Bowerbird — Bower as Extended Phenotype Display
Bowerbirds (family Ptilonorhynchidae) build and decorate elaborate structures — bowers — not for nesting but entirely for mate attraction. The bower is a visual signal extended outside the body: a constructed artifact that advertises male genetic quality, cognitive ability, and aesthetic taste to discriminating females. This is Richard Dawkins' extended phenotype principle made literal in a signaling context.
What the Signal Is
Male bowerbirds construct two bower types:
- Maypole bowers — sticks placed around a sapling, sometimes with a hut-like roof (genus *Amblyornis*)
- Avenue bowers — two parallel walls of vertically placed sticks (*Chlamydera*, *Sericulus*, *Ptilonorhynchus*)
In and around the bower, males place collected objects: shells, leaves, flowers, feathers, stones, berries, plastic fragments, coins, glass. Each species has characteristic decoration preferences (satin bowerbirds prefer blue objects; great bowerbirds prefer pale/reflective objects). Males spend hours arranging the collection. Items are frequently stolen from neighboring bowers — both as a competitive tactic and as a test of other males' ability to defend.
Forced perspective / optical illusion: Male great bowerbirds arrange objects from smallest to largest in the bower court, creating a forced perspective that holds female attention longer. Males with stronger optical illusions have higher mating success. This is documented object manipulation for a visual effect — and it works specifically on receivers inside the bower, suggesting the male has some model of the female's visual perspective from within the structure.
The Extended Phenotype Argument
In the rest of the animal kingdom, sexual selection acts on the body — feathers, colors, songs, horns. In bowerbirds, the selection pressure has been transferred to the bower itself. There is an inverse relationship within the family between plumage elaborateness and bower complexity: species with the most elaborate bowers tend to have the drabest males. The hypothesis: ornament has been evolutionarily "transferred" from plumage (which increases predation risk) to the bower (which doesn't move with the bird).
The bower serves both as honest signal (quality reflects cognitive investment and resource-holding capacity) and as a female-controlled evaluation arena — females can approach and inspect the bower while the male is visible but constrained inside it. Males adjust their display intensity in response to female signals of discomfort (robot female experiments at Berkeley); bowers may have evolved partly to give females safer evaluation conditions.
Female Mate Choice Mechanics
- Females visit multiple bowers, often returning to preferred ones several times
- Multiple females select the same top-performing males; many males breed not at all
- Females who mated with top males tend to return to them the following year and search less
- Young females are more easily threatened by intense male courtship; they prioritize bower quality over male courtship intensity
- Experienced females weight courtship performance more heavily
The female's choosiness drives an arms race in bower quality, object collection, and cognitive sophistication that substitutes for the arms race in plumage that characterizes other bird-of-paradise and pheasant lineages.
Cultural Transmission
Marked learned because individual bower-building quality improves with practice and is not fully specified by genetics. Young males take 7 years to reach sexual maturity and spend these years learning construction and decoration norms partly by visiting and stealing from older males' bowers. Species-specific decoration preferences are likely partly genetic (color preference) and partly acquired through observation.
Some species (MacGregor's bowerbird, satin bowerbird) are superb vocal mimics as part of their courtship display — imitating pigs, waterfalls, and other species — adding an acoustic dimension to the visual bower.
Open Questions
- How does a male "know" what a female can see from inside the bower? The forced perspective effect requires a model of the female's viewing angle
- The "plumage transfer" hypothesis is not well supported — species with vastly different bower types have similar plumage. Alternative: bowers evolved from an initial function of protecting females from forced copulation
- Do bower quality norms drift culturally across populations, analogous to humpback-whale-song?