"tzappar צפר" is ornithologist, but i'm not sure if it can also mean a casual birdwatcher. As for "Agrostology" I don't know what that even means lol.

As Aysar said, "bird watcher" is "צַפָּר"(tzappar). I also had no idea what "agrostologist" means, so I googled it and found out that Agrostology is "the branch of botany dealing with grasses." In Hebrew there is no equivalent word, so maybe you can use the word Botanist(בּוֹטָנַאִי - botanaiee).