The crows seemed to use the spikes differently, turning the sharp pins toward the interior of the nest. Although the idea remains unproven, positioning the spikes this way might provide the nests with more structural support, Mr. Hiemstra speculated. It is not entirely clear whether the birds are simply using the spikes because they are available — in the urban wild, they might be easier to come by than thorny branches — or whether they might be even better suited for the job than natural materials are. But the use of artificial nesting materials is common across the avian universe, according to a new review of the scientific literature by Dr. Mainwaring and his colleagues, which was published in the journal Philosophical Transactions of the Royal Society B on Monday. They found reports of tens of thousands of nests — built by 176 different bird species, on every continent except for Antarctica — that contained artificial materials, including plastic bags, cloth straps, fishing line, paper towels, dental floss, rubber bands and cigarette butts.

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Persons: Hiemstra, Mainwaring Organizations: Royal Society