Background to the rat poison problem

  • Background Rat Poison Barn Owl RatRats and Barn Owls
    Barn Owls have lived in and around farm buildings for thousands of years and when Brown Rats arrived in the 18th century, Barn Owls helped to keep their numbers down. Some farmers positively encouraged Barn Owls by providing owl-holes in buildings. Despite damage caused by rodents, farming remained profitable for hundreds of years.
  • Farm assurance schemes 
    These days the UK food industry imposes strict quality assurance requirements on farmers, including the control of rodents. Schemes such as ‘Red Tractor’ require farmers to have a Rodent Control Program but fail to explain to farmers that this does not mean using rat bait all the time.
  • Permanent and preventative baiting
    The constant use of rat bait where there are no rats, or keeping it down when the rats are dead, results in other animals such as Wood Mice and voles being poisoned. This unwanted poisoning is a major cause of wildlife contamination.
  • Background Rat Poison RodenticideGamekeepers poison wildlife
    Currently, around 70 million pheasants are reared annually in the UK. They continue to be provided with food, even after their release into the wild. 50% of the food that gamekeepers put out for released pheasants is taken by rats, despite the fact that 72% of gamekeepers use SGARs. Thus, most gamekeepers are creating rat infestations and using a lot of poison yet failing to deliver rodent control. This causes widespread contamination of wildlife.
  • Rats are not the only route of contamination
    Most Kestrels tested in the UK contain rat poison, despite the fact that Kestrels don’t eat rats. It is obvious that the main route of their contamination is through eating Wood Mice and voles that have eaten bait laid for rats. There is little doubt that this is also the main route of Barn Owl poisoning. Red Kites however do prey on rats and, unlike owls and Kestrels, they will scavenge for dead ones.
  • Bait covering does not work
    Keeping baits covered reduces the risk of bait being eaten by dogs, children, pigeons etc., but does nothing to prevent bait being consumed by animals that are smaller than rats. It is impossible to prevent Wood Mice and voles gaining access to rat bait, no matter how it is laid. Whether or not the bait is laid by a Gamekeeper, Farmer, professional Pest Controller or an amateur, makes no difference whatsoever.

How the poisons kill

Background Rat Poison Dead Barn OwlMost rodenticides are anti-coagulants. They prevent the blood from clotting and thin it until the victim eventually dies from internal bleeding. The time taken for a rodent to die after eating the bait varies from 2 to 12 days. A rodent eating a sub-lethal dose (not enough to kill it) may carry the poison in its liver for several months. Before a poisoned rodent dies it may be caught by a Barn Owl which then ingests the poison. This is called secondary poisoning. Creatures which have been killed by secondary poisoning include Barn Owls, Tawny Owls, Red Kites and Foxes. Animals which have been killed by directly eating rodenticide baits include dogs, cats, pigeons and blackbirds.

Research has shown that poisoned Barn Owls either die slowly, or survive and carry a residue of poison in their bodies. Typically it takes 6 to 17 days for a Barn Owl to die after eating 3 mice containing the poison Brodifacoum. Unfortunately no research has been carried out on the effects of sub-lethal doses on wild Barn Owls. There is a concern that it may affect survival during hard times and breeding success.

Levels of SGARs in Barn Owls

Background Rat Poison PbmsMonitoring of Barn Owl carcasses has shown an increase in secondary poisoning; the percentage of carcasses containing residues of second-generation rodenticides increased from 5% in 1983-4 to 53% in 2003 (Predatory Bird Monitoring Scheme). In 2006 a new, more sensitive method of analysis was used. This revealed that 63% of the Barn Owls examined had traces of second-generation poison in their livers. Had the old method been used, the poison would have been detected in 39%. While this shows that levels of poisoning have gone down since the high of 2003, it also suggests that previous levels have been significantly underestimated. What the 2003 figure would have been, had the new method been available then, can only be guessed at. The figure for 2010, again using the newer method, stands at 91%. The latest figure (for 2015 published in 2017) is 95%.

Rodenticides almost certainly have a detrimental effect on Barn Owl populations in the UK and this is an issue of serious concern for Barn Owl conservation.

What’s the difference between First Generation and Second Generation?

The various anticoagulant rodenticide poisons are divided into two ‘generations’ based simply on when they were first introduced and, in simple terms, how toxic they are. First generation rodenticides were first introduced in the 1940s. The second generation rodenticides, that were first introduced in the 1970s, are 100 to 1,000 times more acutely toxic. The currently available anticoagulants are:

First generation Second generation
• Warfarin • Difenacoum
• Coumatetralyl • Bromadiolone
• Chlorophacinone • Brodifacoum
• Flocoumafen
• Difethialone

What is ‘resistance’?

SGARs were introduced because rats in some parts of the UK became resistant to first generation products (FGARs). Some rats were eating it, surviving, and then producing young that were also resistant. Some rat populations are now resistant to the earliest SGARs Difenacoum and Bromadiolone. Ever-increasing resistance is a huge problem. It is accelerated by the over-use of poison (e.g. permanent baiting) and the widespread use of  poisons that are stronger than needed. Ideally, SGARs would only be used in areas where rats had developed resistance to FGARs. However, once a poison is authorised for use, the licencing authority (HSE) has no control over where it is used. At least 76% of farms across the UK use SGARs.

Background Rat Poison LabelRat poison labelling

All rodenticides are toxic and can kill Barn Owls; however the instructions provided on the containers do not mention the risks of secondary poisoning, or explain how it happens. Labelling generally gives the impression that, provided baits are kept covered and dead rodents disposed of, there is little or no risk to predatory birds. Unfortunately no amount of bait covering will prevent secondary poisoning and research has shown that the instruction to search for and dispose of dead rats is ignored by 98% of users.

Other relevant pages:

There’s more information on the rat poison problem in the Barn Owl Conservation Handbook.