Grey herons are resident UK birds that face high mortality risk during cold winters when frozen waterways prevent fish capture. They stand exposed in cold weather, vulnerable to starvation and hypothermia, but populations typically recover after mild winters.
Herons during severe winter cold stand motionless for extended periods, entering torpor to conserve energy when they cannot feed. Starvation of fat reserves over days leads to progressive weakness and inability to hunt effectively, creating a downward welfare spiral. Individual birds may feed at garden ponds during severe weather, where the welfare benefit of supplementary food availability is demonstrable.