Wonder what kinds of food pigeons, sparrows and grackles would all eat in common.
I've heard of birds getting really sick on chile pequins they eat off of bushes, thinking they're ordinary seeds or berries - and I've also heard that quail and game birds eat them with impunity and their meat is highly seasoned and very good.
Then there are mistletoe berries that are poisonous to birds, but we would've heard of lots of Oklahoma birds also dying if that were the source. I do wonder if this was a bumper crop mistletoe year - hadn't been out in the country to observe.
Then I've also heard of them getting drunk on those little red berries that are non-edible to humans - and always splattered on my patio in bird droppings. Drunk birds fall out of the sky, too.
I don't know what to make of the Austin ones. And we already know there is an animal uprising in Australia, so that's easy.
They are all omnivores so anything is possible, food-wise.
Since those in Austin were in a few block area, I would assume some business got tired of the poop and sprayed. However, there's always UT's wacko Dr. Pianka (he of the let's kill off 90% of the human race with ebola) or one of his converted students testing an idea for something bigger.
As you said "out in the country". There's not that many bushes or trees in the middle of downtown Austin. And knowing the mentality of Austinites, there would protests against and a permit issued for any berries that might fall on the sidewalks.