Which of the following is not essential for a newly trapped bird?

For a newly trapped bird, the use of a transmitter is not essential. Transmitters can be valuable tools for monitoring the location and behavior of birds, especially for tracking their movements over time and ensuring their safety. However, when initially handling a newly trapped bird, the most immediate tools required are the glove, lease, and swivel.

A glove is essential for safe handling, allowing the falconer to protect their hands while handling the bird, as well as providing a stable surface for the bird to perch on. A leash is important for controlling the bird's movements after it has been trapped, giving the falconer a means to securely attach the bird to a perch or themselves. A swivel is crucial as it allows for free movement of the bird on the leash while preventing tangles, which aids in the comfort and safety of the bird during this critical acclimation period.

In summary, while a transmitter serves specific purposes in monitoring and research, it is not a fundamental requirement in the initial care and handling of a newly trapped bird, making it the correct answer.