Think about some of the plastic litter you ve seen along the shore - fast food containers, foam ice coolers, plastic foam surfboards. Eventually they break down into tiny pieces. Sea gulls often can t tell the difference between plastic pieces and fish scraps so they often ingest trash.
Finches have similar problems with seeds and plastic.
Feeding can be a very frenzied activity. Birds are likely to swallow before they taste. They can t digest plastic. So plastic just builds up in their stomachs taking the place of real food. Birds slowly starve.