Insecticides, Spiders and Birds

By Kay Charter –

Ask a room filled with people who love birds what the following Michigan species have in common: chickadees, titmice, all wrens, most warblers, all flycatchers (including phoebes), all thrushes (including robins and bluebirds), catbirds, thrashers, all members of the blackbird family (including bobolinks, meadowlarks and orioles), tanagers, grosbeaks, buntings, many sparrows, some swallows and Ruby-throated hummingbird. Chances are no one will answer that they all eat spiders.

Prothonotary Warbler – by Doug Tallamy

Ask those same folks what is the most common phobia in the world and they are likely to say that snakes are the most widely feared creatures. The reality is that fear of spiders is the most common universal human phobia, but the fact is that in the United Snakes less than two people die of spider bites on average every year while about six succumb to snakebite. We are six times more likely to be killed by lightening than by snake venom, which means we are at least eighteen times more likely to die by a lightening strike than by a spider bite. All of this is by way of saying that we worry far too much about spiders. And that’s a problem for our birds.

Many people so fear spiders that they not only have insecticide sprayed inside their homes, they also have a twenty-foot (or more) “envelope” of insecticide sprayed outside their homes. But blanket spraying never kills every individual. Dying spiders can be picked up by birds, like the beautiful Prothonotary Warbler in the photo, and either eaten or fed to their nestlings.

Prothonotary Warblers occur in southern Michigan, but are rarely found north of Muskegon. Winter Wrens and Ovenbirds (a ground nesting warbler) also eat significant numbers of spiders, and they breed throughout the state. Both species nest in wooded areas, occasionally relatively near a home, which would put them at risk of ingesting poisoned spiders (or other insects affected by insecticide spraying). Insectivorous birds such as wrens and warblers are also adversely affected by insecticide use by simply losing their food base. Conversely, because many birds predate on spiders, our avian population helps to keep them in check. Decline in those birds that eat spiders ultimately means an increase in spider numbers.

About sixty years ago, brown tree snakes were inadvertently introduced to the island of Guam. Because none of Guam’s native predators eat this reptile, it increased to incredible numbers. And, because brown tree snakes predate on birds and bird eggs, the island has now lost all but two of its avian species. Today, ecologists find as many as 40 times more spiders in Guam’s remote jungle than are found on nearby islands. Scientists have investigated whether the disappearance of birds led to the increase in the spider population on Guam. Many birds there (as here) consume spiders, compete with spiders for insect prey and use spider webs in their nests. In some places on Guam, spider webs fill gaps between jungle trees to the extent that it is impossible to walk through the dense array of silken threads without a stick to knock them down. At least one researcher suspects that spider numbers are increasing in other parts of the world where birds are declining.

Ovenbird by Jeff Kingery

Spiders play an important role beyond serving as food for birds. They are great predators of other insects, and help to control populations of many, including mosquitoes. A little jumping spider clinging to your screen door feasts on pesky fleas and mites. The more daunting wolf spider eats flies, other spiders and cockroaches.

If spiders are at the top of your phobias, give them wide berth by all means. But before heading for the spray can, think of their importance in the web. And remember Ovenbird parents that might pick up poisoned spiders for their nestlings.

Kay Charter is Executive Director of Saving Birds Thru Habitat and co-owner of Charter Sanctuary for breeding and migrating birds. She also sits on the Kirtland’s Warbler Alliance committee.