Home & Garden Columns

I tend to take house finches for granted, as I suspect most birders do. But there’s more to these ubiquitous little birds than meets the eye. 

Biologists have been teasing out the details of their social lives, learning how females—the choosy sex, as is often the case in birds—pick their mates. And a recent study goes farther to examine 

the consequences of mate choice: how females stuck with a substandard male endow their eggs with compensatory resources. 

Mate choice, of course, was a major theme of Darwin’s Descent of Man, and Selection in Relation to Sex (which, as David Quammen points out in his new Darwin biography, is really two books smooshed together). Female birds of many species show definite preferences for males with brighter colors, longer tails, more elaborate plumage. 

All these traits may be indicators of various kinds of fitness, like resistance to parasites. When females with a genetically-based predilection for gaudy males mate with those males, they’ll produce male offspring with their father’s feathers and female offspring with their mother’s tastes. Carry this runaway sexual selection out long enough and you get the baroque extravagances of the pheasants or the birds of paradise. 

It’s simpler for female house finches. According to Geoffrey E. Hill of Queen’s University in Kingston, Ontario, who has studied these birds for years, what they look for is redness. That’s a variable trait in males, and it seems to be determined by diet. In the wild, male house finches range from yellowish through orange to red. 

The colors come from carotenoid pigments, the same substances that make carrots orange and flamingos pink. Three different chemicals are involved: beta-carotene produces yellow feathers, isocryptoxanthin produces orange, echinerone produces red. Biologists have established the carotenoid connection by manipulating the diets of captives. 

Male house finches in Hawaii, probable descendants of northern California birds, are on the yellow end of the spectrum. Some ingredient that mainland birds have access to is missing in the islanders’ diets. Hawaiian house finches have been dubbed “papaya birds” because of their fondness for the fruit, but papayas apparently don’t have the right carotenoids. 

In any case, female finches look for degree of redness and color saturation in potential suitors. Hill says females will actively chase off males that don’t meet their criteria. And whatever color says about the male’s genetic dowry, there’s a direct payoff: brighter males bring more food to their nestlings. 

But what if there aren’t enough bright red males to go around? The females apparently have another card to play. Female birds—and I’m not at all clear on the mechanism here—can vary the level of hormones and vitamins in their eggs. In species that had previously been studied, like the zebra finch, the eggs of females mated to more colorful males get an extra dose of testosterone, which promotes growth.  

When Kristen Navara, a reproductive physiologist at Ohio State University looked at house finches, she found the opposite pattern: females paired with the less attractive males laid eggs with more testosterone and antioxidants (vitamins A and E) than those of females with brighter red mates. Antioxidant levels in the first group were 2.5 times higher than in the second. These substances counter the tissue-damaging effects of free radicals. 

So a female saddled with a loser—a drab male who won’t be as attentive a provider as a brighter one—can slip her offspring a little biochemical insurance. Navara relates this strategy to the house finch’s life span, which is short even by small-bird standards: a year or two at most. That limits a female to only a couple of breeding attempts in her lifetime—all the more reason to give the kids extra resources. 

Not that any of this involves conscious calculation on the female finch’s part, of course. It’s all done with hormones (exactly how remains to be determined). I don’t know whether anyone has figured out how it works on the male’s side: why brighter males should be better providers. Do the carotenoids affect their energy level or general vigor? Let’s hope some Ph.D. candidate is already working on that one.