Be fit or get hit

I've never really debated the idea of what fitness means. Like, there's definitely two sides of its definition, at least, but I just consider the miscibility of those definitions to be a valuable subjectivity. Here's how I see the two sides: in laymen's terms, an individual can be fit based on their physical prowess and ability to deal with circumstances. On the other hand, in regards to biology, an organism is fit if it has more progeny relative to whatever context you want to give it: its population, species, etc. I can see where confusion could come in if you conflate an individual's physical prowess with its reproductive success, which is fair: an organism that is more physically "fit" does indeed probably have a better chance of surviving and thus reproducing. However, I don't think there's a direct causation. An organism may be physically fit but that doesn't guarantee that other organisms are going to mate with it. Maybe a bird of paradise is very fast and capable of getting food and fleeing predators but it sucks at doing a mating dance. Sure, it's fit enough to survive all those environmental competitions but it still fails to win a mate if it can't do the bird of paradise jig to the females' standards. Then again, maybe using that definition is then too subjective; maybe we could call it less physically fit because it didn't do the dance well, and that's what is important in the context of its species. Either way, I think that when you're talking biology, it's best to say an organism is fit because it does good in the baby-making category, period. It's objective and avoids those tricky subjective stipulations that the "physical prowess" definition conjures.

Measuring fitness would thus have to involve measuring an organism's baby-making ability compared to its intraspecific competitors. I think it would be best to use a population scale, based on my general understanding of biology because it would omit variability in ecosystem resources between intraspecific populations. Let's go back to birds of paradise for an example, specifically the magnificent bird-of-paradise, Cicinnurus magnificus. We'll imagine a population on one of the small islands surrounding New Guinea, which we'll assume has no gene flow to the mainland or surrounding islands. These birds are one of the species that depend on elaborate, groovy as hell rituals that females have strict judgement over in order for her to choose a male to boogie with. Since the males are polyamorous, we'd have to track individual males and see which bird bros succeed in mating with the most ladies. I don't know how many clutches females lay in a breeding season, but we'd need to track individual females as well to see if they mate with multiple males over the course of a season. One would have to count the number of offspring all these different birds have, probably over the course of many generations. Then I think it would suffice to find an average every breeding season and maybe between breeding seasons. This way, you could determine fitness within particular years/breeding seasons or within the course of the population's existence. Then an individual would be fit if it had more offspring than the average of the population and differentiations could be made based on resource availability across years. Maybe in a resource-lacking year an individual would be fit in comparison to its contemporaries but not fit or average in comparison to the populations' history. Sorry that was so verbose, thanks for bearing with my thought process! Here's a fun video with cute captions of C. magnificus:

References:

https://birdsoftheworld.org/bow/species/mbopar2/cur/introduction