These are summer visitors, usually arriving in April. Three species can be seen at Gainsborough.
Swallows occur along the whole length of both sides of the river, and they can be seen hunting for insects on the river side of Mercer Wood.
Swallow (pictures from Wikipedia)
House Martins can be seen all over Gainsborough, and I often seen them around the Mercer Street entrance to Mercer Wood.
House Martin (pictures from Wikipedia)
Sand Martins can also be seen o the riverside. The best place to see them is around the Trent Bridge. Stand on the bridge and look over the side and you will see them. They will sometimes venture further downriver, and I have sometimes seen them by Mercer Wood.
Sand Martin (picture from Wikipedia)