Migrant hoverflies

My day was spent surveying a brownfield site just south of Norwich, an area with plenty of flowers which were providing much-needed nectar to a shifting masses of hoverflies, mostly Marmalade Hoverfly Episyrphus balteatus and  Humming Syrphus Syrphus ribesii (pictured). I can't remember ever seeing  quite so many in one location. They are part of a huge migration from the continent, which has been particularly evident along the east coast.