Tuesday, November 11, 2014

Predators and Prey

Bald Eagle adults in nest tree
      Today my sister Nancy, our dad and I took a walk around the middle part of the Deschutes estuary.  This is one of our favorite places and we visit it often in fall and winter.  The walk allows for a great view of a Bald Eagle nest on the eastern shores of the estuary.  Today we lucked out and saw the pair of eagles in their nest tree, perched high in a Douglas fir overlooking the estuary.  Their white heads gleamed like brand new tennis balls in the late afternoon sun.  Just underneath the tree, in the waters of the estuary, floated a variety of overwintering ducks.  I remarked that it was a great place for an avian predator to sit and survey the dinner table below, checking out which ducks would make a tasty midday snack.  This reminded Nancy of an experience she had had.  

     A few days ago she was also at the lake, taking a leisurely walk.  Suddenly she noticed a crow flying in big swooping loops, screeching at top voice, with a flight pattern which seemed to be directed at one spot on top of a nearby cedar tree.  She  immediately stopped her walk and started scoping out that tree:  as an experienced nature watcher, she knew the crow was zeroed in on something that Nancy had not yet seen.  The crow was absolutely right:  here’s what it showed Nancy:

    This is an adult Bald Eagle and most likely one of the mated pair that we saw in their nest today.  At this time of year the young eagle fledglings that were hatched last April are moving into independent life, so this adult is back to its usual winter schedule, perching near the lake edge,  looking for any late salmon in the fish runs up the Deschutes, or going after one of the wintering ducks on the waters of the impounded river. Or when that fails, waiting for one of the local Peregrine Falcons to make a duck kill, then forcing the hunter to dropping its catch.  (This is a mode of hunting called kleptoparasitism.)  

     This eagle is directing a fierce look at the crows who are harassing it. It is clear to me that the eagle would loooooove to get its talons on the taunting crow, but though the crow is smaller it is also far more maneuverable: it can easily evade any eagle attack.  It’s like a Volkswagen beetle dodging a Boeing 727 : this is no contest.  Now if the crow is sick, slow or stupid, all bets are off.  If I were the crow in question, I’d be pretty damn careful:  the look in that eagle’s eye gives me the willies.

     But the crow is not being foolish.  It knows that Bald Eagles eat crow babies and like all crows, it is born knowing how to make life uncomfortable for such predators.  So there is method in this crow’s madness.

      Over thirty years of birdwatching, I have seen similar examples of birds who respond to and give warning of the presence of predators.  One of my favorite examples comes from this same lake.

     I’ve been watching winter ducks at this place for many years.  It often happens that I’ll be watching a beautiful pastoral scene of quiet ducks, nibbling away at vegetation, dabbling for food.  Then all off a sudden every single duck on the lake rises up in screeching cackling cacophony, beating wings high and fast for distant parts, running for their lives.  When I see that behavior I start looking for the predator.  Today we saw exactly this at the Deschutes estuary:  we were
enjoying a beautiful view of the ducks on the water when all of a sudden they got really restless and took off in panicked flapping flight.  We all started looking for a predator and bingo!  both of the Bald Eagles were had left their nest tree and were flying over the lake to hunt that snack.  And the ducks knew it.

American Widgeons in flight
    One another occasion I was leading a field trip for a class of beginning birdwatchers.  We went to our friend Sherry’s house, where she had a fabulous feeding station set up on the edge of wetlands and woods.  We parked our butts on some lawn chairs and watched the activity:  we saw the usual winter guild of black-capped and chestnut-backed chickadees, red breasted nuthatches, songs sparrows, juncos and etc.  The chickadees were especially active, darting in and out of the tube feeder, grabbing a sunflower seed and taking it off to a private cache.  I was in teacher mode, identifying each bird and describing field marks.  Suddenly,  all the chickadees stopped what they were doing, rushed en masse into a nearby thick shrub and started making LOUD alarm calls.  This was clearly a teachable moment:  I told the students to start looking for some sort of predator, probably a sharp-shinned hawk.  And sure enough, lurking in the woods off to one side, we finally saw the sharp-shinned hawk.  But the chickadees had seen it long before we did and took steps to save their lives.

     Then there was the time I was out at Cape Lookout on the Oregon coast.  The Cape has tall cliffs in which gulls nest as a group during spring.  Bald Eagles are infamous for raiding these colonies and making off with shrieking gull chicks.  Not surprisingly, gulls hate eagles.  During one April visit to the Cape, I watched an immature Bald Eagle.  It had been perched low on the cliff and was attempting to take off in windy conditions.  As a novice bird it was struggling in the wind and some nearby gulls noticed this.  They grouped up and mobbed the eagle, striking repeatedly at it and doing their best to drive it into the sea, where it would most likely drown.  I watched for about ten minutes as  they very nearly pulled it off.  The young eagle came perilously close to the high surf crashing against the cliffs before it managed to pull itself up and away to safety.    
     We have no personal pictures of this but here is a link to a fabulous photo/story from Europe, where White tailed Eagles prey on Herring gull colonies:  http://io9.com/5856819/see-gull-take-on-eagle-in-mid-air-piggyback-attack

     There are all kinds of ways to be a birdwatcher.  One of my favorite ways is to watch behavior and in doing so, allow the animals to show you their lives and their concerns.  Today at the Deschutes estuary, we got an eyeful .


Resources:  all photos by Nancy Partlow

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