The hunter is the "alert man" according to Jose Ortega y Gasset, author-philosopher whose "Meditations on Hunting" has become the de facto standard to explain the intrinsic human instinct to hunt. This applies, of course, to the aquatic hunter, or fisher, who is constantly seeking cues to the activity and whereabouts of her/his quarry.
The question arises, alert to what? Alert to anything; especially, able to think clearly; intellectually active. And so, when I emerged from the house one mid-May morning and saw the swallows swarming over the river, I suspected that a hatch was on.
While the swallows were visible enough, their prey was not. Nor was there a conspicuous cloud or swarm of insects over the river. I concluded that the swallows were feeding on very small insects, perhaps midge-sizes, certainly not mayflies larger than tricos.
I seldom carry an insect net afield when I am fishing, but I often swipe my hat at nearby flying insects, to see first-hand what they are, or, at least, how big and what color they are. Skittering insects on the surface of the water frequently indicate egg-laying is in progress. Empty shucks (exoskeletons) of insects attached to plant stems at riverside are further indicators of what kind of trout food is present, or, recently risen.
Just as the hunter is attuned to the squawk of crows or ravens, sometimes heralding the approach of ground-roving creatures, so too does the fisher remain alert to the actions of birds and other riparian indicators of trout food in the making.