That's what it said in one source--I had the same reaction.🤔 Maybe the noise? Maybe because the trees they hunt bugs in tend to be diseased and people thought the woodpecker caused it? Ibis are common birds. They were honored in Egypt which could have been distasteful to the Hebrews. There was an old myth that they give themselves enemas with their long beaks, that's what your source means by "purges itself." Ibis actually are probing the mud for snails and such, not carrion. The manuscript is using ibis as a metaphor, and not a very accurate one. They are mentioned in the Bible in connection with wastelands, along with owls and ravens, all unclean because of eating meat or carrion. So if not actually evil, they all have unclean associations and could cause rital defilement.