By John Upton
The durations of days and years are calibrated by celestial turntables: The spinning of the Earth and its arcing around the sun. Humans and wildlife alike live out rituals according to daily and annual schedules.
But the seven-day week is a human construct. It’s an arbitrary chunk of time that cocoons timetables of work and rest, of television programming and soccer practice. Whenever you see wildlife falling into a weekly routine, you can be confident it’s the result of a human influence.
A weekly schedule plays out among European herring gulls and lesser black-backed gulls nesting in the dunes of the Dutch island of Texel. And it’s a macabre one.
A chick being reared in these dunes may dread Sunday more than a young atheist dreads their mandatory church outings. It’s on Sundays that adult gulls are most likely to cannibalize the young. Saturdays are also popular chick-eating days among the Texel gulls, though not to the same extent as is the case on Sundays.
Sometimes the gulls eat their own chicks — or their own eggs. But more often they steal the unattended young of other birds, in some instances to be shared with their own hungry broods.
That’s not the only weekly pattern that marine ornithologist Kees Camphuysen has discovered during his studies on the island. Chicks tend to grow in spurts during the week, then their growth slows down over the weekends.
The Royal Netherlands Institute for Sea Research scientist thinks he knows what’s going on. He contends that it’s the weekly patterns of the region’s beamtrawlers and shrimpers that are driving the hebdomadal trends.
“[A] very strong weekly pattern in fleet size occurred, with high numbers of boats at sea Monday through Thursday, a much reduced number (mostly homeward bound) on Friday, and near to nothing on Saturdays and Sundays,” he wrote in his Ph.D. thesis.
The Texel Dunes gulls feast on the by-caught scraps of the fishing fleet, trailing the boats to scavenge protein for themselves and for their growing chicks. But when this supply of human surplus dries up over the weekends, the chicks’ growth rates slow, and hunger can drive the birds to cannibalism.
“Only commercial fisheries have a periodicity that can explain the strong, cyclic synchronisation in chick growth,” Camphuysen wrote. “Chick cannibalism rates were a mirror image of the rhythmic cycle in growth increments.”
The following series of photographs was published in Camphuysen’s Ph.D. thesis, showing an attack on an unattended chick by a bird from a nearby nest. The attacking gull can be seen sharing the kill with its own chick. If you would prefer to not see an adorable lesser black-backed gull chick being pecked to death and gutted by its own kind, then stop scrolling now.