When the birds circled on rising air currents the hemisphere connected to the eye facing the direction of the turn was typically awake while the other was aslee

When the birds circled on rising air currents the hemisphere connected to the eye facing the direction of the turn was typically awake while the other was asleep, suggesting that the birds were watching where they were going. “The frigatebirds may be keeping an eye out for other birds to prevent collisions much like ducks keep an eye out for predators,” says Rattenborg. Earlier it said these brief periods of sleep would start as the sun went down; does it mean that they would only catch some sleep at dusk/twilight, or this would continue throughout the night? And if it's at night, do the birds have good enough night vision to see anything flying nearby or swimming on the surface below (such as their prey, squid and flying fish)?