Which of the following birds is considered the largest?

The Jerkin is recognized as the largest bird among the options provided. This species refers to a type of male kestrel that is larger than its female counterpart, but in the context of falconry and the birds listed in the question, the Jerkin generally stands out in size compared to the other choices.

The Jack Merlin, also known simply as a Merlin, is a smaller falcon. Similarly, the Peregrine Falcon is a formidable bird of prey but is not as large overall as the Jerkin. The term "tiercel" denotes a male bird of prey, specifically in this case, the Peregrine tiercel (male Peregrine Falcon), which is smaller than the female Peregrine Falcon.

Understanding the size distinctions among these birds is crucial for falconers, as size can influence handling, training, and hunting capabilities. Thus, recognizing that the Jerkin is the largest bird listed allows one to better appreciate the diversity within falcon species and their respective roles in falconry.