Learn English – the plural form of the phase “play a role”

Although the proper Latin plural would be fora, forum has been adopted into the English language--and in most cases follows the rules of English pluralization. Similar changes can be seen with the adoption of other words like octupus (the proper plural would be more like octopedes, but in English we usually say either octopi or octopuses).

The current entry from the Oxford Dictionary says:

The plural of forum is usually spelled forums; the plural fora (as in the original Latin) is chiefly used when talking about a public square in an ancient Roman city.

Since you're talking about online discussion boards, I would use forums instead of fora when you are talking about more than one. An English speaker might understand fora, but it wouldn't come as naturally.

This is a very good question. In general, you can help yourself work through any confusion by trying to express the idea more precisely or in a slightly different way. I'll use each of your examples to explain this a bit further.

EX1: All lizards have a head width, but that isn't what you measured. You measured the different head widths of a group of lizards, or "the head widths of some lizards." I would at least add the "some", but perhaps it would be better to include the actual number.

EX2: You are talking about the growth rate of all birds, not, say, the growth rates of different species of birds.

EX3: Each of two populations has a mean height, therefore there are two mean heights. You are saying that the mean heights of these two populations are different.

EX4: Dogs have noses is correct, but this is a bit inconsistent. For example, dogs have a sixth sense that lets them know when someone is coming, and they have a nose for trouble. People have minds, but they have a combative streak. So, when you are talking about a shared characteristic of all dogs, you use the singular, but when you are talking about something that each dog has, you use the plural.