Some people tend to take the nests down each year. If they don't like their current nest, I'm sure they will be able to make a new one! Hope this helps.
Regularly check your houses wall insulation, make sure they are all the same temperature as house martins like to nest on warmer points.
Swallows (house martins, swifts, etc) migrate from the northern hemisphere at the approach of Autumn and Winter, to Africa and warmer climes. They are reputed to return to the same nest site each Spring. By migrating, they ensure that insects are available, and they will nest and rear another brood.
No they build a new nest for each brood. However they may use the same nest site year after year.
No, they make a new nest each year.
No. They build a new nest each year because of safety, health, and instinct reasons.
No, a new one is constructed for each nesting.
Bees do not specifically nest in the same place each year. The queen migrates when the colony dies off in the winter. However, bee colonies might pick similar areas.
No. They make a new one for each brood but if you take out the old nest after the young have left the nest they may make a new in that birdhouse.
House sparrows and wrens often compete with bluebirds for the same nesting house. Often time sparrows will break bluebird eggs or make another nest over the current bluebird nest.
It depends on species but territories are generally held by males and they return to the same territory each year, if they can fight-off competitors. The same hen turning-up is far from guaranteed, but re-use of a successful nest-site is quite possible. Disturbance of the old nest is not relevant.
No. Doves and pigeons build new nests for each brood.
Separate them if they can't get a long, they might seriously hurt each other. Why are they fighting? Are they of the same sex? Of incompatible personality types (each one wants to be the leader)?