That is a very big deal observation. Philosophers of science have a concept called "theory ladenness". Until the right tool arrives you may not even be able to formulate the question, and you don't know it. You keep dancing around versions of the wrong question.

The history of heat versus temperature is quite enlightening. The absence of thermometers means you have a hard time conceiving of heat capacity. They knew that phlogiston wasn't right but all of their experiments kept assuming it was measurable. Same with mass versus weight; the difference isn't hard to measure once it occurs to you to measure it.

These can be great aha moments in the history of science. It's both daunting and inspiring.

They swabbed them and sequenced their DNA.

I find it very hard to believe that people in the middle ages (or before or after) looked at birds and actually thought that they really spawned from trees or barnacles. After all, birds can fly, you observe large flocks of birds in the sky right before they vanish, you know that there are warmer regions in the south, so it's not a big stretch to imagine that birds could perhaps migrate southwards. You cannot prove it, but it's a good enough guess.

I think illustrations or stories from the middle ages are to be taken as symbolic or allegorical rather than factual like a biology book would be today. They wrote down a story not because they necessarily believed it be factually true, but because it taught a different kind of truth. For example, no one has ever believed that Red Riding Hood actually happened or that you can cut open a wolf's stomach to pull out a living person.

This is a good video on that medieval belief:

Medieval Science was Baffled by Birds - CuriousCabinet123

https://youtu.be/vgFj-MMTqIc

everyone here is talking about eels but i prefer icktok

I'm just glad that somebody solved this very slippery problem.

tl;dr Eels have a long lifecycle with several stages. They do not develop sexual organs until late in their life, when they migrate back to the Saragossa Sea. This meant earlier autopsies of eels revealed no sexual organs, even though scientists could provoke them with hormone therapy. So, a team lead by Jose Azevedo tagged female eels in the Azores in 2018, and tracked them via satellite [0].

[0] https://www.nature.com/articles/s41598-022-19248-8

I sometimes think about the selection pressures that lead to complex life cycles, like fig wasps. I find myself thinking about it naively, like one existed and the other grew into the niche. But, realistically, everything is changing (slowly) all the time. I just notice it for, say, influenza because their cycle time is so short.