The word “invasion” makes it sound sinister, but when an unusually high number of a single bird species show up in an area, birders call it an invasion or “irruption.” And this winter, Chicago is seeing a lot of long-eared owls, according to Field Museum Ornithologist Josh Engel.

Engel joins Phil Ponce on Chicago Tonight to fill us in, with stunning owl photos and video by Chicago photographer Rob Curtis.

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

(Courtesy of Rob Curtis)

Why so many this year? It’s most likely because of the available prey, says Engel. Perhaps an abundance of rodents in their breeding grounds farther north led to a bumper crop of young long-eared owls, or more food here may have made this a particularly good place to winter this year.

The milder winter is likely not an explanation, he says.

Long-eared owls typically eat smaller rodents, like white-footed mice and voles. Adult rats are too much for them, but they will gladly eat a young one, says Engel. Owls eat their prey whole and then cough up pellets filled with indigestible bones and hair, allowing researchers to analyze their diets.

If you do come upon a long-eared owl, Engel says, it is fine to enjoy it briefly, but then back up. It's no problem to watch from a distance, but if the bird is staring at you then you're probably disturbing it.

Photographer Rob Curtis's video below of the two owls in a pine tree is a great example of what relaxed owls look like: sleeping, preening and generally ignoring the observer. You can see more of his videos here.

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