In Clayden, page 735, Aromatic Heterocycles 1:Reactions:
The other simple five-membered heterocycles are furan, with an oxygen atom instead of nitrogen, and thiophene, with a sulfur atom. They also undergo electrophilic aromatic substitution very readily, although not so readily as pyrrole. Nitrogen is the most powerful electron donor of the three, oxygen the next, and sulfur the least. Thiophene is very similar to benzene in reactivity.
This is in the context of electrophilic aromatic substitution.
My question is, why is nitrogen the most powerful electron donor? Sulphur is the least electronegative, so will it not be less reluctant to donate electrons?
Or are there some resonance/aromatic or other effects I'm failing to see in pyrrole, furan, and thiophene?