Do migrating birds show up on radar, and if so, what do they look like?
Yes they do...and so do insects! They're sometimes called 'migratory aerofauna'. We'll stick with 'migrating birds'!
Keep in mind that migrating birds usually fly at a fairly consistent altitude. The radar beam moves away from the radar at an angle. The beam close to the radar is below the flocks of birds, the beam far away is above the flocks. For that reason, the display on radar looks like a doughnut surrounding the radar...and the colored reflection looks 'speckled'. The movement of birds will show up mainly when the weather is dry and a radar is set to 'clear air mode'...and usually happens in the evening, a bit after sunset. But mass migrations also show up seasonally, during the spring and autumn months.