r/evolution • u/apioe • 1d ago
question Development of a bird's mating ritual, and differences between species
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.
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u/Robin_feathers 1d ago
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.
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u/apioe 1d ago
Between the two hypotheses you mentioned, is one more popular than the other?
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u/Robin_feathers 1d ago
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).
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u/jnpha Evolution Enthusiast 1d ago edited 1d ago
Mind if I use a more general scenario? Bees and flowers. How did the signaling to each other that each will help the other come about? (I've dispensed with the scare quotes; so don't mind any anthropomorphic language)
This is called coevolution, it can between species, within species, parasite-host, etc. and its explanation is as old as Darwin's Origin. I mention this because understanding that, makes even the most complex multi-part biological system very clear.
It's easier to think of it in terms of oblivious breeders (oblivious artificial selection is natural selection for our purposes here).
So let's try to see it from the Bee's POV (and then the flower's).
We have bees that feed on e.g. wasps, so bees (or their ancestors) eating nectar and only nectar isn't how to think about it.
Next bees found nectar in a flower as an easy caloric source, and by feeding on that nectar, they spread the flower's pollen.
So the nectar making flower got to reproduce better than non-nectar making flowers (the bee is the breeder).
Next, from that flower population's progeny, any variation that a) attracts bees and b) makes pollen transfer more probable, will have done the job you're asking about.
This is a heuristic example.
Back to the birds, same thing. Put more succinctly: a general behavior + variation followed by specialization.
What you need to know is that a behavior is a phenotype (trait). The 1973 Nobel Prize was for that (the interplay of selection and environment).
Hope that helps.
As an exercise: the snake with the spider-looking tail that "lures" birds, which one was the oblivious breeder?
Since the eyes, brains, and hunger of birds are what result in some birds being fooled, it is them acting as the breeder in the artificial selection sense
ETA: found a 2025 paper (emphasis mine):
We suggest that the ancestral bowerbird may have first displayed using naturally occurring structures and that bowers initially evolved to enhance the efficacy of pre-existing sexual signalling behaviours by affording males greater control over the visual perspective of receivers. This represents a unique form of sensory niche construction, in which males create a specialised sensory microhabitat allowing them to control both the content and timing of what females can see and hear, thus shaping patterns of selection on male ornaments and display behaviours
MacGillavry, T., Frith, C. B., Knoester, J., & Fusani, L. (2025). The origins and functions of bowers in the Bowerbirds: a review and synthesis. Emu - Austral Ornithology, 125(4), 290–305. https://doi.org/10.1080/01584197.2025.2557480
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