The BTO's Bird Facts page that Robert looked at is not very up to date or helpful.
The number of nests records (to the end of 2015) is 9720, rather than 137. See
https://www.bto.org/volunteer-surveys/n ... nrs-totalsThere is a useful guide to nests, including their timing, available from the BTO
https://www.bto.org/volunteer-surveys/nrs/field-guide. This indicates that Sand Martin first egg dates are around mid April; with a combined incubation and fledging period of about 36 days, that would mean flying young soon after 25th May.
Earlier this year, I visited the Nottinghamshire Trust's Attenborough reserve, where they have a combined artificial Sand Martin bank & bird hide, enabling one to view the birds entering holes at very close quarters. The nests there (you can't see the actual nests as a visitor) are monitored by reserve staff, with information provided on noticeboards inside the hide. In Lincolnshire, other Sand Martin banks not already mentioned that I'm aware of are at Deeping Lakes (in use in 2017) and at a Roger Wardle-designed farm wetland at Kirton in Lindsey (no idea of current condition). There are probably others.
It would be an interesting exercise to attempt a complete count of the county's colonies one year. Information submitted to LBC and in the Bird Report is sketchy to say the least. I get the impression that fewer are now using sand quarry faces (but that could be that I don't go to them so often) ... and how many are in wholly natural sites in river banks and so on?