We've had similar nighttime temps over the same period here in Maryland,
and they're still crowding the water throughout the day.  In the past, I've
found that if the water isn't fresh they'll go off of it, sometimes for the
rest of the season; at times there had been algae lining the bottom of the
birdbath, but at others there isn't any visible contamination.  I remove
leaves and other debris whenever I see it, but it may be that the tannins
leach out if the organic material soaks too long.  At any rate, now I try
to replace the water every couple weeks, with a good scrub in between.

Otherwise, can't really offer anything else, sorry.

Cheers,
Mark Anderson

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