Sorry Jonners but in my opinion and that of many other wildlife photographers you really shouldn't be using flash for nightime shots of nocturnal flying/feeding/hunting species
(save for captive birds which aren't reliant on their own skills).
The latest research suggests that doing so can severely impair the dark adaptation of birds such as Owls and Nightjars for as much as one hour, making it difficult (if not impossible) for the birds to hunt successfully during that period of time.
Such impairment, albeit temporary, can be catastrophic for birds with nestlings to feed - particularly at this time of year when the hours of darkness are at their minimum - and if the same bird is targeted repeatedly may prejudice the success of an entire brood
I hope you don't mind me pointing this out and I'm sorry to dampen your enthusiasm but, as with all wildlife photography, the wellbeing of the subject must always come first
Jeff
(Schedule 1 Licence holder for Kingfishers and Barn Owls)