Adaptive value of bowers

There are several different models that offer possible accounts for the adaptive value of bower-building behavior in bowerbirds.

"Proximate benefit" model

The “proximate benefit” model claims that bowers may provide females with protection from forced copulation or predators (which is a direct, or "proximate", benefit). Because the females could then visit and inspect bowers without risk, the males would likely see increased female visitation.There are 3 main types of protection bowers are proposed to provide: protection from predators, protection from marauding conspecific males, and protection from the bower owner (Borgia 1996). The first hypothesis is largely refuted by the available evidence, and evidence for the second is mixed. The third hypothesis, on the other hand, is supported by a large body of data. According to this hypothesis, bowers serve as a physical barrier between the displaying male and female. This gives the female an opportunity to escape if she decides not to copulate(Borgia 1996).And by granting the female the freedom of choice, there may be a higher probability that a female will return to them for copulation in subsequent breeding seasons (Borgia 1998).


Transfer theory

The “transfer theory”, proposed by Ernest Thomas Gilliard, supposes that elaborate courtship display is transferred to a bower from the bird itself (Borgia 1998), such that the bird does not need bright, conspicuous plumage. The theory is based on the idea that a bird without bright plumage will be harder for a predator to detect. There is mixed evidence for this theory, with some, but not all, groups of bowerbirds showing an inverse correlation between the brightness of male plumage and the degree of bower complexity (Borgia 1995).


Marker hypothesis

The marker hypothesis suggests that the characteristics of a male’s bower can provide females with information on the quality of the male (Borgia 1985). Bower paint, number of decorations, and decoration color are all characteristics that may serve such a function.

Color preferences vary from species to species, and this is likely due in large part to differences in their environment. Bowerbirds may choose colors that offer the best contrast with their natural surroundings (Endler & Day, 2006). Bowerbirds also have nuchal crests of differing colors, and decoration color may serve to complement the crests. For example, great bowerbirds have lilac crests, and show a strong preference for green decorations which contrast with the lilac (Haruyama et al. 2013). Bowerbirds arrange decorations not only aesthetically but also functionally (Borgia 1996), and so these green decorations will be highly concentrated in the area below where the male will display his lilac crest.

The numbers of certain types of decorations can be good predictors of mating success in bowerbirds (Madden 2003), although these types may vary from species to species. More decorations can indicate the male’s skill in locating decorations, a good indicator of fitness. Additionally, in many species of bowerbird, males destroy the bowers of other males and steal their decorations. Thus the amount of decorations a male bower has is also related to the males’ ability to both steal other decorations and defend their own bower from destruction or burglary (Doerr 2010). These are also very strong indicators of the male bowerbird’s fitness. An exception to this phenomenon occurs in the spotted bowerbird. Their bowers are widely spaced, so little decoration stealing or bower destruction occurs. This renders decoration number, as well as overall bower quality, less reliable indicators of male quality, thus another indicator is needed. This may be why the display of spotted bowerbirds is so much more intense than in other bowerbird species- display intensity may serve as such an indicator. In fact, this heightened intensity of display can explain much of why the bowers of spotted bowerbirds are so different from those of the other avenue bower-building species. The straw walls, wide avenue and east-west orientation are well-adapted to their unique display style. To reduce the threat to the female, male spotted bowerbirds court them through one of the walls. The wide avenue allows the female to face the wall, and the see-through walls enable her to view the male through the wall (Borgia 1995).

Females taste the paint on the walls when visiting a bower, and evidence suggests that this paint is likely used as a criterion for mate choice early in the breeding season. There are two main possibilities for the mechanism by which bower paint provides females with information about male quality.

"Paint quality" hypothesis

The “paint quality” hypothesis suggests that female bowerbirds can discriminate male quality based on chemical cues. This would explain why females taste the paint. However, studies have shown that when paint was transferred to a male’s bower from another male’s bower, the male did not spend a significant amount of time repainting over it. Repainting was minimal regardless of whether the transferred paint was higher or lower in quality than the original paint (Hicks et al. 2013). If the females were actually gaining quality information from chemical cues in the paint, then the male bowerbird should repaint to a significant degree if the transferred paint were lower in quality. As this was not the case, the data appear to refute the hypothesis.

"Paint quantity" hypothesis

The “paint quantity” hypothesis suggests that female bowerbirds can discriminate mate quality based on the amount of paint. Again, it could explain why females taste paint if they are able to detect paint thickness through tactile cues received via mouth. It has been shown that females preferentially return to the bowers of males with more paint (Hicks et al. 2013), offering some support for this hypothesis; however, this evidence must be treated with caution. Certainly male bowerbirds with more fitness can likely afford to spend more time painting, and amount of paint is an honest signal of fitness. However males who are more fit probably can also afford to spend more time arranging their sticks or decorations, and these other cues may be what actually attracts the female to return.

Female attraction rate and amount of paint

In one experiment (Hicks et al. 2013), after females had visited bowers for the first courtship, some males had nearly all of the paint on their bowers washed off (washed males), while other males did not (control males). Far fewer females returned to the washed bowers for a second courtship as compared to the control bowers. As a result, far fewer females ultimately copulated with washed males than with control males.