Some time ago at Roath Park Lake I was intrigued to notice Black-headed gulls using Coots as their personal fishers, and I have since noticed this behaviour between the gulls and Tufted ducks as well, though I haven’t yet seen them using this feeding method with the other water bird species at the lake.
These gulls eat different food from the coots and ducks – gulls eat insects, worms, scraps, etc whereas the ducks and coots are mostly vegetarian, though the tufties will also eat some insects – so, presumably, the gulls aren’t actually stealing food the other birds would eat themselves.
The ducks and coots dive down for food, while the gulls float patiently waiting for them to surface. When they do, the gulls paddle over to see if there’s anything interesting for them to steal. The coots and tufties don’t seem to object to the gulls, even though they’re doing all the work. I guess they would be doing the diving anyway and, if the gulls aren’t actually stealing their food, then they’ve nothing to lose. The official term for creatures that take the food other creatures have caught, prepared or collected is kleptoparasitism, hence my title.
When researching this post, I located an excellent article that goes into much more detail than I have about this and similar bird behaviour: ‘Food usurpation by waterfowl and waders’ by Juan A. Amat can be found here.