Here is some advice. Scenario: you’re playing Boggle™ and you’re able to come up with ‘bird.’ Congratulations. You must have read a lot of Highlights as a kid. But wait, what’s that? A suffix…for a noun. ‘-er.’ So now you have ‘birder.’ You have now begun your journey down the dark path of Boggle™, and your gaming partners give you that half-lidded Jon stare from Garfield.

They mentally consider any number of arguable points perhaps including the following:

1. A falcon is birder than a sparrow.
2. An emperor penguin is birder than a non-emperor penguin.
3. An ostrich is birder than a kiwi.

…and so on. But you were thinking of ‘birder’ as in ‘bird-watcher.’ You are now requested to give account for the difference between ‘birder’ and ‘ornithologist.’ The answer?
Money / education.
Consider:

1. farmer vs agriculturalist
2. singer vs vocalist
3. painter vs artist

Basically, ‘-er’ means amateur and ‘-ist’ means professional. However, you can’t just slap either ending on just any occupational description. After all, how far would you trust someone who claimed to be a gynecologer? Armed with the knowledge that the occupations described by the lowbrow / -er words can be carried out by anyone, what noun can not be a verb which can then be turned back into a noun? Floor – floorer, water – waterer, chronicle – chronicler… Okay, there are a few, like ‘cat,’ but you just add another ‘-er’ and you’ve got ‘caterer,’ the less professional version of group-gastronomist.
However, what is a racist, but one who competitively drives automobiles? Sure, there might be multiple meanings for that word, but that’s what linguists are for. (Or, depending on your resources, dictionariers.)
The point here? Don’t be tempted by the the dark path of Boggle™. Once you open the game up to this kind of insanity, your partners will trust you less. In other words, your fellow Bogglists will view you as a mere speller, a worder, an amateur…er.