Since immersing myself in the observation and science of birds, I’ve learned that they might outdo humans in levels of complexity. I must admit, however, that birds are a tad easier to understand simply because they lack the capability to think in abstracts. Unless you watch Loony Tunes’ Wiley and Roadrunner cartoons.
Regardless, one of the many aspects about birds that incites my curiosity is the vast and complex diversity of coloration. This is also a source of understandable but frustrating confusion for many people when attempting to identify birds. Most guidebooks may describe and illustrate only one or two of the color variations that a bird species might show. On the other hand, many bird species may exhibit variations of colored plumage throughout the year depending on the season, their age, and their sex. This often befuddles attempts to identify a bird people see, especially if the bird in question might be a vagrant to their local area, in between-seasonal plumage, or an intermediate between two subspecies.
Understanding the basis for individual variations in a bird species can enhance observers’ identification skills. Consequently, I will address some of the topics that directly and indirectly contribute to bird plumage variations in a series of posts. Additionally, because of my interests in evolutionary biology and genetics, side topics touching on some of the fun things (in my opinion) will also appear. I do this simply because of the ‘why’ and ‘how’ questions that often arise in conversation with other people, which indicate to me that even non-scientists are curious.
Birds of the same feather don’t always flock together
The most direct difference between males and females (sexual dimorphism) is sexual organs. However, examples of secondary differences, those most obvious to our eyes, include body size, behavior, ornamentation and coloration.
For many animals, it’s all about appearances. One of the basic concepts that all of us are aware of, whether we are cognizant of it or not, is what we see. Humans take appearances for granted. Why? Simply because appearances communicate information that we might not be cognizant of when we receive and process it in our brains. That applies to nearly all animals that have and rely on sight.
But humans also communicate in a myriad of other ways: sound, touch, and smell. Although most other animals also possess these senses, humans rely more on one: language.
Sure, birds and other animals might have their own vocal communications that could be described as rudimentary language. Bird songs and other vocalizations communicate basic intent and even distress, etc. Conversely, human kind has a developed a complex network of language rich with abstract meaning, all which we can hear and see (read). This coincides with our species’ capability of abstract thinking and ability to reason. I doubt that a duck could read and recite the Old English Beowulf and understand it.
Not to be outdone, many animals have evolved complex ways to communicate with their own kind. One example is how they look: different colors or different sizes. Although a few explanations can be posited for differences between the sexes, the most prevalent is mate selection. And the most common expression in birds is differences in coloration between the sexes, called sexual dichromatism. With most birds, but not all, the male is usually the most colorful because they compete for attraction to the females. Thus, sexual selection drives the evolution of chromatic variability.
But how does this work in different species? This is where it gets complicated. Birds can’t write love poems or dress up in fancy clothes, but they can look very appealing with their feathers!
Birds can be sexually monochromatic (same color), where the males and females within a species are the same color year round.
Familiar examples are most eagles, Canada geese, and sandhill cranes. Most of our bird species in North America are sexually dichromatic; the sexes are divided into two basic appearances. In the latter case, the male is usually more colorful. In some species, especially the shorebirds, the female may be the contrasting sex, called reverse sexual dichromatism (e.g. Wilson’s phalarope).
Some species are extreme in colored plumage. The male may have several very bright and contrasting colors, whereas the female may be drab. These two differences are what most guidebooks depict. Sometimes all the females in an entire genus may look very similar making it difficult to identify which species they belong to.
But not all mono- and dichromatic species are complete or dramatically colored. Sometimes differences in coloration may be very subtle to our eyes. The male and female red-shafted Northern Flicker look nearly alike except for the absence of the red malar on the female’s face. Similarly, with a monochromatic species, such as the Western bluebirds, both male and female may have the same colors but different saturation. The female’s colors are less saturated than the male’s.
Size does matter sometimes.
Size differences between males and females are more present in monochromatic bird species. In larger birds, such as many eagles, hawks and owls, the female is often larger than the male. Size differences can also be subtle, and unless the two sexes are near each other, one may not easily identify an individual as male or female. In other species, such as the sandhill crane and American pelican, there is hardly any difference in size or color and sexing the birds is difficult. But those birds have evolved other ways to communicate between the sexes!
Secondary sexual traits of birds – coloration and size – are varied and most have evolved to communicate attraction and selection. Some traits, however, are driven more by reproduction demands (physiology) than sexual mate selection. Regardless, birds are much more complex than us simply because of need. Perhaps if we never developed a language to communicate, we may have evolved with more extreme sexual differences. Instead, we create our own 😉
Of course, all this is relatively basic and does not tell the entire story about bird coloration. Anyone familiar with ducks knows that most males have more than one coloration plumage during the year. Observers seeing a flock of red-winged blackbirds in the winter might be confused at all the different color combinations exhibited by the birds, thinking more than one species were mixed in together. And then there is the issue of interbreeding subspecies (and even species hybrids!) resulting in a mix of colors imparted by both parents. It can all get very complicated!
My next post will be about the changing of the guard: molting.
Meanwhile, do you have any additional examples of bird coloration?