Rio Grande Nature, March 22, 2019

The wintering bald eagles have already gone north. They will soon be followed by the ducks that also came south to spend the winter – mergansers, buffleheads, goldeneyes and others. As things warm up, other critters are on the move.

The first mayfly hatches are now happening. What is a “hatch”? An environmental cue, either water temperature or daylength, stimulates the underwater forms of a particular species of mayfly to detach from the river bottom and go to the surface. They do this together, because the point is to find each other and mate. This is also called an “emergence”, and is an event of great interest to fly fishermen. Trout notice a hatch, and feed on the mayflies as they congregate on the surface. And fishermen notice the surface feeding activity and use “dry” flies – flies that resemble the mayflies and which float on the surface – to fool the trout.

But, what happened yesterday during the hatch was that the trout were not feeding on the mayflies. Instead, it was the mallards that were feeding on them, and I saw that as I arrived at the river. The mallards were in eddies, where the mayflies were particularly concentrated, picking them off.

Did any other creature take notice of the mayflies? Yes, the bluebirds did. After mayflies emerge, they take to wing, at which point birds can readily snatch them from the air. Flocks of both mountain and western bluebirds are now present along the river. The mayflies now hatching are very probably members of the genus Baetis, and are known as Blue-wing olives. They are pretty small, but, to a trout, probably bigger than a peanut is to a human.

 

 

Mayflies have transparent wings that they hold up, making them look like little sailboats as they move with the current. These mayflies are getting clustered in the eddy downstream of a rock.