A characteristic of a kestrel that can become a vice when it's trained is...

The characteristic of a kestrel that can become a vice when it's trained is carrying. Kestrels, like many raptors, have a natural instinct to carry their prey. In a training context, this behavior can present challenges. If a kestrel becomes too focused on carrying items, it may distract itself from the intended training tasks or could begin to carry unwanted objects. This behavior can complicate the training process, as the bird may prioritize carrying over responding to commands or working with a falconer. Understanding this aspect helps trainers recognize and mitigate the tendency early on, ensuring a more successful training experience.

The other characteristics like soaring, warbling, and bowsing, while they might describe natural behaviors or sounds of the kestrel, do not typically interfere with training in the same way that the instinct to carry can. Thus, while they are notable aspects of a kestrel's behavior, they do not become vices in a training scenario.