The number of birds killed at one time is staggering. Some estimate as many as 5,000 in a three hour period. This means that many explanations can be eliminated because they are not plausible for such a large number of bird deaths in such a short period. There are suggestions of a common trauma event.
It's not unusual for birds to fly in huge flocks and it's plausible that they all flew within an airspace in which a natural event caught the birds suddenly in flight. Possibilities are temperature or electrical events, but the culprit would vanish as quickly as it appeared, so even though a storm situation is suspect, there is not firm evidence as yet. It may be possible to determine whether the event was hot or cold.
My own guess is that it was a cold event and may be associated with wind.
What do you think after reading the press?
http://www.katv.com/Global/story.asp?S=13791490
http://www.reuters.com/article/idUSTRE70534T20110106
http://www.washingtonpost.com/wp-dyn/content/article/2011/01/05/AR2011010502625.html
Time is, was and will be. Without time we would not be. What a load it has to carry! Time seems eternal! You might even ask whether cycles in time exist? If so read Roger Penrose's book on new attitudes in physics towards possible "Cycles in Time".