Parakeets must be remarkably adaptable birds.  When they escape, they seem to rapidly establish themselves in cities from which they did not originate.  When we lived in Chicago, there was a park on the south side of the city which had a large parakeet colony.  Richmond Park in London is a similar spot.  We heard the parakeets before we saw them, but their bright flashes of color made them easy enough to spot when they got moving.  They just seem to flourish wherever they are, even if they really should not be there.