Massive southern invasions by northern birds linked to climate shifts

I had no idea this even happens, and when I saw the headline I thought that it was odd because I would have thought climate change would have driven the southern birds north as the northern climate started to agree with them more. But on reading the article, it all makes sense now.

Previous studies have found evidence that irruptions are triggered by food shortages caused by the large-scale collapse of seed production in northern pine, spruce and fir forests.