Frolicking Gulls

The Illinois Ornithological Society held it's 13th annual (where have the years gone?) Gull Frolic, as always at the Yacht Club at North Point Marina. So, in honor of that event:

Slaty-backed Gull (Larus schistisagus), North Point Marina, Lake Co, IL 2/15/2014

This Slaty-backed Gull (Larus schistisagus)is a rare visitor to most of the country, and only the first one ever seen in Lake County!

Here he is in context:

Gull spp., North Point Marina, Lake Co, IL 2/15/2014

Gulls, of course, are very social critters, and like to hang out in large groups, both when nesting and when wintering. Watching them at a favored feeding site, it's amazing how much competition there is for food -- if you see one of them with something choice, it's pretty much guaranteed to have a following of several birds trying to steal it. So why hang out together?

Well, this does mean more eyes looking out for predators, which isn't a bad thing at all. But is that enough to overcome the competition problem? In some species, the answer is clearly yes. With gulls, there's likely to be something else going on. Many social species rely on food sources that are abundant, in very local patches. So finding a patch means there's more food than you need, but your chances of finding it in the first place improve with additional sets of eyes.

Exactly what drives this has been somewhat controversial. Early suggestions that unsuccessful foragers were following more successful ones to new food sources (the Information Center Hypothesis) haven't held up too well (1) . However, it seems that those authors arguing against the idea assume (1) that successful foragers are capable of hiding that they were successful, and thus must be intentionally advertising their success, and (2) that successful foragers don't have any other reason to return to the colony or roost (an obvious oversight when dealing with nesting colonies). It may be that Ward and Zahavi made that argument in 1973 (2) , but I don't see that it's really necessary. If foraging success is essentially random, birds aren't capable of hiding their success, and there's no noticeable cost to being followed, then the advantage to social roosting should be clear.

Of course, the location of a useful food patch may not be the only sort of information being exchanged here.  The idea of local enhancement (birds simply spotting other birds foraging and joining in, without gaining any information at the roost site) seems obvious -- we can see this happening with gulls. But for this to work, birds have to be foraging within sight of each other, and for it to be most effective, they should be dispersed in a non-random way to start with. And what better way to figure out where you should look while keeping your fellows in sight than to start with them? In fact, en masse departure to foraging grounds should enhance foraging success by this argument, suggesting another means by which a social roost could be an information center.

I doubt that the gulls spend their roosting time in deep discussion about the evolutionary origins of their behavior, of course, but that behavior does provide some wonderful moments of observation for us birders. And when you find a group of birders around, you can bet that you're looking at an information center in the truest sense!

(1) Richner, H., & Heeb, P. (1995). Is the information center hypothesis a flop?. Advances in the Study of Behaviour, 24, 1-46.

 

(2) Ward, P. & Zahavi, A. (1973). The importance of certain assemblages of birds as "information centers" for food finding. Ibis 115:517-534.