In general, we understand red birds thusly: the brighter the red, the better the bird. (Why? See this post.) The better the bird, the better his genes/parental care/mate, and the better his reproductive success.
But sometimes it’s more complex—and interesting—than that.
Male House Finches come in several colors, from the most familiar red shading through orange to bright golden yellow. They use carotenoids acquired from their diet to make these bright colors, and the colors are a sexual trait used to attract female mates.
The conventional interpretation of this system might go thus: the males try to be as red as they can, and so the best males achieve redness, while intermediate males are orange and poor-condition males are yellow. If this was true, you would expect 1) females to prefer to mate with red males, and 2) red males to have the most surviving chicks. After all, that’s the point—to produce as many healthy kids as possible.
Both of those expectations are true—at first. For birds in their first year of breeding, redder males are in the best condition, females prefer them over the other colors, and the redder males have the most surviving chicks (Badyaev & Hill 2002).
But then things get complicated. First, those yellow males in their first year aren’t just sitting around being bad parents. They breed later than the red males, which is bad because earlier nests do better, but they feed their chicks much more than the red dads do. The red dads’ chicks only survive because the females are willing to work extra hard to make up for their beautiful, lazy red mates. And they are lazy—they’re no less able to help out with feeding the chicks than the yellow guys are, but they don’t. Because they don’t have to do such stressful work, they survive to the next year better than the yellow males (Duckworth et al. 2003).It’s not all good for the red males, though, because things change when you consider birds who are experienced breeders—that is, who have already bred at least once. Experienced females don’t prefer the red males; they prefer males who feed the chicks more (Badyaev & Duckworth 2003). This makes sense when you consider that feeding the chicks is hard work, and the females don’t want to have to do everything themselves. It also has the strange effect that the selection pressure on males to be red can depend on what proportion of females in the population are first-years: the more females are inexperienced youngsters, the more advantage red males will have over other colors, since it’s only the young females who prefer red males.
For older males, color is not determined as much by their condition: an awesome older male might be yellow, instead of red. Or a not-that-awesome older male might be red. It looks like developing red plumage may become easier with age, so that while only great-condition birds can be red in their first year, older birds don’t have to work so hard to get that crimson hue (Badyaev & Duckworth 2003).
Males don’t necessarily stick with the same color throughout their lifetimes. There is a great sequence of photos in Badyaev & Duckworth 2003 showing males over three years. Some start out yellow and turn red, some start red and turn yellow, some stay red all their lives. Males who don’t manage to attract a mate one year tend to be redder the next year, hoping to seduce a young female. Males who breed successfully one year seem to be able to be any color they want in the next year.
In older birds, it isn’t the early-breeding, lazy red males who are the most successful at raising chicks. It’s orange males, who compromise between the colors: fairly early breeding like a red male, a substantial contribution to feeding the babies like a yellow male. The optimal combination of those lets orange males raise the most chicks (Badyaev & Hill 2002).
This is a great example of something we increasingly find in the natural world: that patterns are often state- or context-dependent. Red indicates better condition, better reproductive success, and an easier time finding a mate only if the context is that you’re a first-time male breeder. Orange is the best color only if the context is that you’re an experienced breeder. If you don’t realize that context matters, then everything just looks like a patternless muddle.
When you do appreciate the importance of context, everything starts to look really complex and interesting.
Badyaev AV, Duckworth RA. 2003. Context-dependent sexual advertisement: plasticity in development of sexual ornamentation throughout the lifetime of a passerine bird. Journal of Evolutionary Biology 16:1065-1076.
Badyaev AV, Hill GE. 2002. Paternal care as a conditional strategy: distinct reproductive tactics associated with elaboration of plumage ornamentation in the house finch. Behavioral Ecology 13(5):591-597.
Duckworth RA, Badyaev AV, Parlow AF. 2003. Elaborately ornamented males avoid costly parental care in the house finch (Carpodacus mexicanus): a proximate perspective. Behavioral Ecology and Sociobiology 55(2):176-183.