Background Nests are composed of and built with different materials that are handled in specific ways. These mate? rials must initially have been used de novo before commonly being incorporated into nests. Plastic and plastic bags were invented in the 1950s, and they are widely distributed in the environment. Birds started picking up plastic from plastic used to cover farm produce such as silage, potatoes, beets and other crops for use in their nests in the 1960s.

Methods I recorded the frequency of such plastic use by the Blackbird (Turdus merula) in nests in Denmark, starting in 1966, followed by a peak in use in the 1970s and a subsequent decline.

Results Nests with plastic were initially built earlier in the season than those without plastic, indicating an associa? tion between innovation and early reproduction. Plastic use was subsequently selected against because nests with plastic suffered from higher rates of predation than nests without plastic, probably because nests with plastic were easier to locate. However, the elevated risk of nest predation only applied to outdoor nests, but not to nests inside buildings, probably because visually searching nest predators such as corvids do not enter buildings.

Conclusions These findings show that the dynamics of nest material use depend on the benefits of innovation and the fitness costs of nest predation.