Here are some recent records from Longham:
27th November: Single Goosander redhead on LR North (Chris Parnell).
20th November: 1 first winter Red-breasted Merganser at 7.30am on LR South (Robin Trundle). This is the 4th record for the site. 50 Lapwings settled on the larger island on LR South (Alan Pearce), one of the better recent counts.
19th November: The first Goosander of the season, a redhead on LR North (Tom Carley).
16th November: A very interesting record of 9 Brent Geese flying in during the late morning. This is only the 3rd record, and by far the largest number seen here. Also Greylag Goose and no less than 11 Little Egrets. (Dave and Pat Harris and OWLS group).
I neither have the 200-400mm nor the 500mm, so I can’t relaly give you my preference. All I can say is that I never get close enough for small birds. So I would go for the 500mm, also to avoid the overlap with the 70-300. But the 500mm is also expensive and heavy ( and supposed to be used on a tripod). I would also recommend reading and