Farming May Have Caused These Birds to Become Darker

A male horned lark. Image: Marshal Hedin
A male horned lark. Image: Marshal Hedin

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.

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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.

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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.

Historical and contemporary specimens of horned lark. Image: Nicholas Mason / Fig. 1 in the study in the Journal of Avian Biology
Historical and contemporary specimens of horned lark. Image: Nicholas Mason / Fig. 1 in the study 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.

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“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.

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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.

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“It’s a nice illustration of how natural history collections are invaluable resources for studying how species respond to human-caused disturbances,” explains Mason.

Jake Buehler is a Seattle area science writer with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.

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Jake Buehler is a science writer living on Washington’s Olympic Peninsula with an adoration for the Tree of Life’s weird, wild, and unsung.

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DISCUSSION

dnapl
Dense non aqueous phase liquid

I’ll hazard a guess and say it’s twofold: complete decimation of a natural habitat and total industrialization of farming practices. So whatever bird species remains after the change in habitat, it’s food sources will have completely change. Or...you are what you eat. Larks eat bugs. Central valley uses a lot of pesticides to get rid of bugs. Pesticides don’t focus on a species as well as hoped.

If you want to completely nerd out, here’s a mechanistic and highly controlled study on how and why bird feathers for specific species seem to be changing over time. Mechanistic and controlled kind of means the anecdotes and observations are reduced or outright eliminated.

Melanin-Based Color of Plumage: Role of Condition and of Feathers’ Microstructure

And this conclusion by the study authors is why environmental engineers who count on a golden gut and intuition like to stuff environmental (natural) scientists in lockers and take their lunch money:

Together, these results suggest that melanin-based coloration may in part be condition-dependent, but that this may be driven by changes in keratin and feather development, rather than melanogenesis itself. Researchers should be cautious when assigning variation in melanin-based color to melanin alone and microstructure of the feather should be taken into account.

Anyway....

Put it this way, we’ve gotten better at applying pesticides and that’s a good thing. But we apply pesticides over a larger area upon crops becoming more and more popular. Many of the cash crops coming out of the central valley like almonds and pistachios were almost insignificant compared to fruits and vegetables and grains like rice, alfalfa and corn prior to 1990.

California does a great job tracking pesticide use for its massive central valley industrial agriculture operations at the California Department of Pesticide Regulation

The summary reports show nicely which crops have become more and more popular (e.g. almonds and pistachios) and total pesticides (insecticides, etc) application and acreage applied since the early 1990s.