Commensal feeding: Black swan and Eurasian Coots

A Black Swan (Cygnus atratus) was seen feeding by upending its rear to stir up food items at Lake Pertobe. The black swan was attended by a party of three Eurasian Coots (Fulica atra) made up of a mother and her two juvenile chicks. The coots swam around the swan excitedly, picking up aquatic plants and other small preys (probably insect larvae and small fish). This is known as commensal feeding as the beater (swan) assists the attendants (coots) in their feeding frenzy. The swan does not incur any losses as the coots take this opportunity to feed on food items not within reach of the swan . The attendant birds are able to obtain more food in this situation than if they had fed independently by diving into the water. At this stage the chicks are only able to pick up food materiials a little below the water surface. This observation is commonly seen among shore and marsh birds around the world.

The water level at Lake Pertobe was at a height of 0.5 m when Wong Kais filmed the occurrence on 21 March 2023. View the videos below.

Video 1 was taken on 21 March 2023. Lake Pertobe. © Wong Kais.

Video 2 was taken by Wong Kais on 18 March 2023. Lake Pertobe.

Image 1: Water level at Lake Pertobe, Warrnambool. 21 March 2023. Wong Kais.

Image 2: Mother coot and one of her chicks stayed close to the feeding swan. The chick had picked up a piece of food and mother coot was looking on. Lake Pertobe, Warrnambool. 21 March 2023. Wong Kais.

Image 3: Mother coot and her two chicks stayed at the back of the dabbling swan to wait for food stirred up from the mud. Lake Pertobe, Warrnambool. 21 March 2023. Wong Kais.

Image 4: Mother coot feeding a chick after picking up food material stirred up by the swan. 21 March 2023. Wong Kais

Image 5: Another view of mother coot and one chick staying very close to the swan. 21 March 2023. Lake Pertobe, Warrnambool. Wong Kais

Video 3: Later in the day, Wong Kais noticed the three chicks commensal feeding with the Black Swan in the mother’s absence. The largest chick took on the role of leader and guardian of the two younger chicks. Lake Pertobe, Warrnambool. 18 March 2023.

Posted by Teo Lee Wei.

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