With its retina punishing feather colors this is a spectacular solo mating dance performed by a male bowerbird advertising his male qualities. The independent dilation and contraction of the pupils is a striking part of the choreography at the beginning of the performance. In Humans male brains perceive dilated eyes as a signal for sexual readiness of females. Seeing the exaggerated form here should make us curious about our “inner reptile” since birds and mammals have evolved from independent reptilian ancestors.
Sexual display in birds is a frequent theme here in Nature Documentaries. One of the earlier posts for instance, A Day in the Life of an Elite Male Sage Grouse by Marc Dantzker documents the mating behavior in a lek forming bird species. Similarly a very informative video put together by Cornell University researchers provide biological explanation of such behavior through the short animated documentary An Illustrated Introduction to Natural & Sexual Selection. These documentaries most certainly will not be the last posts here on bird behavior. The following footage of lek-forming “moonwalking manakins” filmed in Costa Rica by Phil Savoie helps us understand group display in a lek:
In nature, males frequently evolve elaborate display behaviors to impress females and increase their ability to compete with other rival males. Displays may take the form of visual ornamentation such as color or physical behavior such as dancing and singing. Additionally males can also extend their displays by building structures or gathering objects that are not part of their body known by evolutionary biologists as “extended phenotypes”.
Mating display is the beginning of pair-bonding. In socially monogamous birds such as zebra finches, pair-bonding has been shown to play a very big influence in breeding success. In an experiment where pair-bonded zebra finches were separated and forced to breed with a random individual reproductive success decreased resulting in many infertile eggs. When zebra finches were allowed to breed with their preferred partner they achieved a 37 percent higher reproductive success compared to pairs that were forced to mate. Moreover, pairs formed by free mate choice were more successful rearing their young from hatching to independence.
In many animals females are less showy and tend to have camouflaging coloration. Alfred Russel Wallace, another influential biologist contemporary to Charles Darwin who also independently came up with the idea of natural selection argued that dull coloration could arise because females face different risks than males, being burdened with motherhood. In his 1867 essay titled “Theory of Birds’ Nest” Wallace elaborated why being less noticeable can be an adaptive trait. Sexual selection is a powerful biological mechanism that can even lead to evolution of new species.
Some argue that Bowerbirds may have a culture. Bowerbirds have 20 species living in Australia and New Guinea. In a review paper socially learned behavioral characteristics observed in the bowerbird family (Ptilonorhynchidae) have been compiled. The behaviors include songs, bower design, decorations, bower orientation and display movements. Indeed when all the behaviors including the nest building techniques and styles are analyzed the conclusion seems accurate. The saying “practice makes it perfect” makes sense when one watches a juvenile bowerbird practice its skills in choreographing his bower as a display stage:
Mating display in birds must be going way back in time and perhaps was even present in their dinosaur ancestors. Some fossilized dinosaur tracks appear to reflect a peculiar dance-like behavior. In 2016, ichnologists studying dinosaur behavior discovered odd shaped foot prints made by a group of dinosaurs called Therapod. Therapods are the ancestors of the bird lineage. These Therapod footprints provide physical evidence of substrate scraping behavior typically seen in modern ground-nesting birds around their “display arenas” or leks. Scientists discovered many large scrapes, up to 2 m in diameter, at several sites in Colorado dating to the Cretaceous which may represent the earliest mating dance behavior.
Why males evolved these clownish traits? The answer could be hidden in a reproductive trade off. Males produce many millions of sperms and can sire many offspring. For females production of eggs are costly and takes time. Females therefore are not always ready to mate. This delay causes a scarcity of females and competition among males.
Sexual display is a complex behavior that requires theory of mind, that is ability to understand or read minds of the others. The ability to analyze and predict others’ behavior is a very advanced cognitive capacity. Male Eurasian jays provide food as gifts to their mates to reinforce pair bonding. They do so in diverse way to prevent females from getting bored from the same meal. If the female ate too much moths for instance a few mealworms in between is a good idea. Males appear to have mastered a simple principle “do not feed what has just been eaten”.