Rats 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.
Rodenticides have had a detrimental effect on Barn Owl populations in the UK and this is an issue of serious concern for Barn Owl conservation.
The more enlightened farmers have given up using rat poison and have turned to using Fenn traps instead. Increasingly, farmers now positively encourage Barn Owls by providing owl-holes in buildings.
Together with the owls, this strategy is amazingly effective.
How the poisons kill owls
Most 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 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 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 Owl
Monitoring 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%.
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