The horned larks of inland Southern California’s Imperial Valley don’t look like their forebears. The birds found in the region today have distinctly darker plumage on the tops of their heads, their backs, and the napes of their necks. But we’re not talking about a shift that occurred over millions of years: New research shows that horned larks in the area were lighter just eighty years ago. The blisteringly rapid color change may be an evolutionary response to the dramatic conversion of Imperial Valley desert into farmland.
In the early 1900s, irrigation first starting flowing into the Imperial Valley’s desert. Within a few decades, the region had been largely converted to agricultural land—which meant darker surfaces consisting of irrigated soil.
In recent years, ornithologists began to anecdotally note that horned larks in the Imperial Valley were also looking darker, at least compared with those in museum collections. Interested in the opportunity to investigate the evolutionary and ecological impact of human activities on wildlife, Nicholas Mason, a biologist in the Department of Ecology and Evolutionary Biology at Cornell University, decided to take a closer look.
With the help of his colleague Philip Unitt, the curator of birds and mammals at the San Diego National History Museum, Mason used museum collections of horned lark specimens that were collected from the Valley to examine the feather coloration of fifty-three different birds. The researchers sorted the birds into “historical” and “contemporary” groupings; those collected before 1940, and those collected after 1980. They measured the relative pigmentation of the feathers on the back, crown, and nape of the birds using a spectrophotometer, a device that measures the amount of reflected light of a given wavelength.
On average, they found that the larks collected from the Reagan administration and onward were indeed substantially darker on their primary upper surfaces than relatives dating back to the FDR years and earlier, according to the study published recently in the Journal of Avian Biology.
While the timing of the feather darkening and agricultural boom may seem like a straightforward match, there is more work to be done to hash out how this seemingly evolutionary event happened, particularly when it comes to the genetic basis for darker feathers. One possible explanation is that slightly darker larks native to the area could have more easily blended in with the newer, darker surroundings, and their success in avoiding predatory birds like hawks and falcons led to an increase in dark feather genes in the population (i.e. the famous peppered moth scenario). However, the story may be more complicated than that, given the larks’ lifespan.
“It’s pretty uncommon to see natural selection occur on 80 year timescales in vertebrate organisms that have life cycles of approximately one year,” Mason told Earther. Rapid evolution like this is easier to spot in organisms with very short lifespans—like insects or bacteria—because so many more generations can pass through the filter of natural selection in a given number of years.
What may be more likely is that a different, darker subspecies of horned lark recently found its way into the Imperial Valley, mated, and spread its genes into the population; a process called “introgression.” It may be that after farming boomed, the new variation in the local gene pool was suddenly a big hit, and natural selection did the rest. It’s a puzzle that Mason plans to investigate soon, by comparing the DNA of the larks from both historical and modern times with DNA from other geographically neighboring subspecies.
Either way, it’s likely this change wouldn’t have happened were it not for radical modification of the desert by humans. The findings add horned larks to a growing list of organisms that have seen rapid physical changes within one or two human lifetimes, from sea snakes adapting to pollution, to birds temporarily wallowing in industrial soot.
The study also drives home the importance of natural history collections for understanding changes in Earth’s biodiversity, since collections provide data points over large stretches in time.
“It’s a nice illustration of how natural history collections are invaluable resources for studying how species respond to human-caused disturbances,” explains Mason.