Now that I'm thinking about it, birds must change their diet through the seasons, according to what's available. Guess seeds and bugs are pretty darned scarce in the early spring, so it's "whatever looks good" ? I can't imagine there would be a whole lot of nutrients in a flower bud, but . . better than nothing.
Luckily, here we have bugs all year round, and there's usually lots to eat so I haven't had any birds eating flower buds. I did have some body snacking on my snap peas yesterday, though . . .