Plastic ingestion by birds

Plastic ingestion by birds

Ingestion of plastic has probably been studied more in birds than in any other group of animals. First observations of plastic pieces inside seabirds date back to 1960s, and studies made between the years 1962 and 2012 have revealed that 59 % of examined seabird species had ingested plastics. From these studied individuals, nearly one third (29 %) had plastics inside their gut. Models have shown an increasing trend of plastic ingestion rate when more plastic is introduced to the ocean.

It has been suggested that birds intentionally select marine litter when mistaking it for prey, but the role of selectivity is not yet fully understood. Some types of plastics resemble natural materials floating on the sea surface: for example Styrofoam and other spongious plastic have observed to contain similar peckmarks than cuttlebones in the North Sea. From the sampled 100 plastic items washed ashore 80% contained peckmarks, which were suggested to have been done by northern fulmars (Fulmarus glacialis) at sea and other birds, such as gulls, when stranded on the beach.

Birds’ vulnerability to marine litter is affected by the foraging behavior, diet, breeding, molting and distribution of the species. Birds belonging to the order Procellariiformes, which include albatrosses, petrels and shearwaters, storm petrels and diving petrels, are observed to contain more plastics compared to other birds probably because they feed in the open ocean and may mistake floating plastic for prey. In addition Procellariiformes have small gizzards and many of them are unable to regurgitate indigestible items, which makes them even more vulnerable to the effects of plastic ingestion.

The foraging behavior may influence the probability to encounter and ingest plastics. For example the most species that had ingested plastics in the subarctic waters of Alaska were surface-feeders, such as shearwaters, petrels and gulls. Foraging behavior may be related to the plastic types ingested, since in general surface-feeders seem to swallow mostly user plastics compared to diving species, which catch more pellets. The size of ingested plastic has also been observed to be positively correlated with body size.

The plastic caught by adult birds from the sea is also transferred to the young when they are fed by their parents. 90% of examined Laysan albatross (Phoebastria immutabilis) chicks had been shown to contain plastics in their upper gastrointestinal tract already in the late 1980s. From these studied chicks, dead birds contained a plastic load weighting 76.7 g on average per individual, whereas the mean load was lower for alive chicks (35.7 g per individual). Also in Hawaii the larger amount of plastics in the gut of Laysan albatross chicks has been linked to elevated mortality. Sometimes plastics can form a significant proportion of the body weight: for example one Australian flesh-footed shearwater (Puffinus carneipes) fledgling weighting 445 grams had 276 pieces of plastic in its stomach weighting 64.1 grams and accounting for 14.4. % of its total body mass.

It has also been observed that young birds contain more plastics than adults. For example in shot-tailed shearwaters (Puffinus tenuirostris) the incidence of plastic ingestion was higher in juveniles (85 %) compared to adults (63 %) and also juveniles had ingested more litter by count than adults. The higher amount of plastic in young birds have suggested to be partly a consequence of being fed by both of the parents who transfer their own plastic load to the chicks. In addition, young individuals might have less developed grinding action in their gizzards which may slow down the removal of ingested plastic items. It has also been suggested that young birds are naïve consumers and have not yet learned what to feed on. On the other hand, also reverse trends have been observed: for example in tropical North Pacific adult birds were reported containing more plastics than young birds. It was suggested, that older and more experienced birds might spend more time in areas where prey and also plastics are concentrated.

Prevalence, mass and amount of ingested plastic pieces in some seabird species

Species	% of birds that ingested plastics	mean mass of ingested particles	mean amount of ingested particles	Area
Northern fulmar	93 %	0.28 g / individual	26.5 pieces/individual	North Sea, Netherlands
Short-tailed shearwater	63 % adults 85 % juveniles	3.86 g / individual	4.5 pieces/adult 7.14 pieces/juvenile	North Stradbroke Island, Australia
Flesh-footed shearwater	90 % fledglings	2.7 g / individual	17.5 pieces/fledgling	Lord Howe Island, Australia
Wedge-tailed shearwater	0 % adults 21 % chicks		0 pieces / adult 3.2 pieces/chick	Heron Island, Australia
Northern fulmar	82 % adult 67 % non-adult	0.13 g / adult 0.14 g / non-adult	5.3 pieces / adult 9.7 pieces / non-adult	Iceland

The long time series on the interactions of birds and plastic litter in the oceans are helpful when investigating the changes in litter composition and their ingestion. For example it has been appeared that even though the numbers of ingested plastic particles by seabirds has stayed stable, the composition of these particles has radically changed in less than two decades. The proportion of virgin pellets has decreased 44-79 % in all studied seabird species including great shearwater (Puffinus gravis), white-chinned petrel (Procellaria aequinoctialis), broad-billed prion (Pachyptila vittata), white-faced storm petrel (Pelagodroma marina) and white-bellied storm petrel (Fregetta grallaria). ­It is unclear whether this trend is a result of decrease in pellet density or increase in other types of litter. Similar change in ingested plastic composition has also been observed in short-tailed shearwaters caught from Bering Sea between 1970s and 2001 and in northern fulmars (Fulmarus glacialis) in the North Sea since the 1980s.

