Evolution of Bowers

The "Birth" of the Basic Bower

Bowers very possibly could have first evolved for the purpose of protecting females from forced copulations from the bower owner. According to this theory, the ancestors of the bowerbirds probably displayed on a decorated ground court. Females would prefer courts that happened to have natural barriers, as this would provide them with an opportunity to escape should they decide not to copulate. Males who enhanced these barriers would receive more female visitors, and their reproductive fitness would in turn be increased. These simple structures were likely similar to the maypole bower of the MacGregor’s bowerbird (Borgia 1996).

The Rise of the Avenue Bower

The tendency to use sticks to enhance bowers eventually made it possible to build protective bowers without saplings. This provided males with a greater opportunity to choose a bower site and to decorate the bower freely. The second wall in avenue bowerbirds likely evolved to orient females to well-illuminated parts of the court where decorations were arranged (Borgia 1998).

Bowers over Evolutionary Time

Bower-building likely evolved only once. Phylogenetic modeling suggests that there was an initial separation of the lineage that led to the monogamous catbirds and the lineage that led to the other polygynous bower-builders. The lineage leading to the bower-builders eventually split into two: that leading to the maypole bower-builders and that leading to the avenue bower-builders (Borgia 1996). Bowers were probably co-opted for females to assess males based on the quality of the bower after the initial bower-building behavior evolved (Borgia 1998). Changes in male display and bower characteristics over time have likely been driven by divergent female choice. Some scientists believe that certain aspects of bowers may not have evolved for any functional purpose, but rather are due to runaway sexual selection. Female bowerbirds show a strong preference for symmetry, and this may have played a role in the emergence and divergence of unique bower shapes.

 

Phylogeny of selected bowerbird species

Phylogenetic tree of selected bowerbird species and comparison species

Losing the Bower

Based on phylogenetic models, it is theorized that tooth-billed catbirds and Archbold’s bowerbirds lost bower-building behavior independently of each other, with each species having their own unique bower-building alternatives (Borgia 1998).

Tooth-billed catbirds

The toothbills have courts that are aggregated in what is known as a lek. Because the courts are within fairly close proximity to each other, the more dominant males can easily interrupt calls from other males on adjacent courts. Additionally, toothbills use large, conspicuous leaves as decorations. Using these vocal and visual cues, as well as the males’ relative position within the lek, the females can often choose their mates without even needing to visit the courts. This means that the females can immediately proceed to land on the court of their choice, with no risk of unwanted mating. This in turn makes protective bowers unnecessary (Borgia 1998).

Archbold's bowerbirds

Archbold’s bowerbird courts are spread very far apart, so males cannot steal decorations from other courts. In the absence of this relative fitness indicator, females may be using an alternative indicator: male athleticism. The mating display of the Archbold’s bowerbird involves the male chasing the female around the court multiple times (Borgia 1996), and a bower wouldn’t be of much use to this display, in fact, it might constrain it. Males maintain a low position relative to the females while courting, and this replaces the bower as a protective mechanism which gives the female a greater opportunity to escape an unwanted mating(Borgia 1998).