As far as anticoagulant poisons go, warfarin is relatively tame. Developed in the 1940’s, it is what is known as a “first generation anticoagulant rodenticide (AR).” These AR’s take multiple feedings in order to kill the target pest and have a relatively short half-life. These attributes also make it so that animals can, and do, develop a resistance to the toxin and then they are no longer effective. The short half-life and low toxicity also means that dead or intoxicated target animals are far less dangerous to non-target species then the AR’s that were developed later. (Note that “less dangerous” does not mean “completely harmless”.)
In an effort to combat the resistance that developed with fist gen AR’s, second gen AR’s were came anout. These are far more toxic, requiring only a single dose to cause death, and have a longer half-life which can, and absolutely does, cause secondary poisoning in other animals. I’m not going to re-read the book tonight, but I remember somewhere in “Prairie Dog Empire: a saga of the short grass prairie” Paul Johnsgard mentions that these second generation AR’s have been linked to deaths in animals far outside the typical predator/scavenger-prey food web, such as song and shorebirds, in areas where they were used to control prairie dogs.
Does that mean one should NEVER used rodenticides or other pesticides? Absolutely not. However it is CRITICALLY important that they are used cautiously and ONLY as specified on the label. The label on any pesticide, whether it’s Round-Up or warfarin, is a legal document and failure to follow it can result not only in damage to human health and the environment, but you can be held liable for that damage in court.
On the note that ANY predator can and should be killed, that is also legal issue. Without the proper permits, it is unlawful to harm or kill any protected species. An animal doesn’t have to be a listed threatened or endangered species to qualify as protected either. For example, ALL birds of prey (threatened or not) are protected in the U.S. by several laws including the Migratory Bird Treaty Act and the Bald and Golden Eagle Protection Act. Most mammal predators such as bobcats, lynx, mountain lions, foxes, coyotes, raccoons, and various weasels are covered by state laws, which vary in regards to livestock/personal property protection. It is vitally important to be familiar with these state laws before you start shooting or poisoning these animals. Both state agencies and the U.S. Fish and Wildlife Service take wildlife crimes VERY seriously and violators should expect to be held accountable.