The following is a graphic that shows the deaths in different regions of the planet during the said period.
Thousands of birds were seen fall out of the sky in Arkansas, Nashville, Louisiana, Texas etc. The first reports of dead birds came out of Arkansas around 11.30pm on New Year's Eve. At about 11.30 pm, thousands of red-winged black birds began falling from the sky over Beebe, Arkansas. The Little Rock radar station, located 30 miles from Beebe, picked up mysterious turbulence over Beebe, Arkansas, four times between 9.00 pm on December 31st and 1.00 am on January 1st  .
There were several theories put out over the incident such as the smoke theory, the fireworks theory etc., to explain the mysterious echoes captured by the radar. Both theories could likely be ruled out, since no major fires were reported in Beebe, ruling out the smoke theory and fireworks are too small to be picked up by the radar which rules out the fireworks theory as well. Moreover, many birds were found to have liquified internal organs after falling from the sky.
Hundreds of birds have been found dead in Nashville and nearby counties. Around 150 grackles were found dead on the road side and have deteriorated so badly that even a lab could not determine the cause of death . Experts believe these birds to have hit the power lines after becoming disoriented somehow.
Bird deaths have also been reported from other places in the US such as Louisiana, Kentucky and Texas while countries like Italy and Sweden also reports mysterious bird deaths.
In Italy, countless turtle doves were found scatterd on the streets, in flower beds and hanging tragically from trees like "Christmas balls" in the town of Faenza .
Apart from birds, millions of fish turned out dead and washed ashore in Maryland, Brazil and New Zealand. Moreover, the Thanet Coast in the UK is now reported to be littered with tens of thousands of dead crabs that have washed ashore , .
There were several theories dished out by the mainstream media on the cause of death although a detailed investigation is underway. What was striking about these theories were they were all outright silly, from 'experts' blaming the New Year's fireworks, power lines, massive trauma and indigestion for the death of birds while cold-water stress and freezing temperatures were blamed for the death of fish and crabs.
'Experts' Solve the Mystery
Despite the mysterious death of 1000s of animals happening across the globe in a week's time (within last week of Dec. 2010 and first week of Jan. 2011), 'Experts' say that all events are likely unrelated. Really??
They went on to blame lightening, freak hail storm, mid-air collision, power lines, massive trauma and New Year's fireworks for the death of birds, while cold-water stress and disease were blamed for the death of fish and crabs. No matter which mainstream sources you go after, in general, they all cry out the same thing and that is: Fireworks, likely to have caused many of the dead birds to fall from the sky in U.S., 'stomach ache' caused the death in Italy and unusually cold weather is believed to be responsible for others. Also, According to the AP, we are told mass animal deaths are not that uncommon .
The following are some of the mainstream explanations dished out by 'experts':
Here's what the Italian sources have to say:
"We are fairly confident the birds died as a result of massive indigestion brought on by over-eating", Rodolfo Ridolfi, a director at the regional zoological institute, told The Daily Mail.
.. a bad stomach ache?
Nadia Caselli of an Italian bird association corroborated Ridolfi's findings, telling the AP that sunflower seeds from a nearby oil factory are likely the culprit, as they damage the birds' livers and kidneys, though full tests results are still yet to come.
1000s of birds simply do not fall out of the sky due to fireworks Mr. Expert. That's Bull$h!t.. Haven't you had fireworks before?
Birds and fish can be susceptible to subte frequency alteration. Could a directed energy weapon or device be the culprit?
Explanations from Louisiana
Another 450 birds were also found dead along the highway in Baton Rouge, Louisiana. They are believed to have hit the power lines after becoming disoriented.
Dan Cristol, a biology professor and co-founder of the Institute for Integrative Bird Behavior Studies, said the Louisiana birds may have all been ill or startled from their roost before hitting the power line.
"They don't hit a power line for no reason,' he said. - Yes, they don't...
What do 'Experts' from New Zealand have to say?
In New Zealand, hundreds of dead snapper fish washed up on Coromandel Peninsula beaches, many found with their eyes missing, The New Zealand Herald reports. A Department of Conservation official allegedly claims the fish were starving due to weather conditions. 
Starving fish... missing eyes??? Please connect me the dots ...
How about millions of dead fish found in Maryland, U.S.?
Maryland Department of the Environment spokeperson Dawn Stolzfus says "cold-water stress" is believed to be the culprit. She told The Sun that similar large winter fish deaths were documented in 1976 and 1980. 
Hollywood's 'Scientists' Explains the Mystery
It looks like the Hollywood scientists outsmarts the so-called experts and might just have the answer to these bizarre incidents. Somehow, they [Hollywood] always manage to predict such disasters like 9/11 (The Lone Gunmen), Hurrican Katrina (Oil Storm) and several others (few yet to be announced to the public) through their movies and TV shows, weeks, months and years in advance. Likewise, "hollywood scientists' may have an explanation to the mysterious death of thousands of animals, predicted through their movie "The Core".
Global Birds Death -"The Core" Turning Into Reality
 Mystery deepens: Radar captures mysterious turbulence over Beebe, Ark. as doomed birds take flight
 100s of dead birds found in Nashville area
 Birds Dying in Italy: Thousands of Turtle Doves Fall Dead From Sky
 Dead Crabs Wash Ashore By Thousands on England Beaches
 40,000 dead crabs washed up on Kent coastline as UK is latest country to be hit by bizarre animal deaths  Dead Birds Fall From Sky in Sweden, Millions of Dead Fish Found in Maryland, Brazil, New Zealand