Cardinals: why they’re red and their ability to act as PFAS sensors

Birds are enviable. Although they can’t integrate the Lambert function, they’re far more musical than rappers and far more in tune than most of us. They build nests without mortgages and fly despite COVID. In fact they will always fly without having to sit, without hoping for a window seat. We can only dream of the true bird’s eye view; it’s at modest heights, unlike that of Icarus, and it shifts with a wavy motion as every bird surfs on an invisible medium.

On Monday, I was on my skis and with a camera strapped over my shoulder. A passerby complained about how the squirrels who were stealing from the sunflower seed-filled feeder were scaring away the cardinals. After a lap around the woods, the squirrels were gone, and I decided to hide behind some reeds and wait for the cardinals to appear. It didn’t require much patience because a woodpecker and chickadee appeared in the meantime.

Here’s the first arrival.

Ever wonder why cardinals are red?

The source material for the red pigments are carotenoids from their diet. Carotenoids are yellow, orange, and red pigments synthesized by plants from basic isoprene units ( 5 carbons). The most common carotenoids in North American human diets are α-carotene, β-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene. Of these 6, are four non-red pigments that are also part of the cardinal diet. They are the following.

Some mutant cardinals, which are yellow in color, can only oxidize the oxygen-hydrogen group to a a double-bonded oxygen. Since the keto group’s π bonds are not conjugated with the rest of the π bonds, the gap in energy levels for electrons isn’t reduced enough to change the color of the molecules to red. Here are the compounds the mutants form.

Red cardinals, unlike their yellow counterparts, do not lack a ketolase enzyme known as CYP2J19. What this enzyme does is attach a keto group in the right place to conjugate the rest of the π bonds, forming the following reddish pigments:

Cardinals can be used to monitor Per- and polyfluoroalkyl substances (PFAS) in a certain region because they don’t migrate, and the birds’ large size means you can take blood samples without harming them. PFAS are a group of man-made compounds that have been manufactured since the 1940s to act as water repellents and to serve a variety of functions in industry. The class of compounds, due to their strong C-F bonds, are very persistent in the environment and can accumulate in the human body over long periods from consistent exposure to very small concentrations. There is evidence that exposure to PFAS can lead to adverse human health effects. Concentrations of the following types of PFAS: PFOS, perfluoroheptanoic acid (PFHpA), and perfluorononanoic acid (PFNA) in rivers correlate strongly with human population density. So the research group of Marie C. Russell hypothesized that they would find a higher concentration in the blood serum of cardinals in Atlanta than in those of the Big Island in Hawaii. Surely, enough they measured significantly higher median concentrations of four PFASs and significantly higher detection frequencies of seven PFASs in the cardinals from Atlanta.

One third of all birds in Atlanta are carriers of the West Nile virus. One of the reasons that the transmission rate to humans is very low could be the presence of cardinals. Cardinals, even though infected with the virus, have immune systems that greatly reduce the amount of virus circulating in their blood. It’s why cardinals are dubbed as “supersuppressors”. But given that in higher concentrations, PFAS can compromise immune systems, it’s one more reason that concentrations of PFAS should not be allowed to keep bioaccumulating. Otherwise cardinals may no longer protect us.


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