The crossword solution for the term 'ailing avian' is 'ILLEGAL'. This term refers to the ILLEGAL trade of wild birds, which is a major threat to avian species. The ILLEGAL trade of birds is driven by the demand for exotic pets, as well as the use of feathers and other body parts in traditional medicines and rituals. This ILLEGAL activity has caused a decline in the population of many bird species, leading to their endangerment or extinction.