...At least one has been known to have a vocabulary of 950 words (when over 70% of communication in English is carried out using only 7000 words), and able to greet Jane Goodall with the question, "Got a chimp?" based on seeing pictures of her with a chimp.
Wow! That is smart. That's not just mimicking words or sounds, that's understanding the concept represented by the words, understanding the basic structure and concept of a sentence and inquiry, and putting it together to ask a question. That's impressive!
I kind of like the concept of a Dragon intelligence being related, evolutionary wise, to the way some birds have developed.
First you'd have to define "intelligence", and trying to do that is apt to start an argument.
I hear you.
How about this then. For sake of the thought experiment, we make the following assumptions:
1. Some species of Dragon (or maybe just one) have evolved an intelligence comparable to humans (at least in self-awareness, cognition, and communication), but yet
different from humans.
2. Although Dragons don't seem to have a mouth structure for effectively using a human-like language, based on myths they have been able to communicate with both each-other and with humans. So, communication with Humans is done much like a bird emulating Human speech (though with significantly more tonal range), but is quite different when communicating with other Dragons (maybe like the way velociraptors possibly communicated, but more complex and expansive).
3. Dragon senses are different from Humans. Enhanced sight (for spotting prey while flying). Comparable hearing and smell (though either could be greater or less than humans). An almost insignificant sense of touch, or at least significantly less than humans (hide is just too thick or scaly for comparable tactile sensing, except for sensing hot and cold).
4. No need to build structures for shelter or tools for hunting.
So, trying to avoid putting Human motivations or values onto Dragons (or at least as much as possible), and avoiding using Magic or Psychic abilities to explain things, maybe:
Dragons mate for life, and Dragons protect their eggs and raise/teach their offspring. (as based on some Dinosaur fossil evidence)
Dragons do not reproduce as quickly as humans. A significant factor in their eventual demise.
Dragons have developed an aesthetic sense, especially as to appreciating the beauty of nature (rather than just seeing nature as an environment to be dealt with), but since they don't use tools, they have no art of their own. Maybe this is why they appreciate, and possibly crave, treasure and human ruins.
Dragons could possibly indulge in musical singing (in a Dragon language, of course).
Probably a strong oral tradition, as Dragons do not write (no tool use) or read. They probably have only a spoken language. (Although if they have developed a written medium, it's most likely carving symbols into stone using their claws. Carved symbols would probably be relatively large due to the medium and the lack of a precision writing instrument, and could be either course or relatively intricate, but most likely some kind of pictogram or ideogram rather than an alphabet.)
Although Dragon Intelligence could have developed while Humans were still living in trees, their level of sentience and intelligence has not progressed any farther than the above. Human Intelligence, and what Humans have accomplished with it, has far surpassed Dragons from the bronze age onward.
As concerns religion, Dragons could have some type of spirit or ancestor worship. Possibly highly formalized and structured. Could possibly have seasonal and astronomical aspects.
Dragon morality would be based around Dragon survival and instincts. Killing or even eating a Human would not seem
wrong to a Dragon. Though some Dragons may have developed a sense of balance with their environment, purposely not overusing resources and maybe even cultivating some resources, and may have a compunction about killing/devouring sentient creatures. They may see Humans as a growing scourge at the same time they see them as part of the natural world (a contradiction they/some struggle with).