Some science behind the scenes
An avicide is any substance (normally, a chemical) which can be used to kill birds.
Commonly used avicides include strychnine, DRC-1339 (3-chloro-4-methylaniline hydrochloride, Starlicide) and CPTH (3-chloro-p-toluidine, the free base of Starlicide), and Avitrol (4-aminopyridine). Chloralose is also used as an avicide. In the past, highly concentrated formulations of parathion in diesel oil were also used, applied by aircraft spraying over the nesting colonies of the birds see Alan Woods Compendium of Pesticides
Bird repellants on the other hand are simply intended to repel birds and not kill themselves - see Alan Woods Compendium of Pesticides