Gulls are so common that it can be easy to overlook them, except in the case of the occasional rarity. Yet they are fascinating creatures with complex social behaviors. A recent paper documents how groups of gulls watch for potential predators.
Within any given flock of gulls, we may see some birds standing with their eyes open, some eating, some sleeping, and others preening. The last three activities, of course, reduce any individual gull's ability to watch for potential danger. To remedy this, it appears that gulls engaged in one of those activities monitor what their neighbors are doing for cues.
The paper's author watched groups of Ring-billed, Herring, and Great Black-backed Gulls to observe how they changed their rest states in response to nearby gulls. While gulls slept, they opened an eye from time to time to see what their neighbors were doing. If the two nearest gulls were asleep, a gull would spend more time sleeping. On the other hand, having nearby gulls engaged in active scanning would lead a gull to scan for danger as well. The farther a gull was from its closest neighbors, the less likely it was to step and the more likely it was to scan. Gulls also spent less time sleeping when they were in larger flocks.
This may explain why flocks of gulls are so quick to depart once a few of their members depart. For a birder, this can be a frustrating behavior; possibly rare gulls are just as likely to leave as the common ones once a few spooked birds take flight. For the gulls it helps them avoid becoming a meal for potential predators.