Wolves are notoriously difficult to hunt because of their elusiveness, their sharp senses, their high endurance in the chase and ability to quickly incapacitate and kill hunting dogs.[253] Historically, many methods have been devised to hunt wolves, including the killing of spring-born litters in their dens, coursing with dogs (usually combinations of sighthounds, bloodhounds and fox terriers), poisoning with strychnine, and foothold and deadfall traps.[254][255][256] A popular method of wolf hunting in Russia involves trapping a pack within a small area by encircling it with fladry poles carrying a human scent. This method relies heavily on the wolf's fear of human scents, though it can lose its effectiveness when wolves become accustomed to the smell.[256] Some hunters are able to lure wolves by imitating their calls.[256] In Kazakhstan and Mongolia, wolves are traditionally hunted with eagles and falcons, though this practise is declining, as experienced falconers are becoming few in number.[256] Shooting wolves from aircraft is highly effective, as it allows greater visibility of wolves than hunting on the ground,[256] though this method is controversial, as it allows wolves little chance to escape or defend themselves.[257]