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).