Thursday, May 23, 2013

Bob!!!! - The Case For and Against Mobbing Behavior

This is the fourth post from Karyn and my recent trip to Yellowstone National Park. A few more to come so check back!

Post 1: Eye to Eye - Sandhill Cranes
Post 2: Bullying the Bully - 3 Coyotes v. 1 Wolf
Post 3: Under the Gaze of the Phantom of the North - Great Gray Owl!

Upon our arrival in the Lamar Valley, we observed a bison carcass in the middle of the valley. This bison appeared to have died of natural causes as the carcass was largely intact. A bison carcass has the potential to be the center of the wildlife action for many days. As a result, we spent a lot of time watching and waiting for the big predators to come in. The wolves, one at a time, eventually arrived. The bears were all spotted up high and not on the valley floor. The carcass was visited regularly by a local coyote pack, Bald Eagles, Common Ravens, and Black-billed Magpies. The Bald Eagles had a nest in the valley which we could also observe - with two nestlings!

Bald Eagle, Lamar Valley, Yellowstone National Park.
Bald Eagle in nest with 2 nestlings, Lamar Valley, Yellowstone National Park.

The bison carcass provided a great opportunity for our national bird / national animal to exhibit its style by sticking its head in the butt of the bison. Since no large predators had yet found the carcass, most of the meat remained locked inside the tough hide. Only bears and wolves have the ability to "open" the carcass. Until they arrived, only the butt was available to the birds and coyotes. Aren't we impressed!

Two Bald Eagles on Bison Carcass, Lamar Valley, Yellowstone National Park.
Two Bald Eagles on Bison Carcass, Lamar Valley, Yellowstone National Park.

This brings us to the topic of mobbing behavior. Many small birds are known to mob larger birds, particularly predators. This may seem illogical, but in most cases the smaller birds are quicker to respond and can easily stay outside of the reach of the predator. Theory suggests that it is better to know where the danger is than to just know that there is danger. This is why birds fly toward alarm calls instead of away. Also, in many cases, the smaller birds can chase off the larger birds. The same theory as my previous post of three coyotes attacking a lone wolf.

While waiting in the Lamar Valley for the predators to arrive at the carcass, the local Bald Eagle took a pass by the carcass. This pass caused over 30 Common Ravens to rise from the carcass and begin mobbing the eagle. The birds swarmed around the eagle, tagging it in flight. The ravens essentially forced the eagle to the ground. Just as the eagle hit the ground, the ravens began landing as well. One landed a bit too close. In an instant, the eagle was on the raven. "Cousin Bob" was done for. The ravens screeched in horror and a few attacked the eagle grabbing his / her wing and tail. This continued until the eagle began plucking the raven. With feathers flying, apparently the remaining ravens determined that "Cousin Bob" wouldn't be returning to the clan. They gave up the fight and returned to the bison carcass.

Bald Eagle eating Common Raven, Lamar Valley, Yellowstone National Park.

After plucking the raven the eagle tried to take flight with the prize. It was not to be. An eagle weighs about 9.5 pounds, while a raven weights 2.6 pounds. This was just too much. The eagle began consuming the raven and then tried again to fly. No luck - still the same weight, just transferred into his stomach. The eagle crapped twice, but this was still not enough. He ate some more and tried again. This time he landed in a small ravine. A water-logged raven wasn't much better. He had to abandon what was left of "Cousin Bob"...

I was asked by some friends as to why the eagle would eat the raven with a full bison carcass available. There are a couple of possible reasons. First, attacking the mob seems like a great way to decrease the mobbing behavior. Second, and possibly more important, is that many raptor species require a diverse diet to survive especially as nestlings. For example, the bird I study, the Northern Goshawk, cannot survive on mammals alone or birds alone. Delivering a raven to the nest would be a great compliment to the Uinta Ground Squirrels that the eagles had been feeding their young.

Now to the question of mobbing behavior. Did the eagle decrease mobbing behavior by killing one of the mobbing birds? In my opinion it's a mixed result. The eagle clearly selected against the mobbing behavior by killing one of the mobbers. However, the "average" raven is now different as a result of this bird being taken out of the population. The "average" raven is likely faster (assuming the slowest bird was removed) and will likely keep a larger distance during mobbing (assuming the closest bird was removed). This is just a single animal, but provides great insight into the basic mechanics of evolution by natural selection. Each animal removed from a population changes the "average" for the population. If the slowest and closest are consistently removed, the population attributes can change fairly quickly.

This is one reason I find wildlife watching and field work so fascinating. It is not always about what you go there to see, but what you see while you are there.

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