The gravel pits at Titchmarsh and Thrapston hold a large population of Cormorants.
On my last outing they were particularly active frequently flying from pit to pit, some times as many as thirty + birds in the air at the same time.
You can not say a flock as their formation is very loose and detached, whilst they are strong flyers, on the wing I think that they look a bit ungainly.
Perhaps this is because they have a dual purpose design and construction to accommodate life in the air and also under water where they are certainly very aquadynamic.
In the right lighting conditions their plumage really come to life.