I Saw a Mobbing in Canada

Last weekend, Jill, Sara, and I took a mini-road-trip up to Thunder Bay in Ontario to spend Easter weekend with Jill's boyfriend, Peter, and some of their awesome Canadian friends. The weather was unbelievably terrible, so most of the weekend was spent indoors watching Game of Thrones, or in various Canadian coffee shops, restaurants, or markets playing cards and hanging out.

This is what a Canadian Easter looks like (photo credit: Sara Sloan)

That's okay, though, because we got a huge dose of nature awesomeness during our drive north. Shortly after crossing the border (I successfully managed not to say anything stupid to the border agent), we noticed what is becoming a very familiar sight this year: a great gray owl perched on the road-side telephone wires. That trip marked my fifth and sixth individual great gray owl sightings of the year! Naturally, we pulled to the side of the road to get a better look ("classic facial discs" -Jill). All of the sudden, two ravens came flying out of nowhere, dive-bombing the formerly peaceful owl. The owl immediately flew to a different perch, but the ravens were persistent, flying at the owl no matter where it flew. Eventually, the owl ended up flying down to the ground under the cover of some trees, and the ravens settled down, but remained perched nearby, seemingly ready to pounce should the owl attempt to relocate again.

As surprising an event as this was to witness, I was even more surprised to discover the birds mobbing owls is a fairly common event. Given that owls regularly eat songbirds, the mobbing is a defense mechanism when an unwanted owl is hanging around. Some researchers have managed to incite mobbing behavior in songbirds by simply playing an owl call in their vicinity. Turns out that corvids (ravens and crows) are both the most common mobbing culprits and also the most vicious; the "guarding" behavior that we observed is normal. This is also why one of the names for these groups of birds is a "murder" of crows and an "unkindness" of ravens.

Mobbing potential owl predators offers three benefits for songbirds. First, it will hopefully result in the removal of a harmful predator. Second, it is a way for parent birds to teach offspring which birds are dangerous predators. Third, if young notice this behavior is happening, it will typically act as a warning sign for them that a predator is nearby and they should take appropriate caution.

When songbirds want to initiate a mob, they will give a distinctive call, which acts as a "call to arms," harkening to the other birds in the area to assist in the mobbing. Birds of all sizes have been observed to participate: an Anna's hummingbirds (4.3 grams) was once observed mobbing a spotted owl (1.3 lbs), a 142-fold difference in size (for a comparison, think of a very small child poking an elephant in the leg with a stick). If owls are being mobbed by "harmless" birds, they will often ignore the event, sometimes even sleepily closing their eyes until eventually the mobbers give up.

In a study by Frederick Gehlbach, 60% of mobbings on screech owls happened during the breeding season. This is the time of year when owls are feeding themselves AND fledglings, so the rate of preying is significantly higher. Also, resident and wintering birds were found to initiate a mobbing more frequently than migrants - an owl is a greater threat to a bird that lives near it permanently than one that is simply passing through.

A couple days ago, I hiked up to Caribou Falls, a short one-mile loop of the Superior Hiking Trail. The ice is finally starting to break up and flowing water is beginning to appear again. While watching the half-frozen water fall, I once again saw some ravens ganging up on a dangerous predator, this time an immature bald eagle. Demonstrating very similar behavior to what I saw in Canada, they continued to launch themselves at the eagle until it eventually flew away.

This is what April in Northern Minnesota looks like.