Friday, February 06, 2015

Line-Up: Barred Owls, Great Horned Owls, and Bald Eagles

At A Glance
How do our birds compare? While bald eagles are clearly the heavyweights of the group, barred owls and great horned owls are territorial and can be quite aggressive. All three are generalist feeders. Bald eagles build stick nests, while barred owls and great horned owls nest opportunistically.
Barred Owls
Great Horned Owls
Bald Eagles
Length
16.9–19.7 in
43–50 cm
18.1–24.8 in
46–63 cm
28–37.8 in
71–96 cm
Wingspan
39–43.3 in
99–110 cm
39.8–57.1 in
101–145 cm
80.3 in
204 cm
Weight
1.03–2.3 lbs
470–1050 g
2.0–5.5 lbs 
910–2500 g
6.6–13.8 lbs.
3000–6300 g
Diet
Small mammals, birds, amphibians, reptiles, fish, and invertebrates. 
Diet
Small mammals, birds, amphibians, reptiles, fish, and invertebrates.
Fish, birds (especially waterfowl), small to relatively large mammals, reptiles, amphibians, crustaceans, and carrion. 
Nests
Lays eggs January to April (Iowa). Nests in cavities, other birds' nests, squirrels' nests, and nest boxes.
Lays in February (Iowa). Nests in other birds' nests, squirrels' nests, cavities, dead snags, deserted buildings, cliff ledges, and human made platforms.
Lays eggs mid-February to early March (Decorah). Builds large stick nests and nests in them for multiple years. 
Habitat
Mixed forests of large trees, preferably near water.
All over, although they have a preference for spaces that combine open habitat with forest.
Forests of large, mature trees adjacent to bodies of water 

This line-up makes it pretty clear that the area is as desirable to owls as it is to eagles. Like much of the #driftless, the area encompasses forest, blufflands, running water, ponds, and open spaces. Food is abundant and nests and potential perching spaces abound.

We know how eagles and owls interact. But how do great horned owls and barred owls interact? It looks like great horned owls are a bigger problem for barred owls than they are for bald eagles. The good people at Cornell Lab of Ornithology write that ...A barred owl's most dangerous predator is the Great Horned Owl, which eats eggs, young birds, and occasionally adults. A video made by Dragonlainey on the night of February 3rd features the sounds of great horned and barred owls. It opens with great horned owl calling. Instead of the familar pair hooting, this sounds a bit more like a harnk call - another great horned owl call that isn't especially well-understood (this is a guess on my part and I will add more information if I get it). Familiar GHO hooting starts at about 1:05 into the video, and at about 3:15, we hear an unearthly noise that sounds a lot like the alarm call of barred owls as recorded at The Owl Pages.


Could the great horned owls displace the barred owls or vice versa? The jury is out on that question. Cornell writes that barred owls are...Territorial all year round, Barred Owls chase away intruders while hooting loudly. They are even more aggressive during nesting season (particularly the females), sometimes striking intruders with their feet. We've seen the barred owls attack the eagles in the nest before, and barred owls are displacing spotted owls in the northwestern United States, but I couldn't find any evidence of them displacing great horned owls. While Cornell mentions that great horned owls can shift barred owls on their range, I couldn't find any documentation of outright displacement there, either. I did learn that barred owls are real homebodies. In a study of 158 banded barred owls, researchers sound that none of them displaced more than about six miles away. Compare that to D1's 900+ mile odyssey, or even Four's 100-mile journey!

At present, I suspect the owls and eagles will live uneasily together in inter- and intra-order competition. If we can find and reach the barred owl nest, it would be interesting to band the young and study the family's movements now that great horned owls have arrived.

Additional Owl blogs from RRP
Things that helped me learn about this topic
Did you know?
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