Coastal and Marine Birds

Many birds in the marine environment dive for food and thereby come into contact with plastic debris. The greatest causes of entanglement by seabirds are fishing lines and six- pack rings. Both materials are often transparent and difficult to see. If seen, they can be mistaken for jellyfish and other food [21]. The gannet is one marine bird species that is endangered by plastic debris. As a “plunge-diver,” the gannet dives from great heights into the ocean and can thereby be caught by ghost nets or other debris. A study at the Island of Helgoland in Germany, which hosts a large gannet colony, showed that between 1976 and 1985, 29% of dead gannets found had become entangled in net fragments [65]. High proportions of coastal and marine bird species (36% of the 312 species worldwide) ingest plastic fragments [54]; see Figure 2.4. Allsopp et al. [21] reported that 111 out of 312 species of seabirds are known to have ingested debris, and this can affect a large percentage of a population (up to 80%). Moreover, plastic debris is also known to be passed to the chicks in regurgitated food from their parents.

One harmful effect from plastic ingestion in birds is weight loss due, for example, to a falsely sated appetite and failure to put on adequate fat stores for migration and reproduction [21]. Although plastics are mainly ingested by birds because they are mistaken for food, they may also already be present in the gut of their prey or may be passed from adult to chick by regurgitation feeding. Some species feed selectively on plastics fragments that have a specific shape or color [66]. Robards et al. [67] examined the gut content of thousands of birds in two separate studies and found that the ingestion of plastics by seabirds had significantly increased during the 10-15 years between the studies. A study done in the North Pacific by Blight and Burger [68] found plastics particles in the stomachs of 8 of the 11 seabird species caught as by- catch. Harmful effects from the ingestion of plastics include blockage of gastric enzyme secretion, diminished feeding stimulus, lowered steroid hormone levels, delayed ovulation and reproductive failure [69]. The food uptake causes internal injury and death following blockage of the intestinal tract [70-73]; the extent of the harm, however, varies among species. Laist [38] and Fry et al. [74] observed that adults that manage to regurgitate plastic particles could pass them onto the chicks during feedings.

The harm from ingestion of plastic is nevertheless not restricted to seabirds. For Procellariformes such as the albatrosses (Figure 2.2), shearwaters or petrels, the appearance of eroded plastic pieces are similar to many types of food they consume [68]. Small plastics such as bottle caps are often mistaken by seabirds (Procellariformes) for food. In several studies, it was found that diving birds that fed on fish in the water column had less plastic in their stomachs compared with those that were surface eaters [68,75]. This could be because birds that maintain a diet of zooplankton may not be able to distinguish between plastics and their primary source of food owing to the color or shape of the plastic pieces [76]. Since most adult birds regurgitate what has been ingested as a way to feed their chicks, they pass the bolus containing the plastic to the first young. Birds such as the albatross and shearwater had more plastic in the first region of their stomachs and gizzards, indicating that when these plastics were regurgitated, they would be passed to their young during feeding [66]. Juvenile albatrosses and shearwaters were found to ingest more plastics than adults [76,77]. Similar to other marine life, swallowed plastics can obstruct and damage a bird’s digestive system, reducing its foraging capabilities. Ryan [72] concluded that ingested plastics could reduce the fitness, growth rate and food consumption of seabirds. Based on the results form a study using domestic chickens (Gallus domesticus), the amount of plastic ingested by different species of birds may be an indicator of the accumulation of plastics in an area.

 
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