After the last Dead Bird Quiz featuring blue wing patches, it seems an apt time to address the structure and function of such color in birds. Blue is, of course, not the only color a bird may display; even outside the outrageous coloration seen in many tropical species like parrots, we here in North America are treated to many birds in these color classes:
In such cases, the colors in the feathers are built from dietary pigments, particularly carotenoids. Because the brightness of the color in the feather depends on the quality of the diet, such reds and oranges are termed “honest signals” of the quality of the individual–birds that are better foragers sport brighter colors. A female selecting a mate can use the brightness of a male’s plumage as an indicator of what sort of provider he will be, both in terms of genetic contribution, and, in species where males contribute parental care, how well he will provision the chicks.
Blue pigments, on the other hand, are not derived from diet, but are instead what are known as structural pigments. Proteins in the feather, when aligned properly, will reflect blue light back at the observer. These proteins typically “self assemble” and a more genetically fit bird would not be bluer than a lesser individual. Blue feathers either are, or they aren’t, as the conventional wisdom goes, and there is no spectrum of brighter or duller blue jays, or ducks, or what have you. There has been work to challenge this, focusing on what can affect the brightness of structural color. Feather mites, for instance, could damage or dishevel the feather, leading to a loss of brightness. A more fit individual might have fewer such parasites, and so the blue in these birds might appear brighter than a heavily infested bird. So a duller bird might be less fit, but is there any such thing as a “super blue?” A blue that is brighter than the average because that bird is fitter?
I encountered a study done on turquoise-browed motmots in Mexico on this subject. In this species, both males and females sport a blue tail with a racket shaped projection at the end, though the tail feathers are longer in males. The researchers analyzed the brightness of the tail feathers across the sexes and within the sexes, between individuals. They were interested in determining whether the brightness of the blue could, in fact, be linked to fitness in either or both sexes. Not only did the study address the variation in brightness across individuals, they also measured how quickly the feathers grew. Growth rate is considered an indicator of fitness since a bird’s energy reserves, and thus diet, determine how much new feather a bird can lay down in a day.
The research team indeed found that the brightness of the blue in the tail feathers was greater in birds whose tails grew faster, but only in males. In females, no link between the two was identified. It seems that, in males, blue may indeed be an honest signal of fitness. In females, the tail is shorter overall, and there is no correlation between brightness and growth. This is consistent with the hypothesis that the tail in female motmots is not used to signal fitness, but may serve a different purpose, like distracting predators while the bird makes a swift escape, or may simply be a genetic holdover between the sexes.
This subject of honest signaling also came up while I was preparing notes for a new course I am teaching this semester. My students will be looking at the dominance hierarchy in house sparrows. In these birds, the size and darkness of the bib in males is an indicator of his position in the hierarchy. Both males and females will defer to larger-bibbed males, and the dominance extends not only to access to mates, but to nest sites and food. Researchers have understandably been interested in what underlies these bibs. The black color of the bib is caused by the pigment melanin, which is expensive to manufacture. Because of this, black coloration is considered an honest signal, much as reds and oranges are, since only a well fed, highly fit individual, would have the spare energy to lay down that pigment. Since the bibs are tied to dominance behavior in males, testosterone seemed a likely target for analysis in this case. Interestingly, I found one study that showed no link between high testosterone and large, dark bibs. That study also noted that the bib is produced in fall when the birds molt, but is not used in terms of mate selection until the following spring. Testosterone levels can fluctuate substantially over that time, so birds that had high circulating testosterone in fall when they grew the bib may well have lower levels than their rivals come spring. So if females use those badges to select mates, and if rivals are choosing not to challenge another male based on badge size, they are not using a reliable indicator of testosterone at the time of breeding either.
A more recent study following up on this work looked at testosterone again, but measured it at night rather than during the day. The results indicated that testosterone was, in fact, linked to badge size. Over the course of 24 hours, testosterone levels peak at night while the birds are asleep, and fall off rapidly as soon as the birds wake up. When measured at night, peak testosterone levels did correlate with individuals with the largest bib size. Perhaps, suggest the researchers, badge size reflects peak production of testosterone, rather than average level. This still leaves the question of seasonal changes–does this nighttime testosterone production hold true for the time of year when the badges are being grown, or only during the season when they are being used for breeding signaling? The plot thickens. Or, since we’re talking about dark splotches on birds, shall we say, the blot thickens? Sorry.