Definition of "bellwether"

Several old ladies forthwith proclaimed their intention of following him; but, as one or two of them were deaf, and another had been threatened with an attack of that mild, but obstinate complaint, dementia senilis, many thought it was not so much the force of his arguments as a kind of tendency to jump as the bellwether jumps, well known in flocks not included in the Christian fold.

1861, Oliver Wendell Holmes Sr., chapter XXXI, in Elsie Venner

I suspect that, after all, […] —that all this has been known, perhaps for ages, to certain ones upon this earth, a cult or order, members of which function like bellwethers to the rest of us, or as superior slaves or overseers, directing us in accordance with instructions received—from Somewhere else—in our mysterious usefulness.

1919, Charles Fort, chapter 12, in The Book of the Damned

In December, after everyone had weighed in, Nature, a bellwether of mainstream scientific thought for more than a century, cautiously supported a future of automated scientific scrutiny in an editorial that addressed the Statcheck controversy without explicitly naming it.

2017 February 1, Stephen Buranyi, “The high-tech war on science fraud”, in The Guardian‎

A bellwether election in Germany’s most populous state has shown a further eroding of support for populist parties on the far ends of the political spectrum, thrown up questions over the pulling power of the chancellor, Olaf Scholz, and entrenched the growing authority of the Greens.

2022 May 16, Philip Oltermann, “German state elections show populism in decline on left and right”, in The Guardian‎