Yesterday after school i went to wilstone res, hertforshire, We herd that there was alot of black tern's there so we thought we would go and check it out. We arrived to find terns everywhere! Black terns were everywhere. I managed to count 52 black terns but another birder there counted 56! What a sight it was!The black terns were spread out over the res but every few minutes they would suddenly all fly up and circle around, quite high but then they would suddenly drop. Each time they did this more seemed to join the flock. It was obvious that there were going to fly off but i couldnt understand why they kept suddenly flying up and then flying back down. Has anyone seen this behaiver before? It was like they didnt want to go? what was they waiting for? The wind, the temperature? Eventually at about 9pm they flew up and got higher and higher untill they were gone.It would be handy if anyone new why they were doing this!