Rightly or wrongly, flash is generally frowned upon where I live. In fact the only ones who ever use it are foreign photographers, who are soon told to turn off their flash. The same local bird photographers will, however, happily blast playbacks (recorded bird calls or songs) and provide a variety of bait to entice the unpaid models to stand upon the set-up perch.
I personally never use flash but more due to my inexperience than an absolute objection to it. I do object to hundreds of photographers surrounding a nest, each triggering the flash dozens of times. This has happened before and was reported by the local media, resulting in much PR damage to bird photographers, not to mention straining the relationship between birders and shooters.
A bit off-topic.