A prompt executive Bird is the Jay — Jay-Hawker

A prompt－executive Bird is the Jay－¹
Bold as a Bailiff's Hymn－
Brittle and Brief in quality－
Warrant in every line－

Sitting a Bough like a Brigadier ⁵
Confident and straight－
Much is the mien of him in March
As a Magistrate－

(F.1022/J.1177)

[1] Jay:: jay-hawker, "a name given to members of the bands who carried on irregular warfare in and around eastern Kansas, in the free soil conflict, and the early part of the American civil war, and who combined pillage with guerilla fighting: hence, generally, a raiding guerilla or irregular soldier" (OED jay-hawker 1865).
[2, 3] Bold, Brittle and Brief:: like a guerrilla.
[3] Brittle and Brief:: the way he fought in the field.
[5] Bough:: a place for ambush; a gallows, a hint on the jayhawker's ending.
[6] straight:: of an aim, a stroke, a throw, etc.: Directed precisely to the mark (OED 13c). Confident:: in the shooting.
[7] March:: border, frontier.
[8] As a Magistrate:: as one but he was not a true magistrate.

Colonel Charles R. Jennison was born in the County of Livingston in this Stage; but while quite a young man removed into the Territory of Kansas. As he was very much linked by the ladies of that region, he gained from his companions the soubriquet of the "gay Yorker,: which, during the Kansas-Missouri border troubles, was corrupted into "Jayhawker." ─ The Portrait Monthly (1864)

Would he have removed the gallant conservative generals, Sturges and Denver, from command in Kansas, for having arrested the notorious Colonel Jennison, the abolition jay hawker, border robber, and field thief, for the crime of mutiny? ─ The Old Guard (1863)