Imam al-Shafi'i (may Allah have mercy on him) said: "The masburah is a sheep that is tied up and then shot with arrows."
And Abu Ubayd (may Allah have mercy on him) said: "It refers to a bird or any other living creature that is tied up alive and then shot at until it is killed; the root meaning of sabr is to confine (and hold captive)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / Q19481