Why do black redstarts exhibit migratory restlessness at night for fewer days than common redstarts?

Answer: Blact redstarts are associated with circadian clock. Seasonal melatonin and circadian clock lead to migratory nocturnal restlessness. The birds that exhibit migratory restlessness at night is due to high levels of melatonin at nights because melatonin are high in the night.

Explanation:

Migratory restlessness is the locomotive performance that birds engaged in before migration. It is common in birds with circadian rhythm.

The statistics given has shown that night-migrating birds are driven by autonomous circadian clocks which is made possible by sunset cues. This timekeeping that is the autonomous circadian clocks system is probably the key factor in the overall control of nocturnal songbird migration.