Friday, July 11 2008 @ 04:22 PM EDT
Contributed by: karenbills
The following questions were sent to David Hancock from Chris Rickwood:
I’ve just seen a spectacular wildlife event. It was about 2pm on Tuesday 2008-Jul-08 on White Rock’s East Beach. I heard a bird at the edge of the water giving a loud cry which sounded like “craaak”. The bird was a blue heron. It was being attacked by a bald eagle. The heron flew along the coast for a short distance and then it seemed to have found a thermal. It started circling upwards with the eagle following. By this time, two other birds had joined the fray. One was a crow and the other was a gull. Both the crow and the gull started harassing the eagle. They buzzed it but didn’t seem to make actual contact. (The eagle completely ignored them.) The heron kept rising in the thermal and slowly the eagle fell behind. Eventually the eagle gave up the chase and the crow and the gull flew away.
This raises some interesting questions: Was the warning cry given by the blue heron a universal cry which all bird species respond to? Are all birds capable of giving this same warning cry? Why would a crow and a gull come to the assistance of a blue heron? Do all birds assist other bird species when they are under attack? Was the bald eagle trying to kill the blue heron or trying to force it to regurgitate its prey? Does the bald eagle make a habit of trying to force other birds to regurgitate fish (like a frigate bird)? How can a heron out-climb an eagle? - Does it have more powerful wings or a larger wing-area or a more efficient glide?
Here is David Hancock's reply:
RE: Eagle / Heron / Crow / Gull!
Great observations and well told.
I think what you witnessed is quite typical but getting all the elements together and witnessed -- quite an opportunity.
Bald eagles will hunt herons but I think you hit the nail on the head. I suspect the eagle saw the heron nab a fish and attempted to cause it to be given up. Much of the eagles' summer food is derived this way. Usually they watch the crow find the intertidal midshipmen in the grass and, when the crow reveals a big enough one, the eagle swoops in and takes it. They regularly do this with herons as well.
At the dump where the gulls and crows do the scavenging for food it is the same. When they turn up a large item they can't immediately swallow, one of the eagles is in like a bird dog to steal it.
With the small turning radius the crows can attack the eagles with impunity. They have learned that they are vulnerable in the 4-6 days before fledging and never forget that experience.
You are quite right about the calls. Just as I always know the call of a gull when it calls out, "Eagle", other birds will be even more sensitive to this.
In your incident, like most, the eagle seems to have not succeeded. But enough eagles, crows, gulls and herons seem to succeed so that all are surviving and doing very well.