Tuesday, February 15, 2011

Snowy Egrets Feeding with Ducks, Grebes and Cormorants in Morro Bay

More than just ticking species off,  I like to see bird behavior that is new to me on my daily birding. For example, I have observed Snowy Egrets feeding with two species of ducks as well as cormorants and grebes in Morro Bay.  On 3/1/10, I watched a Snowy Egret feeding with an adult male Red-breasted Merganser (pictured). The merganser fed by swimming below or mostly below the surface of the shallow water and the egret followed it, apparently looking for animals stirred up by the duck. In turn, when the egret stirred up sediment by moving it's foot around, the merganser came over and appeared to be watching for anything brought up by the egret. Both were clearly following the other and feeding off the efforts of the other species (mutualism). I watched this for about ten to fifteen minutes in Cuesta Inlet before walking on.

Cattle Egrets feeding on insects stirred up by cattle is well known, but this was the first I had seen or heard of a Snowy feeding cooperatively with ducks. I did find an article on Little Blue Herons commensaly feeding with White Ibis (The AUK 95: 667-681, October, 1978). Eric Johnson, a friend and former Cal Poly ornithology professor, referred me to an article published in a late '60's ornithological journal (The Auk, Condor, or Wilson's Bulletin) on dozens of egrets and herons feeding with Red-breasted Mergansers in Florida.

On 2/21/10, and again on 3/3/10, I observed a Snowy Egret (the same one?) standing next to a group of Blue-winged Teal (pictured below) at Cuesta Inlet. It was clearly watching for anything it could eat as the teal moved their bills through the sediment both below and slightly above the water level.  It was a low tide both times. The teal appeared to ignore the egret both times I observed the egret feeding next to the teal. It appeared to be a very one way relationship (commensalism) as the egret ate things stirred up by the teal, but it contributed nothing and the teal appeared to be oblivious to the egret's presence as they were concentrating on their feeding in the muck.

On May 23, 2010,  I observed a Snowy Egret feeding with two Double-crested Cormorants. I was at the Audubon Overlook in Los Osos at low tide and numerous birds were feeding in or around the channels of water that snaked through the exposed mudflats at low tide. The Snowy Egret appeared to be following the cormorants as the cormorants swam in the channels. The Snowy would follow on the dry land next to the channel (walking and flying to keep up with the cormorants) and occasionally go into the water after something near the cormorants. There were Great Egrets nearby that were not behaving in this manner. I watched this for about twenty minutes and then I left.  It appeared that the egret benefitted from the cormorants, but I did not see the cormorants follow the egret at all.

On February 11, 2011,  I watched a Western Grebe follow a Snowy Egret around as it stirred up sediment with its feet.  The grebe would watch and occasionally go under water in the direction of the egret, when it was actively moving a foot under water. The grebe was totally submerged or partially so when it was under water in the vicinity of the grebe. This was at Cuesta Inlet in Morro Bay and at a higher than average tide. It went on for about ten minutes and then the Western Grebe stopped following the egret.

On March 14, 2011, I again observed a Double-crested Cormorant and a Snowy Egret interacting. I was on my morning walk along Cuesta Inlet (Morro Bay) when I saw an adult Snowy Egret and an adult Double-crested Cormorant following each other around at low tide in a tidal channel at about 7:45 AM.  The cormorant would swim under water and the Egret would follow.  The egret would stir up sediment with its foot and the cormorant would come over and feed.  They both fed several times during this behavior.  This lasted about ten minutes and then they went their separate ways.

On March 26, 2011, at about 9:30 AM, an adult Snowy Egret was feeding with two female Red-breasted Mergansers in the water near the edge of the main channel from Cuesta-by-the-Sea into the bay. The mergansers and the egret seemed to be following each other around, all three feeding.  After about five minutes the egret flew over to another egret and left the ducks.  After a minute or two of crest raising and flapping of wings by the egrets, the same egret went back to one of the mergansers and they resumed feeding together.  The merganser and the egret started going up a shallower side channel.  It was shallow enough that when the merganser swam under water you could always tell where she was by turbulence on the surface above her.  As the merganser worked further up the channel, the second egret joined them - one egret would feed on either side of the merganser, sometimes snagging fish within inches of the side of the duck.  It looked like the egrets were harassing the duck, but several times when he got away from the egrets the merganser would return to them. The egrets were walking around in the water, but I saw none of the foot shaking feeding technique often used by Snowies.

They all seemed to be doing well at catching fish and two more Snowy Egrets flew in.  Now the merganser was really surrounded as she worked further back in the shallow channel.  The egrets mostly stayed within two feet of the merganser.  The duck seemed to be doing most of the work of stirring up fish from the bottom and then the egrets and the duck would feed.  This went on for about ten minutes, with one of the egrets leaving and the remaining three following the merganser about 50 yards back to the main channel, where the merganser went into deeper water and the joint feeding ended.  All of this observed behavior occurred in a twenty minute period. Great Blue Herons and Great Egrets were in the vicinity, but they were not feeding with the mergansers or any other birds.

On April 5, 2011, Snowy and Great Egrets were out in number near and in Cuesta Inlet of Morro Bay.  Tide was low and egrets and Double-crested Cormorants were feeding in the channels.  Starting at about 7:30 AM I watched three Snowies feeding as they followed three cormorants as they dove and fed while swimming under water in the channel.  This went on for about ten minutes, during which time one cormorant flew off and two egrets continued their pursuit (pictured).  About two hundred yards away, five different Snowies followed a single cormorant as it fed in another channel.  Five nearby Great Egrets did not join the pursuit, although one flew over the cormorants and another Great Egret landed nearby.  So, at least eight Snowies in Morro Bay engage in this type of behavior and Great Blue herons and Great Egrets are in the vicinity during such joint feeding, but they do not appear to join.  I watched for about ten minutes before I had to go.

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