A Bird’s Tale

Aerial plankton

Posted:  Saturday, August 29, 2015 - 1:00pm
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Plankton. When you see or hear the word you think of the ocean, right? Have you ever heard of “aerial plankton?”

That’s a term coined by some unknown person (we have seen reference to it in a research paper from the 1960s) to describe the mass of insects and spiders, mostly tiny, that can be found up in the air at varying heights above ground.

Small, flying insects are typically not strong enough flyers to fight against thermal updrafts and gusty winds. They, along with newly hatched tiny spiders, can be carried aloft and moved great distances. Many insects hatch in swarms that form at particular times of year under particular temperature and humidity conditions. The insects in these swarms can be carried into the air to become this “aerial plankton.”

Bird enthusiasts will not be surprised to know that aerial plankton is the major food supply for many birds, especially those that ornithologists term aerial insectivores including swallows, swifts, and nightjars.   

Earlier in the year, in spring and early summer, we have all probably noticed the hatches of stoneflies and mayflies that emerge from streams, rivers, and ponds and can coat the windshields of our cars and the gas pumps at the local convenience store. We’ve all seen the swarms of unknown midges and other flies hovering around a spot along a woodland trail or maybe the back lawn. Well, this time of year, late summer, is when we see the massive numbers of flying ants — the form of the lifestage in ants that leaves the colony to reproduce.

This form of aerial plankton seems to be very popular with many birds, not just those aerial insectivores! Last week we watched an impressive air show over Wharton Point in Brunswick. Flying ants were thick in the air and on the ground, as perhaps 50 ring-billed gulls hovered around snapping up the ants hundreds of feet in the air, sometimes over the water, sometimes over land. They were joined by a few laughing gulls and, believe it or not, a number of common terns, all performing flying stunts as they grabbed the insects from the air. Later, a flock of about a hundred swallows appeared — mostly tree and barn swallows — and joined in.

A little over a week ago, a flying ant swarm over our neighborhood in Gardiner attracted about 50 chimney swifts and a single common nighthawk. All over Maine in the past week and probably the next week to come, observers have been reporting groups of common nighthawks gorging on flying ant swarms, including one report of at least 50 of these migrating birds, which are related to the whip-poor-will.

Birds are not the only creatures to feed on aerial plankton. Dragonflies, too, are adept at snatching smaller insects from the air and eating them. Many people are surprised to learn that some species of dragonflies themselves migrate and will form large swarms as they do so. Our friend and colleague, John Fitzpatrick, director of the Cornell Lab of Ornithology, was once witness to a dragonfly swarm in Chicago that he and others estimated to contain over a million individuals. And dragonflies themselves can become part of the aerial plankton, becoming prey for birds. Merlins and kestrels can catch and eat dragonflies, but perhaps the most well-known dragonfly pursuers are the kites—Mississippi kites and swallow-tailed kites often catch dragonflies and can even eat them in flight!

If you notice a hatch of “aerial plankton,” be sure to keep an eye out for “nature’s insecticides.” Observing them can be a fascinating spectacle.

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology. Dr. Wells is one of the nation's leading bird experts and conservation biologists. His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization. Both are widely published natural history writers and are the authors of the book, “Maine’s Favorite Birds.”