Spring traps with metal jaws were also effective, though poisoning by strychnine became more common by the late 19th century. Strychnine poisoning was initially problematic, as the solution had a bitter taste, and could only work if swallowed. Consequently, many jackals learned to regurgitate poisoned baits, thus inciting wildlife managers to use the less detectable crystal strychnine rather than liquid. The poison was usually placed within sheep carcasses or in balls of fat, with great care being taken to avoid leaving any human scent on them. Black-backed jackals were not a popular quarry in the 19th century, and are rarely mentioned in hunter's literature. By the turn of the century, jackals became increasingly popular quarry as they encroached upon human habitations after sheep farming and veld burning diminished their natural food sources. Although poisoning had been effective in the late 19th century, its success rate in eliminating jackals waned in the 20th century, as jackals seemed to be learning to distinguish poisoned foods.