Back in January, we ran a short post about how pine siskins were showing up at Marin County bird feeders in surprising numbers and then dying. At that point, it seemed like the feeders themselves might be at fault, as vectors carrying pathogens like Salmonella.

But thanks to a newsletter article by Diana Granados, who serves on the board of Mount Diablo Audubon Society, we learned that this is a more complex, but apparently natural phenomenon called an irruption–a sudden surge dispersal of an animal from its core range out into many places and habitats where it is not usually found.

We received comments from worried readers in Santa Cruz, Arkansas, Georgia, North Carolina, Texas, and Washington state. All had found dead siskins at their feeders. Clearly this is a national event.

I called Granados this morning to get more detail from her about what’s going on. In addition to serving on Mount Diablo Audubon’s board, Granados runs Native Bird Connections, a bird education group, and worked in wildlife rescue for many years.

In a nutshell: Those dead birds at your feeder are evidence of the kinds of population dynamics that allow species to disperse into new habitats, and also reduce stress on their core habitat. Every few years, siskins flee their home ranges in droves and go south.

The Audubon Society (here) says there’s no definitive answer to why pine siskins irrupt some years and not others, but they do say this about the closely related common redpoll: “There is speculation that this variation in food production is an evolutionary strategy that forces these birds south every few years, thereby reducing their long-term impact on the plants. The same may be true for Pine Siskins. In years when Pine Siskins appear in either Southern California or North Carolina, food abundance in their typical wintering grounds may be low.”

Granados explained to me that the siskins we’ve seen in California likely came down from their core ranges up in Oregon and Washington, where recent drought conditions and widespread fires might have reduced the favored food source, the seeds of pines and other conifers.

So they came here, and other birds fled south to Texas, Georgia, etc., looking for food. At that point, it was “move, adapt, or die,” a common theme when food sources suddenly disappear.

“A lot of us would be thrilled to see pine siskins in our yards, maybe for the first time,” Granados said. “But then when we started to see three or four dead birds, it became ‘What am I doing?’ In a sense, we weren’t doing anything. Salmonella is a common bacteria. The bird could carry it for years, but under a barrage of different kinds of attacks on that species, it can bloom.”

As for bird feeders causing the spread, Granados explains that regularly cleaning feeders is a good idea in any season, but pulling out feeders won’t save any birds during an irruption. The pine siskins are irrupting because that’s what they do.

To birders watching their backyard feeders, it might seem like substantially more birds are dying at the feeders. That’s possible, especially if the birds were without feeding territory anyway and therefore looking for food. But it’s just as likely that siskins are dying in the woods, and we just don’t know it. If a tree can fall in the woods and no one knows, then a bird that weighs half an ounce can certainly die unnoticed.

But the news isn’t all bad. In prime siskin habitat up north, home ranges will be wide open for the young birds that survived there, who will likely do quite well now that there’s less competition for home ranges and food.

And some of the birds that have visited California, Texas, or North Carolina might just stick around. “The pine siskins could decide, wow, this is a habitat we haven’t explored yet,” said Granados. “Mount Diablo has some wonderful pines, and people have planted plines, even if they’re not always native. So in irruptive situations we could have a component of birds decide that they could actually live here.”

If that happens, we’ll have resident pine siskins, at least for a while, because of this one spring when we all saw a lot of dead birds. Or as Granados says, “It would probably have come about due to these territorial changes that come back to this idea of Move, Adapt, or Die.”

You can see Granados’s original article on page 8 of this PDF and read about irruptions generally on the national Audubon website.