Nature follows the Law of Unintended Consequences. The peregrine falcon, after decades of being crushed by DDT and guns, is riding the upside of the population seesaw driven by conservation.
More peregrines increase the danger for shorebirds, and can affect prey behavior, in this case Western sandpipers in the Strait of Georgia. Those changes may have longer-lasting effects across a wider area.
Dr. Rob Butler, adjunct professor of biology at Simon Fraser University, spent two decades studying sandpipers’ behavior at migratory stopover sites.
Researchers captured, weighed and banded Western sandpipers at Sidney Island in the Georgia Strait and in the Fraser Delta.
“The data indicated that the weights of birds on Sidney Island during the northern migration were going down over time. At the same time, those birds we caught in the Fraser Delta were not losing weight,” Butler said.
The presence of a peregrine falcon creates a fear alert among shorebirds. But for several decades, generations of sandpipers were unfamiliar with peregrines. Now the game has changed.
The increase in peregrines coincided with a steep decline in sandpiper numbers on the island.
“It’s more of a fear factor than actual killing,” Butler said.
The island’s small bays concentrated the sandpipers and attracted more peregrines; the close-by forest provided cover for the falcons and less time for the flock to react. The sandpipers needed to be there to fatten up for the migrations.
Researchers concluded that the presence of an increased number of falcons caused the sandpipers to spend less time on the island, which was reflected in less body weight. The weight of sandpipers in the Fraser Valley stayed the same.
“It’s very dangerous for a fat sandpiper because it is slower and less agile,” Butler said. “Over time, individuals spend fewer and fewer days there and got leaner. The lean and nimble can afford to go to Sidney, fatten (some) and quickly move to wide-open Fraser Valley, where it’s safer because the sandpipers can more easily spot peregrine falcons.”
Over a decade, the length of stay for northbound sandpipers on Sidney declined by 68 percent.
When the researchers discovered the predator link, Butler said, “It was quite a shocker! There were lots of skeptics. Many reviewers said, ‘Wow!’ I never thought of this. I don’t believe it.’ ”
On Sidney Island, more falcons means more stress on the shorebirds, less weight gain and shorter stays.
If the peregrine-sandpiper predator-prey connection is applied to other shorebirds, it could be a game-changer.
“The actual number of Western sandpipers migrating (to Sidney Island) may have changed little,” Butler said. “The shorter lengths of stay have a tremendous implication for collection and interpretation of data on trends in shorebird population on a broad scale.”
Another predator’s numbers: Will the uptick in the state’s wolf population change any of its prey’s dynamics? Only time will tell, but for now, there’s been a 30 percent increase in wolves in 2014, bring the population to 68, according to the Department of Fish and Wildlife. Four new packs formed, another indication that wolves are on the road to recovery.
Columnist Sharon Wootton can be reached at 360-468-3964 or www.songandword.com.
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