What comes to mind when you think of a bird’s nest? Twigs, probably dry leaves, maybe some cattail fluff—perhaps even cigarette butts. Industrious creatures, birds construct their nests from just about any material they can get their claws on. From koala fur to shreds of plastic, if birds can make a nest from it, they will. For birders, these indiscriminate construction practices provide a fun opportunity to take part in the nesting process by providing raw materials for birds to collect and use. Problem is, many of the items birds find on their own or that are provided by people pose risks to them and their chicks. What's more, which items are potentially dangerous isn't always obvious. Take it from Jennifer Gordon, executive director of Carolina Waterfowl Rescue, a center in North Carolina that cares for everything from donkeys to injured songbirds. Gordon says her team treats dozens of adult birds and chicks every year due to hazardous nesting...