Why Are Some Birds So Colorful?

UCSB Anthropology professor Steven Gaulin delivered a fascinating talk to our local Audubon Society group at the Museum of Natural History. Asking us to consider why some birds, but not all, are so colorful.

The broader question is about the evolution of flamboyance, which goes beyond just color. He started with the King of Saxony Bird of Paradise. Gaulin explained that this bird’s head feathers are the most modified of all feathers in all 11,000 bird species. The feathers also have muscle control.

He also showed these birds that seem to have elaborate tails. Gaulin noted that these “tail feathers” are actually extended flank feathers.

We observe such flamboyance mostly in male birds, with females mostly being drab. Evolution usually reduces waste, which raises the question of how such “unnecessary” frills evolve.

For example, in a place with few predators, birds can lose the ability to fly. Cave dwelling animals can lose their ability to see. If it is not needed, why waste resources on it?

Evolution is driven by survival, fertility and mating. Darwin offered his theory of Sexual Selection in 1871. He noted that mating success can outweigh survival and fertility.

Flamboyance is not wasteful if it increases mating success.

But why not both sexes? Some species of birds don’t have sex differences. He gave the examples of the oak titmouse and Bewick’s wren. Neither the male nor female are ornamented.

But if there is a sex difference, it is almost always the male that is more ornamented. Though there are some species in which the females are more ornamented.

He asked his UCSB students and they answered that the males are “hornier” than the females. But this does not explain why evolution would cause this to be true.

Gaulin offered three answers that have been suggested:

Darwin in 1871 claimed it was due to sex ratios

Robert Trivers in 1972 claimed it was due to parental investment

Tim Clutton-Brock in 1990 claimed it was due to reproduction rates

But Darwin was mistaken. There was no surplus of males in ornamented male bird species. Males and females are equally common.

Trivers explanation seemed to make better sense. There is often more investment in reproduction for one sex than for the other. In hummingbirds females not only lay the eggs and sit on them. They also build the next and feed the young.

Trivers claimed that the less parental effort, the more mating effort is needed. It seems to fit the data well. But there are some exceptions. Trivers was on Gaulin’s PhD committee. Gaulin noted the exception of the three-spined stickleback fish. This fish can rear the young of multiple females in its nest. By making an elaborate nest, the male can entice multiple females to lay eggs in his nest. He can fertilize and tend them all.

Clutton-Brock and Vincent offered a more nuanced theory: Fast vs slow sexual reproduction. One sex can produce more offspring per mating season. That is the faster sex. Doing all the work slows you down. The faster sex is the one that will be more flamboyant. This has the best fit to the data.

The ostrich and rhea are birds that behave like the stickleback. The male is able to care for eggs of multiple females at once.

Gaulin raised a bigger question: Why be choosy? Why mate at all? If you can reproduce without mating, you pass all of your genes along. Isn’t that the best option? The question is what does it mean to be better or best?

There are two categories of challenges. One set is abiotic: Climate, altitude, terrain. These challenges don’t change as the organism adapts.

But the other set is biotic and these are hostile challenges. Consider the dynamic of prey and predator. This forms an evolutionary arms race. As predator or prey evolves to adapt to the challenge, the other side changes, too.

Evolution by natural selection happens by generations. Organisms with a short generation have an advantage in this arms race. Pathogens have that advantage.

Gaulin made the analogy of the Red Queen in “Through the Looking Glass”. The Queen explained to Alice: “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

Sex allows parents to choose genes that work best right now. A clever explanation. Is it correct?

Gaulin said there are five predictions if this idea is correct.

1. Mate choice provides reproductive benefits. Elizabeth Agey reviewed the research. She claimed choice gives an 8.5x advantage. In the case of mallards, more eggs hatch and more survive. In the case of the crested ibis, more eggs are fertilized and more survive.

2. Sex will be deleted by selection where pathogens are less of a threat. At high latitudes, high altitudes and deserts. No birds have done this, but some reptiles have. He gave the example of the New Mexico whiptail lizard.

3. An experiment was conducted with a nematode host and a bacterial parasite. If the same parasite is presented to the nematode, the nematode will use asexual reproduction; it can adapt once and keep using the same strategy. But, if the parasite is allowed to co-evolve with the nematode, then the nematode switches to sexual reproduction.

4. Bright colors are a signal of low infection. Infection reduces flamboyance.

5. Flamboyance is more likely in higher pathogen environments. This occurs closest to the equator. And less so towards the poles and in deserts. Birds that live in the US that are colorful actually migrate to the tropics.

He gave the example of the Inaccessible Island rail. Living in a remote location with no predators, it has lost the ability to fly.

He then took questions and this was an energetic crowd for questions.

One woman asked how food fits into evolution. Gaulin noted that food is turned into offspring. Cheetahs turn gazelles into baby cheetahs. When males feed females they are signaling to the females that they will feed their offspring.

Another woman asked how it is in the tropics where food is plentiful. Gaulin said that in some tropical birds the males and females both care for the young and both are colorful.

One man noted that flamboyance will also attract predators. Gaulin agreed. All factors work at once in evolution.

Another man asked about the adaptability of the immune system. Gaulin said that choosing a brightly colored male means choosing a good immune system.

Another man asked about auditory vs visual flamboyance. Gaulin said the principle is the same: It all takes extra energy. This is a signal of being healthy and not infected. In humans we have dancing!

– Robert Bernstein