Magic carpet

On 6 September 2012, I was conducting a bird survey on Salmon Creek Beach.  Right away I could see an unusually large number of sandpipers on the beach.

 

While scanning ahead in preparation to count these birds, I noticed an intriguing sight offshore.

 

A very large flock of shorebirds was in a tight formation flying low and quickly over the water.  I'm guessing it consisted of thousands of sandpipers.  They didn't show any sign of heading towards the beach — unusual behavior at high tide, when many sandpipers are resting after feeding during low tide.  

I was puzzled about why they were staying so far offshore.  And why they were staying so close to the surface of the ocean.  Suddenly, all of the birds on the beach took off at once and a potential answer was revealed.  Can you find the clue in the photos below?

 

 

The larger bird above the shorebird flock in both photos is a Peregrine Falcon.  Here's a closer view where you can see the gray plumage and dark hood.

 

I'm guessing that the falcon had been actively pursuing shorebirds along the beach, and that the offshore flock was trying to avoid this fast-flying raptor that specializes in capturing birds.

The nearshore flock eventually settled down again.  The birds seemed pretty tired and immediately got into resting positions upon landing.  I was fortunate that they landed very close to me, resulting in some nice shots of the dominant species — Western Sandpiper (Calidris mauri).

 

 

 

 

The falcon came back several times, and on each occasion the sandpipers flushed, wheeling back and forth across the waves.

 

 

When I ended the survey and was leaving the beach, I heard the call of a Peregrine Falcon and turned to see the flock over the water once again (see next photo).  This might be tricky, but can you spot the falcon?

 

[The falcon is the dark bird at the top edge of the photo (left side), silhouetted against the white swash zone.]

Today I started to research coordinated flocking behavior in shorebirds.  The first paper I found was called, 'Magic carpet' flight in shorebirds attacked by raptors on a migrational stopover site (Michaelsen and Byrkjedal 2002, Ardea 90: 167-171).  

The authors described a very similar type of behavior with Dunlin and Common Ringed Plovers avoiding Peregrine Falcons in Norway.  The flocks "rushed in straight line out over the sea, forming a flat 'carpet'...this flight pattern seemed to prevent Peregrine Falcons from effectively using their preferred stooping mode of hunting." 

'Magic carpet' was a perfect description of this flight formation and behavior in Western Sandpipers!