Flying arboreal theropod venom mimicry

Question

I have an idea for an alternate world where "mythical" creatures exist, but are based on biology. For instance a unicorn is more closely related to ovines (sheep/goats/deer) than to horses and dragons/drakes are similar to prehistoric proto-mammals than say Komodo dragons. Assuming that cockatrices are arboreal theropds, is there any reason why a species of them might have developed the ability to mimic venoms of other creatures and/or helpful/harmful chemicals produced by local flora?

• What would the drawbacks to this be?
• How would this particular species "sample" the compound/venom in order to produce it?
• What other considerations might I have overlooked?

Answer 1

Your birds sequester toxins that they eat.

Biologically copying a toxin is difficult. Really that would be convergent evolution. But not getting poisoned by poisonous things you eat and sequestering the toxin for your own use is a things done by various animals including birds.

https://en.wikipedia.org/wiki/Toxic_bird

Toxic birds are birds that use toxins to defend themselves from predators. No species of bird is known to actively inject or even produce venom, but some birds are known to be poisonous to touch or eat. These birds usually sequester poisons from animals and plants that they feed on, especially poisonous insects.

The pitohui, the ifrita, and the rufous or little shrikethrush all sequester batrachotoxin in their skin and feathers.[1] The African spur-winged goose is toxic to eat as it sequesters poison in its tissues, from the blister beetles that it feeds on...

That is what your cockatrice does. It eats poison things but is not poisoned, instead sequestering the poison and moving it out to its own tissues.