A Grey Heron can occasionally be seen flying upriver or downriver at Gainsborough (including by Mercer Wood), and I have seen them flying overhead in the town centre. One flew over Gainsborough’s bus station on Feb 13th 2013.
They often turn up at Beckingham Marsh, and there were 3 there on Mar 11th 2018.
A Grey Heron flew low over Morton’s church on May 30th 2018. Another flew over the Riverside at Gainsborough on Jul 6th 2018
Grey Heron (pictures from Wikipiedia)
Little Egrets were once rare vagrants to the UK from the continent, but in recent years they have become resident in this country.
One or two can often be seen at Beckingham Marsh (with 3 there on May 3rd 2017, and again on May 28th 2018).
One flew over John Coupland Hospital on Oct 8th 2018, while another was on the Field Lane Allotments at Morton on Apr 28th 2019.
Little Egret (picture from Wikipedia)