[CT Birds] Gulls feeding behavior??

[Boletebill]

     I know that this subject has been discussed before here and I apologise for not paying enough attention to the outcome of the thread but I'm too curious to be embarassed enough not to ask again.
     Today, watching Ringbilled Gulls in the CT river, i was fascinated by their behavior.  A gull would bob his head underwater, lift his head out and shake his bill back and forth and immediately bob under again, over and over.....while doing this the gull would shake his tail side to side to hold his position against the current.  It seemed to be important to maintain position.  If the gull lost his position (about once a minute) the gull would flap his wings enough to lift off and then drift back to his former position and settle back in the water and repeat the head-bob, bill shake, tail wag again, over and over.
     There were a dozen gulls doing this.
     This has the appearence of feeding.  But what are they feeding on? Isopods? Plankton?  This behavior would seem to have a high energy cost for the posibile return if the food was isopods or plankton. Or not?  Or is this some other behavior rather than feeding?
 
I'd like to hear some ideas about this. Thanks.
 
Bill Yule
Clinton CT

"For those who hunger after the earthly excrescences called mushrooms."