Avian flu could be a factor, though I’ve not heard of any egret corpses being found locally.
Numbers can fluctuate day to day, and there would appear to be some mobility between the roosts in the area. Peak roost counts have been down and, whilst sample dates may not record true peaks, the counts at Leighton Moss in the 3 peak months of August – October were all down on 2020. December counts at Leighton were 28 in 2019, 51 in 2020, then the 0 and 10 this week.
It would be interesting to know whether numbers have decreased in the other roosts around the Bay.
A local factor at Leighton could be increased disturbance from the presence of marsh harriers throughout Autumn and Winter. Without disturbance the roost normally empties a few birds at a time with many sitting out on higher branches before departing. If a harrier flies over the roost all remaining birds will erupt, and depart en masse. 3rd November was a prime example; no stirrings in the roost, then suddenly 38 birds in the air and flying straight out when a harrier went over.