I have a huge and very old oak tree in my garden, home to what must be hundreds of thousands (millions?) of insects which attracted housemartins and pipistrelle bats every year. It was lovely to sit in the garden and watch both species swooping around day and night. They both disappeared twelve ago after the orchard opposite went under housing development. Despite much campaigning, the bat roosts (unoccupied at the time) were allowed to be destroyed so long as the new houses had articifical bat boxes attached. I have not seen a bat or had housemartins nesting since so human encroachment in urban environments, as well as weather, is a factor.