPN. “This means that if changes in spring arrival are not uniform along migration routes and between wintering and breeding habitats, birds have trouble keeping up with the shifts.”
This is especially troublesome for birds flying long distances, such as Neotropical migrants that winter in the Caribbean, Central and South America and travel to North America to breed in the summer. “We found that for the two species that we examined as case studies, the whooping crane and Blue-winged Warbler, for the Blue-winged Warbler, there was little shift in spring conditions in their winter range but a large shift to earlier spring conditions in their breeding range,” said Eric Waller, a former USGS scientist and the lead author of the study.
A species such as the Blue-winged Warbler, which travels first through the southeastern U.S., might respond to local environmental cues and slow its travel north, only to arrive at the breeding ground after the peak in its food source has passed.
Missing out on critical food sources has implications for the birds’ ability to nest and successfully raise young, and may lead to long-term population declines in these species.
Getting there depends on your flight path
To demonstrate how these data might be applied to specific bird ranges, the researchers applied these analyses within the breeding and non-breeding ranges of two migratory bird species with different migratory patterns, ecological requirements, and population status – the Blue-winged Warbler (Vermivora cyanoptera) and the Whooping Crane (Grus americana). For both species, while spring onset advanced in their breeding ranges, the shift was not significant in their wintering ranges.
The findings from this study can be used to predict the potential for mismatches between other migratory bird species and their habitats to help determine which species might need help adapting to change along their migratory pathway. Knowing how spring onset has changed can help refuge managers make effective decisions on conservation, resource management, and visitor services.
“The study findings, and resultant dynamic tools, have direct management application by helping staff better align the timing of management actions to changing environmental conditions, potentially increasing the efficacy of these actions,” said Jana Newman, National Inventory and Monitoring Manager, National Wildlife Refuge System, U.S. Fish and Wildlife Service.