This is pretty easy and quick to grind out on level 1-25. Aim each of the three birds just below the small planet (the one above the 'UFO' made out of blocks and a pig), and they will arc round the planet exiting its atmosphere at the upper right.
It took me about 10-15 minutes to get this. I had to do it for about 150 birds though - the blue bird might not be counted if you activate its special power (split into three smaller birds), which I did each time.
Edit: You might not need to grind this depending how far you play into the game. I just wanted to get it out of the way. It's also worth pointing out you must enter then exit the atmosphere - birds that start in atmo. then exit don't appear to count.