The most obvious external factor is weather, where birds are blown off their usual course: this might be a gradual drift or a seemingly spontaneous deviation caused by extreme weather. “They might have no option but to go with the flow and get taken a very long way out of range,” says Lees. “You can’t stop. You stop, you die.” Wind might also cause birds to “overshoot” — they stay on their correct course but keep going further than planned.

Humans can also drive birds outside their normal range, both directly (for example, escaped captive birds, such as the flamingo found on the Ottawa River in 1997) and indirectly (changing climactic conditions could lead birds to extend their foraging range due to a lack of food).

Internal factors involve the inner compasses that birds use to navigate — and the compass errors that send birds wildly off course. Before a migrating bird sets off on its journey, Lees explains, it has to identify a reference point using its inner compass (i.e., north) and then angle itself with respect to that point. Some birds mistake north for south, or east for west, and end up flying in the opposite direction, a phenomenon called “reverse migration.” Others get the first step right, but then orient themselves in the wrong direction, resulting in “mirror-image misorientation.”

Lees cautions that “ultimately, there’s just a lot of uncertainty about what causes these things, and there’s a lack of hypotheses that are being tested.” But, he says, we’re now entering an age where we have the tools and data to understand more.

As of this writing, Canadian birders are once again aflutter at the vagrant Steller’s sea-eagle, which was most recently sighted in Newfoundland. “It’s at its normal latitude. It’s got all the cues it needs there: nice boreal forest, and there are other eagles around, and there’s fish,” says Lees. “Everything is there for it, except another Steller’s sea-eagle.”