Birdfall

[Catsmate]

                        We were three hours at sea
                        When the birds began to fall;

Technically bird kills are localised event's that cause death of a large number of birds over a short periods. They're not even that uncommon, with 15-20 events per annum having more than a thousand dead birds. Usually it's triggered by bad weather (especially sudden changes in condition), outbreaks of disease, poisonings or air pollution.

Last Thursday's event in the Dorchester area of Boston wasn't even on that scale, only about fifty dead birds (all grackles) have been recovered, along with two cats that may have consumed one or more sick or dead birds. A Boston resident noticed her cat was ill and that the street was littered with birds described as "lethargic, trying to roll over, not able to stand up or fly". Other witnesses saw birds dropping out of the sky.

Game possibilities.
1. What killed the birds? A purely natural phenomena perhaps? Or something stranger, such as the testing of a piece of Mad Science that generated a sudden drop in temperature in the air that killed the birds while not being noticed by humans on the ground below. Or did something nasty leak from a Mad Scientist's lab or the hideout of an alien?
2. Perhaps the birds aren't from around here, and arrived from somewhere/somewhen else via a rip in space-time, or the wake of an inter-dimensional transmat or time-ship. What oddities might be found when the bodies are necropsied? Unusual pollutants? Genetic differences from known species? Tracking implants?

• I refer you to Steve Stirling's Conquistador which employed a similar plot device.
  

3. Maybe it's just a red herring, suitable for distracting the PCs or plaguing them with the attentions of conspiracy theorists.

More.
Boston Globe
Boston Herald