The bowerbird is known for its unique courtship behavior in which the male builds elaborate structures called bowers to attract females. The bower serves as a display area where the male can showcase his colorful plumage and perform courtship dances to visiting females. The complexity and artistry of the bower reflects the male’s fitness, and females will mate with males that have the most impressive bowers. This unique social behavior raises interesting questions about the evolution of artistic displays, cognitive abilities, and sexual selection in the bowerbird.
Bower Building and Maintenance
Bower building is an intricate process that requires significant time and effort by the male bowerbird. Bowers are built from twigs and sticks, and then decorated with colorful objects like flowers, berries, feathers, stones, shells, or even man-made items scavenged from human environments. Some species build avenue-type bowers in the shape of a tunnel or avenue, while others build maypole bowers consisting of a central platform of sticks surrounded by a wall of decorations. The quality and complexity of the bower reflects the male’s skill, effort, and aesthetic taste.
Maintaining the bower is equally important. Males devote several hours a day to upkeep by re-arranging decorations, restoring disturbed display courts, and even painting the interior walls using chewed plant material, charcoal, or clay. Females likely use bower upkeep as an indicator of the male’s commitment and work ethic. Males also keep the areas around their bowers meticulously clean, removing leaves and debris that could detract from the overall presentation.
Display Behaviors
An equally intricate set of display behaviors complements the bower structure itself. Males use a range of visual signals like stereotyped movements, body postures and elaborate plumage to catch a female’s attention. For example, the male satin bowerbird faces away from the female and spreads his wings to emphasize the dramatic purple gloss on his back feathers. Other common displays include posing, bobbing, fluttering wings, and mimicking vocalizations. More active displays like dances let the male show off his athletic vigor.
One of the most remarkable courtship behaviors is the bowerbird’s ability to imitate sounds from other species. Males incorporate learned vocalizations of other birds and even human-made sounds into their courtship displays. The ability to mimic novel sounds likely demonstrates the male’s auditory-cognitive skills in learning new vocalizations to appeal to visiting females.
Sexual Selection
The bowerbird’s elaborate courtship behaviors have evolved through sexual selection, in which traits give males a reproductive advantage because they are preferred by females. Darwin first proposed the concept of sexual selection to explain exaggerated male ornaments like the peacock’s tail. Traits evolve when they help males compete with rivals or appear attractive to discerning females. Sexual selection can lead to elaborate and seemingly maladaptive traits that actually decrease male survival.
In bowerbirds, the bower and courtship displays are sexually selected. Females preferentially mate with skilled males that build intricate bowers and perform complex display behaviors. By choosing males with better bowers, females likely gain indirect benefits like superior genes for their offspring. Research shows that males with symmetrical bowers also have more symmetrical physical traits, indicating good developmental stability.
The bowerbird’s brain size has also expanded to control and integrate the male’s complex courtship routines. Sexual selection for brainpower may have driven the evolution of greater intelligence and abilities like vocal mimicry. In satin bowerbirds, males with larger cerebellums relative to body size build better bowers.
Female Mate Choice
Females are the choosy sex that evaluate male quality based on the bower and courtship performance. Females visit multiple bowers, carefully assessing the structures and interacting with displaying males before making a final mate choice. They destroy bowers that do not meet their standards, spurring males to make improvements.
One clue females use is the number of decorations in the bower, which signals male foraging skills. Females also prefer symmetrical bowers built with precision, and react more strongly to males associated with these high-quality bowers. The males’ display intensity and coordination are also judged during their courtship dances. Females likely integrate all available cues before selecting the fittest mate. It is still unclear how females weigh the relative importance of the physical bower versus male visual displays.
Interestingly, female preferences can drive microevolutionary changes in male traits. When females were experimentally provided with objects of a specific color, males incorporated those colors into their bowers within a few generations. This demonstrates how male signals can rapidly evolve to match female sensory biases.
Cognition and Artistic Displays
The bowerbird’s complex skills in constructing and decorating bowers raise intriguing questions about avian cognition, aesthetics, and artistry. The level of detail, craftsmanship, and visual design indicates advanced mental abilities.
Bower building relies on excellent visuospatial skills and attention to structural symmetry. The males also exhibit planning abilities, collecting materials up to a year in advance. Decorating with diverse objects of specific colors and arranging them in an aesthetic display requires classification learning, object memory, color vision, and artistic taste. Males preferentially acquire rare, visually striking objects which are likely more appealing to visiting females. Some have called bower decorating an ancient form of art rooted in sexual selection for creativity.
The bowerbird’s brain regions involved in spatial memory, object memory, and sensory processing are enlarged compared to related females. The males require excellent working memory to build the bower’s twig foundation. Next, they utilize reference memory to collect decorations matching a mental template. Finally, trial-and-error associative learning helps refine decorating techniques.
Mating Systems and Sexual Dimorphism
Bowerbirds have a polygynous mating system where males mate with multiple females while females mate with only one male in a breeding season. The skewed mating system intensifies sexual selection on males to compete for access to mates. It also selects for choosier females since they invest heavily in reproduction.
This breeding system has led to some of the most extreme sexual size dimorphism in birds. Males are up to 70% larger than females in satin bowerbirds, reflecting stronger selection on male body size for dominance interactions. Males also have more colorful plumage for courtship displays. The smaller female body size may be better for egg production. The size difference also reduces resource competition between the sexes.
Group Display Sites
In several bowerbird species, males build bowers clustered together at communal display sites rather than being dispersed across the habitat. For example, up to a dozen males may construct bowers in close proximity. Females visit the display site to visually compare male quality before choosing a mate.
