Climate change might be reducing the population of red-winged blackbirds and also increasing the ratio of females to males produced by the birds.
At a research station in Ontario, Canada, where University of Illinois Professor Patrick Weatherhead has studied the blackbirds for the past quarter century, the bird's breeding population has dropped dramatically by 50 percent in recent decades, a decline apparently related to weather trends as far south as the Southeastern United States.
Moreover, in what may be another warning sign of the potentially broad ecological impacts of global warming, Weatherhead found that the red-winged blackbirds summering and breeding at the Canadian location are producing more females than males. The population ratio was formerly about 50-50.
He said the birds appear to have extended their breeding season with the warming trend. But the extension has occurred at the season's beginning and end, when the birds' tendency is to produce more females, not in the middle, when more males are produced.
The situation isn't endangering red-winged blackbirds, which are near the top of the list of the most abundant birds in North America. Males of the species mate with more than a single female, for one thing.
But most types of birds are monogamous, and some face greater population pressures than the blackbirds, which, despite their advantages, eventually could be affected as well if the trend continues, Weatherhead said.
He characterized the finding as a "warning bell. Here's a heads up about things that might happen in the future. Less abundant species could get into trouble a lot faster."
Weatherhead said the red-winged blackbird population decline he identified stems not from the change in female-to-male ratio, but from something that's increasing the mortality of the birds when they winter to the south or in their migrations back and forth.
He thinks the answer might be, in large part, the type of storms produced in warmer, wetter winters, which can soak the birds before temperatures drop into the sleet- and ice-forming range, conditions that make them especially prone to heat loss and deadly hypothermia.
"You can get massive bird kills," Weatherhead said.
Weatherhead is a behavioral ecology professor in the UI Natural Resources and Environmental Sciences and Animal Biology departments, as well as associate department head of the former. He specializes in studying how birds and snakes address challenges to their reproduction and survival.
He began studying the red-winged blackbirds around the Queen's University Biological Station on the northeastern side of Lake Ontario as a student and collected data on the birds from 1975 to 2000 and in 2005.
He collected the information not for climate studies, but as part of studying the bird's breeding structure and also looking at its impact as a crop pest. Red-winged blackbirds may feed on corn crops after breeding, although they generally stick to a high-protein bug diet, including corn borers, when producing young.
But the growing concern over global warming and studies showing that some birds are migrating and nesting earlier as a result of climate change gave him the idea of comparing his blackbird data with climate records to see if patterns emerged.
In particular, Weatherhead said, other studies hadn't done much examination of the potential impact on the numbers and sexual mix of a bird population.
"I had the data to do that," he said.
What he found was a definite correlation between the changes in the red-winged blackbird population and the North Atlantic Oscillation.
Kind of a lesser-known cousin of the southern El Nino effect, the North Atlantic Oscillation is nonetheless the biggest influence on winter climate variation from central North America to Europe and much of Northern Asia. It has been on a positive trend, which generally means warmer winters in the Southeastern U.S. and Europe, for the past 30 years.