As it happens with many birds, NBI gather nesting materials from the neighbourhoods and those available materials are sometimes rubbish. Thus, plastic bags and other refusal can be found inside the nests.
Occasionaly, some of those materials are fishing lines made of nylon, wich is a very longlife material.
Unfortunately, this choice has very dramatic consequences on the survival of chicks.
Every year some fledgings cannot filght and die of starvation or are even strangled