Group display likely intensifies male-male competition for the best display sites. Males aggressively defend display sites from intruders, leading to frequent confrontations. Males also destroy competitor’s bowers or decorations. High-status males with bowers in central locations have greater mating success. Group display sites can even become sinks where the skewed mating system leaves bachelor males that fail to attract females.
For females, visiting a lek-like display site with multiple males reduces search costs. She can efficiently compare male traits and bowers to find the highest quality mate. But group display may also allow low-quality males to parasitize the courtship efforts of attractive neighbors. Females must exercise mate choice wisely.
Cooperative Display Sites
Some bowerbird populations have an even more remarkable social behavior where males cooperate to build courting arenas called maypole circles. Several males construct their bowers around a communal maypole consisting of a tower of sticks up to ten feet tall. They jointly decorate the tower with fruits, flowers and ornamental objects to lure visiting females.
This cooperative display behavior is extremely rare in birds. It appears to have evolved in habitats where suitable display sites are limited, forcing males to aggregate. Why males cooperate rather than fight over sites is still unclear. It may minimize damage to bowers or reduce predation during courtship.
Joint maypole construction also showcases males’ coordinated teamwork in building an imposing structure. Females likely favor males skilled at cooperating towards a common goal. However, the benefits of teamwork must be balanced against increased mating competition. It remains puzzling how cooperation is maintained instead of a “cheating” strategy.
Duetting Display
An unusual form of acoustic display has been observed in the spotted bowerbird. Neighboring males jointly produce a “duet” of alternating vocal mimicry to attract females. One male starts making a mimicked call, and the neighboring male replies with a different mimicked vocalization. The pair essentially take turns in an antiphonal duet.
This cooperative display behavior seems analogous to coordinated duetting in some tropical bird species. Males may alternate rather than overlap calls to avoid signal interference at display sites. The antiphonal duet allows females to better localize the displaying males. Females also likely prefer males capable of precisely coordinating their interactive duets, which is cognitively demanding.
However, the alternating duet means males share the females attracted to their display site. Why males cooperate instead of vocalizing independently remains uncertain. Duetting may help males assess rivals before engaging in combative signaling.
Evolutionary Tradeoffs
The bowerbird’s elaborate courtship strategy reveals evolutionary tradeoffs between natural and sexual selection. Bower complexity and brain size are sexually selected, but come with survival costs. Larger brains require more energy and maintenance. Devoting time to bower building also leaves less time for foraging and vigilance against predators.
During the intense display period, competition and attraction of predators may decrease male survival. Bright plumage and loud display calls can also increase predation risk. In some species, males reduce feeding to only one hour per day during courtship. The evolutionary arms race of sexual selection can push male traits to the limits of survivability.
However, enhanced reproductive success outweighs any survival costs. Sexual selection pressures are often stronger than natural selection. As Darwin noted, elaborate traits like the peacock’s train can persist even if they jeopardize survival, as long as their reproductive benefits outweigh their costs.
Female-like Males
Intriguingly, some bowerbird populations include “female-like” males that do not build bowers and have female-like plumage. These males likely have elevated estrogen levels during development. They mimic female behavior and appearance to parasitically mate with females attracted to normal male bowers. By avoiding the high cost of bower building, these cheat males can invest more in survival. “Sneaker” males represent an alternative mating strategy when attracting mates directly is too costly.
However, female-like males sire fewer offspring than normal males. This is because females can discriminate against them and prefer to mate with males associated with elaborately constructed bowers. The lower mating success of female-like males maintains the equilibrium ratio of normal males to cheats in the population.
Hybridization
In areas where two bowerbird species overlap, some males build intermediate bowers with features of both species. These males also sing an intermediate song. This provides evidence that some bowerbirds occasionally hybridize where their ranges meet. For example, the great bowerbird and the Fawn-breasted bowerbird interbreed to produce hybrids with mixed display traits.
However, hybrid males have low reproductive success. Hybrid females mate randomly with hybrid or purebred males. But purebred males mate only with purebred females, limiting gene flow between the species. Strong preferences for elaborate species-typical bowers help maintain reproductive isolation between bowerbird species and prevent complete hybridization.
Threats from Humans
Several bowerbird species are under threat due to human activities. Deforestation has damaged habitats and reduced population sizes. The destruction of old-growth forests hits bowerbirds particularly hard since they may require decades-old trees for displaying and nesting.
Invasive species like cats, foxes, and rats also prey on bowerbirds and have contributed to extirpations. Other introduced animals like pigs degrade habitats and disturb display courts. Some species are threatened by overcollection for the pet trade due to their bright plumage. Pesticides reduce insect food sources while also impairing reproduction.
Anthropogenic climate change will likely worsen these impacts through increased droughts, wildfires, and vegetation shifts in Australian ecosystems. Conservation actions like habitat restoration and control of invasive species are critical to protect endangered bowerbird species.
Conclusion
The unique courtship behaviors of bowerbirds provide a captivating example of how sexual selection can shape elaborate traits and social displays. Males compete in an aesthetic arms race to build ever more elaborate bowers and compile more attractive decorations. Females choose mates based on the quality of bowers and courtship performances. These preferences drive the evolution of remarkable maledisplay adaptations like artistry, vocal mimicry, cooperation, and intelligence.
Yet for all their beauty and intricacy, such sexually selected traits reduce male survival. This illustrates the power of sexual selection to push traits to extremes, even when they are detrimental to natural selection. The bowerbird’s complex social behaviors offer intriguing insights into how brains, beauty, and behavior interact in the never-ending evolutionary race to find a mate.
