One of the most annoying things about trying to photograph things that are rare or unusual in the British summer is that grey skies are practically guaranteed. I don't think it is being entirely paranoid to feel that there is a direct correlation between the rareness of a photographic opportunity and the probability of grey skies.
In this context, it was inevitable that the one chance I get to photograph gannets diving at sea was accompanied by dull grey skies. Oh well...
So how to make a picture of a white and black bird against a featureless grey sky a bit different? Before I went on the trip, I had a vague feeling in the back of my mind that it must be possible to get white birds against a white sky - a high key effect. Some years ago, I'd been extremely impressed with a picture of a gannet flying close overhead of the photographer in a cruciform pose with feet splayed and seaweed in its mouth. The sky was almost white and every feather detail in its underneath was clearly rendered. A very beautiful print - you can tell it really left an imprint on me. (I can't remember who it was by - perhaps Deborah or Duncan Armour?)
As we headed out from Bridlington, Steve Race gave us a tip that immediately brought these inchoate, back-of-the-mind, thoughts into focus. He suggested overexposing pictures to push the grey sky to white to get a high key effect. While our boat made its way out to sea, occasional gannets and gulls came by: I made some trial exposures, looking at the histogram and the blinkies (highlight warning display on the rear screen) to judge where the sky would be blown, but retain all the bird's detail. Depending on exactly how the clouds were, setting between +3 and +4 stops on my Nikon D500 seemed to work well: eventually, I settled on +3 stops as being enough. This is using Nikon's matrix metering, and the overexposure values will certainly vary depending on which camera you have.
The highlights warning display on the back of my camera. The display quickly flicks back and forth between a normal preview (shown on the left) and showing blown highlights (right) in black. The normal display makes it look as though detail is being lost, with the bird merging to white with the sky. But the the entire bird is revealed as fully recorded against the blown sky.
The big thing is that because you are looking up at the bird against a bright sky, even though its feathers are white, they can never be as bright as the sky itself, so it is possible to retain all the detail in the bird against a blown sky.
Keeping to 1/3200 sec to minimise motion blur meant that the ISO values went way high, but, in the end, that simply doesn't matter.
I'll have this technique in mind during the next winter when photographing birds in birds in flight against typical English winter bland grey skies.