The native Toyon is one of Nature’s cornucopias. Its berries (“pomes”) have sustained creatures ranging from birds to humans to bears for millennia. Here a female House Finch (Haemorhous mexicanus) feasts on the Toyon in the Native Plant Area that I featured recently in “Signs of the Season” Oct 16 2022. The bird chews up the berry thoroughly in her beak, making a mess of them. She doesn’t seem interested in the flesh or the juice. She shakes the flesh and skin off. She’s only after the little seeds inside. She’ll swallow those, and some fraction of them will pass through her digestive system and come out the other end to possibly start new Toyons in the area.
You would think that with eating this bright orange/red fruit, the bird would definitely develop the red plumage that the males of her species display in maturity. But no, her grownup feathers will stay the same dull drab grey and brown. It’s almost maddening to read the standard bird texts explaining that the males’ red feathers come from eating carotenoids, never asking why females stay drab. Even the otherwise exhaustive Birds of the World from the Cornell Bird Lab barely touches on the issue.
One researcher has noted that research on feather coloration “has focused almost entirely on males.” Source. I’ve found no studies that squarely ask why female house finches generally remain drab while males develop color. The nearest approaches that I’ve found are two papers by G.E. Hill of Auburn University, who has published very widely on House Finches. In one paper, he concludes rather vaguely that the difference between males and females is due to “the influence of foraging behaviour, gut parasites or food quality.” Source. In a more recent study (Source), Hill divided captive female finch populations into two groups, feeding one of them a standard finch seed diet, and other the standard diet supplemented with a carotenoid. He found, with some variations, that females fed a carotenoid-supplemented diet did grow red feathers on their crowns, breasts, and rumps, “indicating that all females possess the capacity for such display.” He hypothesizes that the finch sexes make different food choices. Males seek out carotenoids because it makes them more attractive mates. Females seek out foods heavy in calories, fats, and other nutrients important for egg laying. But, Hill concludes, field studies that observe what the sexes actually eat are needed to test this hypothesis. Indeed. How do the birds know what foods contain carotenoids? Moreover, the right kind of carotenoids, since only one of the 13 known carotenoids appears to be key to color? Source. How do they know that eating carotenoids makes red feathers? They’re smart, but it’s a stretch.
There are literally hundreds of papers studying House Finches. I may have missed a more definitive study and would be grateful to readers who are able to delve more deeply.