A flock of black-headed gulls decides to rest
As darkness falls and thoughts turn to slumber, waves of sleep wash over seagulls huddling against the elements.
This is not poetry, but a discovery made by a scientist who has been studying sleep in bird colonies.
He found that seagulls learn from each other when it is safe to nod off, resulting in "waves of sleep" passing through seagull colonies as the birds enter differing states of vigilance.
This is the first time such behaviour has been documented.
The work is reported in the journal Ethology.
It was not obvious if temporal waves would occur. It had never been documented before
Dr Guy Beauchamp
University of Montreal
Like many other species, seagulls open and close their eyes periodically while sleeping. That allows them to monitor what is going on around them while they are resting.
"But not to the extent that they could if they were awake," explains Dr Guy Beauchamp of the University of Montreal, Canada.
So sleeping is risky, as it makes the birds vulnerable to predators.
Yet, until now, it has not been clear what information seagulls use to decide when to sleep.
For example, do they base the decision on their own experiences, or do they monitor what other seagulls are doing?
If many birds are sleeping, that may be a sign that it is safe to nap; equally, if few are sleeping, a seagull may decide that it will be more vulnerable to attack if it is asleep while more vigilant group members are awake.
Dr Beauchamp investigated this puzzle by studying how the sleep patterns of seagulls (Larusspp) change over time at loafing sites in the Bay of Fundy, New Brunswick, Canada.
Scientists are still struggling to understand exactly how many species there are of Larus gulls, the group that includes Herring gulls and lesser-black-backed gulls
He noted how often individual birds slept within a colony over fixed periods of time.
"Sleeping is easy to score because gulls usually sleep with their bills tucked into their [feathers]. Every minute or two, I calculated the proportion of sleeping birds in the group."
These counts revealed that gulls with more alert neighbours opened their eyes more often while sleeping.
"So seagulls do pay attention to what their neighbours are doing, and adjust their sleep pattern accordingly," he told the BBC.
What is more, as the gulls tended to copy the behaviour of their neighbours, Dr Beauchamp recorded waves of sleep passing through the colony, with the proportion of sleeping gulls rising and then decreasing through time.
"It was not obvious if temporal waves would occur. They are predicted to occur when copying is important, but it had never been documented before," he says.
Dr Beauchamp's results add weight to a growing view among biologists that vigilance in animals is a social phenomenon.
Individual animals adjust their behaviour - for example by deciding when to sleep - according to their own perception, but also in response to information gleaned from the behaviour of their companions.Such behaviour then leads to a collective phenomenon, in this case waves of sleep.