Date: December 21, 2016
Source: Central Ornithology Publication Office
A new study in The Condor: Ornithological Applications documents the steep decline of a population of endangered Southwestern Willow Flycatchers over 16 years -- and the change in the sex ratio that has left the birds' future hanging on a dwindling number of males.
Changes in sex ratios can cause problems in small, declining populations, reducing individuals' ability to find mates and reproduce. From 2000 to 2015, Barbara Kus of the U.S. Geological Survey and her colleagues monitored federally endangered Southwestern Willow Flycatchers on Marine Corps Base Camp Pendleton in southern California, collecting data as the population declined from 40 individuals to only five. They found that the number of adult males was stable until 2004, but then began to decrease sharply until females outnumbered males at least two to one from 2012 on.
As the number of males plummeted, more and more of them became polygynous, mating with multiple females. Kus speculates that this may have prevented even faster declines. "It was particularly amazing to watch two or three males manage 10 or so females between them," says Kus. "They seemed to be able to increase their individual efforts such that every female was mated."
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