A marbled teal (Marmaronetta angustirostris) and a great egret (Ardea alba) in an outdoor zoo aviary died without premonitory signs of illness. Gross necropsy revealed internal hemorrhage and liver samples from both birds tested positive for brodifacoum, a potent second-generation rodenticide.
Bait stations were used within the aviary to control a heavy infestation of rodents, but the bait blocks were not directly accessible by birds. Stations consisted of a 12” x 14” plastic box with a single, round opening where rodents had to enter the box completely in order to access the bait within. The bait consisted of 0.005% brodifacoum in a solid, cereal-laced block. Bait blocks were checked every day by zookeepers and replaced when almost completely consumed. A zookeeper noted that she often saw slugs on the bait blocks. Slugs were likely attracted to the cereal component of the bait.
A stone curlew (Burhinus oedicnemus) and a black-crowned night heron (Nycticorax nycticorax) were found dead 1 month and 6 months respectively, prior to the teal and egret but were not tested for rodenticide levels at time of necropsy despite evidence of internal hemorrhage at necropsy.
Molluscs (slugs and snails) randomly caught throughout the aviary tested positive for brodifacoum. Subsequent feeding experiments illustrate that slugs and snails can bioaccumulate brodifacoum and bromadiolone, suggesting that they could serve as vectors for rodenticides in molluscivorous birds. It is important to note that sensitivity to rodenticides varies tremendously depending on bird species.1
The authors would like to thank zookeeper Dina Pettit for her observations, and for collecting slugs and snails for the bait feeding experiment.
1. Godfrey MER. An evaluation of the acute oral toxicity of brodifacoum to birds. In: Proceedings of the Twelfth Vertebrate Pest Conference. 1986:78–81.