[Tweeters] Purple Martin Pre Dawn Behavior

[area weatherman]

For the past few weeks members of my local Purple Martin colony have been taking flight at a ridiculously early hour. I'm used to swallows and like well before sunrise, but these ones have been chortling away as early as 3:50am a few hundred feet above my house, itself a hundred feet above their sea-level nests. I've read about martins stepping outside their nests early to do their attracting thing, but its dark at the beginning of nautical twilight. The stars are still bright!

Can anyone point me to a resource that explains how these birds are physically able to see when it's so dark? It's almost as if they know that if they go high enough - and chortle loudly enough - that they can just fly around blindly for some time. I hope they're not that desperate for reproductive attention!

Thanks - James