Add to that, how close together the targets are to one another affects the intensity of the return. Birds migrating at night are really spread out compared to rain drops or hail stones.
Tree swallows sometimes aggregate in the millions in the willows in the swamp just west of the radar here in Melbourne, Florida. They make diurnal (daily) foraging trips while staying in the area. They fly up and out from their roost early in the morning. There will be a very bright return at the roost as the all lift off and then the return spreads out and dissipates as they disperse. Look up "Purple Martin Roost Ring" radar images for an example. When they return in the evening they are generally flying under the radar and aren't detected. But, the colony in Melbourne is so close to the radar that when they mass up in the evening over the roost the radar picks them up and it produces a bright red spot for a couple of frames.
You can see a nightly version of the same thing from Mexican Free Tailed bats in central Texas during certain times of year.