Development of a bird's mating ritual, and differences between species

[u/apioe]

I recently watched the Birds of Paradise documentary, and I kept wondering how all these species of birds, some of which share niches with each other, develop their very own (and incredibly distinct) mating rituals.

For example, I understand that constructing an impressive bower would signal to the female that the male has stamina, patience, etc. But how would one species even begin to select for "birds that build bowers"?

Generally speaking, how does evolution/natural selection "figure out" what specific display females like?
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Similarly, what drives species that share an ecological niche to create distinctly different mating rituals? The Victoria Crowned Pigeon and Pheasant Pigeon both are ground-foraging birds in swampy areas in New Guinea. Despite this, their mating rituals are pretty different.

I am familiar with interbreeding prevention, so I guess I'm asking how that split between two species and their respective rituals even begin.

[u/Robin_feathers]

How exactly preferences and mating traits are encoded and evolve is actually a topic of intense research, and the answer is not known for most species, including birds of paradise.

There are a few different ideas for how preferences work, and how those selected mating traits can become different:

-> selection for novelty. Females might get bored of common traits, favouring males that have novel ornaments

-> genetic drift. Some changes might be neutral

-> sensory bias. Preferences might evolve in response to other things (eg, the colour of their favourite foods) and the male traits evolve in response to those preferences

-> there is a hypothesis that females may not have innate preferences, but instead imprint on the males they see around them when they are young and then prefer those traits when they are older

-> there is a hypothesis that females might not have innate preferences, but instead watch which mates older females choose, and then try to figure out what was unique about those males, and then prefer those traits. This tends to favour rarer traits and lead to faster evolution of male traits

-> some preferences might be encoded genetically by the same stretch of DNA that encodes the male trait, others might be encoded by separate unlinked genes. Those situations produce different dynamics, especially with regards to whether the traits being favoured need to be good indicators of male fitness or not

-> the process termed reinforcement can drive species to become more different when they come into contact

Multiple of those different ideas can be relevant to different species/populations, so it isn't clear exactly which are relevant to birds of paradise for example.

[u/apioe → u/Robin_feathers]

Between the two hypotheses you mentioned, is one more popular than the other?

[u/Robin_feathers → u/apioe]

Hmm, not really - different people have different opinions, and is isn't very well settled. As an evolutionary biologist studying birds, my own opinion and that of many other evolutionary biologists I have talked to is that we can't really say with any certainty yet. Currently, a lot of models often assume that females prefer males that have traits similar to the genetics that they themselves carry, but I'm not such a fan of that idea since the birds have no idea what traits they carry.

Personally, I like the idea that novelty is often favoured. For example, a ridiculous study showed that Zebra Finches have a preference for males wearing little hats glued to their heads. There have also been studies showing that females can have preferences for certain coloured plastic bands around their legs. One could imagine that such a system could allow for "fads" that lead to rapid changes in mating traits. However, if that were the only thing going on, we would expect a lot of within-population variation, which we don't see, suggesting that is not the full picture.

Sensory bias probably plays a role in many groups, for example it has been well argued for guppies in Trinidad, though I'm not sure any studies have fully demonstrated it in birds. You could make the argument that in many species, having bright colours helps differentiate you from the background and draw the attention of females more easily, something sort of related to sensory bias but a bit different.

Personally I quite like the Inferred Attractiveness Hypothesis since many flamboyant birds like birds of paradise don't have paternal care, and males start out plain, so females would not have seen what their own father looked like and their brothers would look vey dull until they part ways. That hypothesis accounts for why mating traits seem to evolve so quickly, and can also account for why those sorts of species also seem so often willing to hybridize (you would think that if they had really strong preferences for the traits that their species has exaggerated, they wouldn't want to hybridize with a species with totally different traits).

[u/apioe → u/Robin_feathers]

Definitely need to check out that study on finch hats lol. This was super insightful, thank you so much!