Secondary poisoning in captive barn owls Tyto alba javanica from feeding with rats poisoned with chlorophacinone and bromadiolone

The poten­tial sec­ondary haz­ards of two anti-coagulant roden­ti­cides com­monly used in Malaysian oil palm plan­ta­tions were eval­u­ated through feed­ing tri­als with cap­tive barn owls. A total of 12 adult barn owls (six pairs) were assigned to two roden­ti­cide treat­ments and a con­trol. The two treat­ments were rats fed with bro­ma­di­olone (0.005% a.i) and chloropha­ci­none (0.005% a.i) respec­tively. All roden­ti­cide treated owls received four poi­soned rats at Day 1, 3, 5 and 7 and a non-poisoned rat on each inter­ven­ing day for 30 days. Each barn owl of the con­trol group received a non-poisoned rat through­out the study. The reli­a­bil­ity of a non-invasive tech­nique such as an esti­ma­tion of anti-coagulant roden­ti­cide residue in regur­gi­tated pel­lets and blood sam­ples, were also eval­u­ated. Barn owls showed behav­ioural aber­ra­tions such as coarse breath­ing, fre­quent clos­ing of the eyes and reduced fly­ing activ­ity as early as Day 5 after con­sum­ing three poi­soned rats. The weight recorded at Day 7 after treat­ment showed that all treated owls reg­is­tered a reduc­tion in weight. The owls in the con­trol group on the con­trary gained weight. Bro­ma­di­olone and chloropha­ci­none were found to have high degree of tox­i­c­ity on cap­tive barn owls. After feed­ing the birds with as few as four poi­soned rats in a week the signs of tox­i­c­ity in birds such as haem­or­rhages (beak) and haematoma (wing) were found. This find­ing is very cru­cial since barn owls have been reported to con­sume up to three rats per night and this would cer­tainly increase their poten­tial expo­sure to lethal sec­ondary poi­son­ing. The detec­tion of residue in the pel­lets regur­gi­tated by barn owls can be used to indi­cate expo­sure of the lat­ter to both com­pounds. How­ever, as the blood residue method is lim­ited to the expo­sure dura­tion of the com­pounds, this tech­nique can only detect recent expo­sure to bro­ma­di­olone and chloropha­ci­none. The amount of residue detected in the pel­let sam­ples for chloropha­ci­none was 69.9 to 81.6 μg per day or equiv­a­lent to 17.2% to 27.4% of the com­pound con­sumed and cor­re­spond­ing value for bro­ma­di­olone was 27.2% to 34.5% (72.24 – 85.77 μg per day). This sug­gests that the amount of bro­ma­di­olone retained in the tis­sue of the rats was higher than that of chloropha­ci­none. Thus, barn owls face a greater poten­tial for sec­ondary poi­son­ing from bro­ma­di­olone rather than chloropha­ci­none as can be deduced from this study.

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