(CNN) Dead and injured birds covered in tinsel and other decorations have been turning up in New Zealand, and animal rights campaigners are worried.

The Royal New Zealand Society for the Prevention of Cruelty to Animals (SPCA) said it received up to 30 calls in recent weeks reporting sightings of sparrows and pigeons with tinsel intentionally tied to their bodies.

"Many try to pry the foreign objects off their bodies with their beaks and feet, becoming further entangled and preventing them from eating, drinking and flying. With others, the decorations are wrapped so tightly it completely cuts off their blood circulation," SPCA spokeswoman Jessie Gilchrist told CNN.

"Those that do survive and arrive at our center are often in a very bad state, they are so malnourished and distressed that we have had to humanely euthanize them."

The decorations can prevent the birds from eating, drinking and flying, leaving them to starve to death.

Since 2015, the charity said there has been over thirty cases of "decorated" birds found dead or with injuries severe enough to require euthanasia.

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