Balloons are more deadly than hard plastics for seabirds, according to a new Australian study.
Scientists from the Institute for Marine and Antarctic Studies, based at the University of Tasmania, and the Commonwealth Scientific and Industrial Research Organization (CSIRO), also in Hobart, Tasmania, examined the cause of death of 1,733 seabirds from 51 species and found 32 percent had ingested marine debris.
Other birds examined died from other identifiable causes, such as starvation, disease, injury or unidentifiable causes.
Hard plastics, both fragments and pellets, accounted for 92.4 percent of debris items ingested.
Although only 2 percent of the 2,671 items collected from dead birds was balloon fragments, the study found soft plastic packaging, including balloon fragments, rubber and synthetic foams, responsible for 42 percent of probable and known mortalities.
The research found balloons are 32 times more likely to kill seabirds than ingesting hard plastics.
Balloons or fragments of them were the known or probable cause of death in 18.5 percent of balloon-ingesting seabirds. There were five known or probable deaths from 32 balloons ingested.
The study said seabirds may select balloons when foraging because of their resemblance to prey, especially squid. All balloons in the study were ingested by species that eat squid, suggesting squid-feeding species are likely to have higher mortality rates, the report said.
The study, published in the journal "Scientific Reports," said albatrosses and petrels were particularly at risk because they mistook plastic for food.
The data showed a seabird ingesting a single piece of plastic had a 20 percent chance of mortality, rising to 50 percent for nine items and 100 percent for 93 items.
Study leader Lauren Roman said the relationship between the amount or type of debris seabirds ingest and mortality is poorly understood.
"Among birds we studied, the leading cause of death was blockage of the gastrointestinal tract, followed by infections or other complications caused by gastrointestinal obstructions," Roman said.
Roman said hard plastic fragments may pass quickly through the birds' guts but soft plastics are more likely to become compacted and cause fatal obstructions. Similar results were found in 1995 research into plastic ingestion by sea turtles.
CSIRO co-author Chris Wilcox said the study's approach was first developed for turtles before being applied to seabirds.
"These two applications are the first time there has been a robust estimate of the impact of plastic ingestion on free-living marine species," he said.
Roman said although the study showed soft items like balloons are more dangerous, all plastics pose a mortal threat to seabirds.
"If seabirds eat plastic, their risk of mortality increases, and even a single piece can be fatal. While hard plastics are less likely to kill than soft plastics, they were still responsible for more than half the seabird deaths identified in our study," Roman said.
The dead birds were collected from Australia and New Zealand.