Nowadays most of the plastic ingested by seabirds are other than virgin pellets. The ingested plastics have been reported to contain for example monofilament line, bottle caps, plastic bags, fragments of plastic bottles and containers, balloons and buttons, cigarette lighters, Styrofoam and magic markers.

It has been suggested that color might have an influence to particles’ probability of becoming ingested. For example seabirds studied in Australia had ingested predominantly dark plastic particles over any other colors. However, these suggested color preferences should be compared to the color proportion of plastics in the foraging area, which is seldom possible due to the lack of sufficient data. It is possible, that the differences in particle colors reflect their abundance in the environment rather than selectivity by color.

The sizes of ingested plastic items has been reported to vary between 0.5 and 51.5 mm and up to 11.3 cm. Some ingested nylon lines have been even longer reaching the length of 18.6 cm. In addition to plastics, also other man-made objects such as glass shards and copper wire has been found inside birds.

There has been an attempt to create models that could predict the ingestion rates of seabirds in different areas. Researches have used the monitoring data of the plastic abundances and predicted the risks they may cause to the local seabird populations across the globe. According to their estimations, plastic ingestion is increasing in seabirds resulting in 99 % of all species by 2050. The predicted areas of high impacts are strongly influenced by species richness rather than the plastic concentration. Therefore, the impacts of plastic ingestion are estimated to be highest in the northern boundary of the Southern Ocean.

Physical damage caused by plastic ingestion

In the 70’s polystyrene particles were found from the regurgitated indigestible food material of terns and gulls. Back then plastics were thought to be harmless, and even later on it has occured being hard to predict the impacts of plastic ingestion, since the residence times in the gut are not very well known and vary between the species.

The physical damage caused by plastics has been experimentally studied with domestic chickens (Gallus domesticus). Polyethylene pellets were fed to chicken to research whether they impair the feeding activity of the animals. The results indicate that ingested plastic reduces the storage volume of the stomach and hence results in smaller meal sizes and slower growth rate. Indeed, the seabirds caught from the tropical Pacific Ocean during 1984-1991 show a similar pattern: the body weight declined with increase in number on plastic particles ingested, and same trend was also observed in fledglings of flesh-footed shearwaters (Puffinus carneipes) studied in Australia.

It has also been suggested that plastic influences the fat condition of the birds: a negative correlation between the amount of plastic and the fat condition have been found in red phalaropes (Phalaropus fulicarius) and Laysan albatross chicks.

The obstruction of gastrointestinal tract may lead to increased mortality among seabirds that have ingested plastics. Necropsies of dead seabirds have revealed that ingested plastic items can cause serious ulcerations in gizzard or block the pylorus so that feeding is impossible and finally lead to death. In some seabird species, plastic ingestion is suggested being the major cause of injury and mortality.

 Harmful substances associated to microplastics

Fledglings of flesh-footed shearwaters (Puffinus carneipes) studied in Australia have exhibited a relationship between ingested plastic and contaminant load. The more the individual has ingested plastic particles, the higher were the concentrations of chromium (Cr) and silver (Ag) in fledgling breast feathers. It is however hard to define whether they have originated from plastics or ingested prey.

Wild short-tailed shearwaters (Puffinus tenuirostris) caught from the northern North Pacific Ocean have shown to contain plastic particles in their stomachs. All of the examined birds had also accumulated PBDEs used as flame retardants to their adipose tissue. In majority of the birds lower-brominated congerners of PBDEs were dominant and their concentration was similar to fish caught from the same area indicating that the contaminants were probably derived from their prey. In some birds, however, higher-brominated congerners were dominant. These were not present in their prey, but have been earlier detected in marine plastics. The authors of this study suggest that the higher-brominated congerners of PBDEs are derived from ingested plastic items.

It has been estimated, that even though the primary route for POP exposure to seabirds would be trough the food chain, plastics could be an additional source of these substances. Since seabirds (other than albatrosses) belonging to the order Procellariformes can retain ingested plastic items for months or even years, the potential leaching time for additives or other harmful substances from plastic in the gastrointestinal tract is long. The load harmful substances in plastic pellets and fragments ingested by Procellariformes species in Brazil support the fact that plastics can carry these substances to birds: even though their effects on birds were not studied, the ingested items contained both PCBs and organochlorine pesticides.