Warmer winters may be pushing raptors northward

Titled "Regional Distribution Shifts Help Explain Local Changes in Wintering Raptor Abundance: Implications for Interpreting Population Trends," the paper was written by Neil Paprocki, who recently earned a master of science degree in raptor biology at Boise State, and Julie Heath and Stephen Novak, faculty in the university's Department of Biological Sciences. Paprocki took the lead in conducting the project's detailed statistical analyses.

The researchers noted a continued increase of wintering raptors in the Boise area over the past 20+ years and decided to take a closer look at how raptors are potentially responding to regional climate change, specifically how their distributions are changing in the areas where they traditionally winter over.

Using data collected during the annual National Audubon Society's Christmas Bird Count -- where citizens over the past 100 years have gone outside over the holidays to count the number of birds they see in a specific area -- they were able to determine that several species were, indeed, wintering far north of their traditional habitats. Most pronounced is the rough-legged hawk (Buteo lagopus), which since 1975 has shifted its winter quarters about 185 miles poleward.

Other species found to winter further north in the study include American kestrels (Falco sparverius), golden eagles (Aquila chrysaetos), Northern harriers (Circus cyaneus), prairie falcons (Falco mexicanus) and red-tailed hawks (Buteo jamaicensis).

"There has been a lot of interest in the scientific community about what kind of effect global warming is having on bird species," said Paprocki, noting that those charged with wildlife management use population trends of species in a region as one factor in assessing how that species is doing.

"Continued shifts in the distribution of species will force us to rethink how we interpret local population trends and determine which areas require conservation action," he said.

While the birds may be reacting to higher average temperatures worldwide, researchers are quick to point out that other factors could also be at play. The raptors might also be responding to other migration factors or even intraspecific competition for nesting sites.

"These shifts provide a lot of insight into what is going on in any one bird population or conservation region," said Novak. "But if we don't understand the shifts and what is causing them, we may end up putting a lot of resources into maintaining or restoring an area and possibly misusing limited funds. These data can help us to better understand what is going on at a more local scale so action can be taken when and where it is needed."

Researchers say these data will inform future studies and lead to new avenues of exploration, such as the consequences of distribution shifts on the survival and reproductive success of species.

"In a more fundamental way, this study adds to an emerging picture of some of the impacts associated with changing climate regimes," Novak said. "If we found this northward shifts during the winter, how do these shifts effect species over the rest of their annual cycle? We clearly need to widen our scope beyond single-season and single-species studies to better estimate the ecological and evolutionary consequences of climate change."