Looking for a fun outdoor activity this winter that will appeal to birders, nature watchers and families?

To get started, head outside and find a field of gone-to-seed Canada goldenrod plants with some woods nearby. These are fairly common in open spaces and meadows, even in the metro area.

Now focus on tall goldenrod plants, those that have a round growth on the stem, and check these balls over carefully to see if any have a small hole.

If you find one, you've discovered the sign of a fascinating story, one that features a bird, a tiny fly and, ultimately, a winter feast. It's a tale of one of nature's synchronicities, happier for the bird than the fly.

Downies love galls

Downy woodpeckers, it turns out, are very fond of the insect whose activity causes the round growths, called galls. The small woodpeckers begin visiting goldenrod in early December, tapping on galls to determine if there's a grub inside. If they detect one, they peck a hole in the gall, then stick their long tongues down to the core to pull out the tasty, calorie-rich grub waiting inside for spring.

These grubs can be an important source of winter food for downies that live near goldenrod fields.

The galls are easy to overlook but once you start noticing them, you'll start seeing them often. Chances are slim that you'll catch a downy at work on a gall: Our smallest woodpeckers are pretty secretive about their goldenrod searches, since they're more vulnerable to predators while out in the open.

A yearlong cycle

The gall you find in the winter got its start last summer, when a tiny female goldenrod fly laid an egg on the plant's stem. When it hatched, the minuscule grub burrowed its way into the stalk, its chewing causing the plant to create a chamber around it. The grub grew throughout last summer as the gall provided both food and housing.

Then, in late fall, an amazing thing happens: The grub chews its way almost up to the surface of the gall, leaving only a thin skin of tissue in place, then crawls back to wait for spring. It does this because the grub will morph into a fly inside the gall, but a fly whose mouth will lack chewing parts, so it would be trapped without that grub-formed tunnel. When May comes around, the little fly needs only to push against the gall's now-thin wall to escape.

Of course, many don't survive to morph into flies, since a number of other insects and the downies are predators. The gall grubs "might be a major source of winter food for downies," say field researchers John Confer and Peter Paicos. They found "intensive predation on large galls on tall stems near woody vegetation," during their work in New York State.

Humans sometimes make use of the goldenrod grubs, too. Several fishermen I talked to recalled that as kids they'd break off goldenrod stalks before heading out for ice fishing. They'd dig out grubs on site to use as bait. It's a wonder that any goldenrod flies are left to emerge each spring, but they do, because the cycle goes on.

Now's the time to head on out to an open area with many goldenrod plants, checking for round growths on the plants' stems. If you find a small hole in the ball, it probably means a downy woodpecker enjoyed a plump grub on a winter's day.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net.

Sites with goldenrod fields

Some sites with Canada goldenrod adjacent to woodlands suggested by John Moriarty, senior manager of wildlife for Three Rivers Park District:

Ramsey County

Battle Creek Regional Park, east prairie area

Tamarack Nature Center

Tony Schmidt Regional Park

Long Lake Regional Park

Three Rivers Park District

Silverwood Park

Elm Creek Park Reserve, near Eastman Nature Center

Crow Hassan Park Reserve, prairie

Murphy Hanrehan Park Reserves, prairies, south portion of park

Carver Park Reserve, prairies and old fields

For more information and park directions, visit ramseycounty.us/residents/parks-recreation and threeriversparks.org.

'Dees get into the act

Downy woodpeckers are the major avian predator of the gall grubs, but in some parts of the country, chickadees get into the act, as well. It's easy to tell which species of bird pulled out a grub, since downy holes are small and round, while chickadees leave a messier opening as they pull off outer layers to get to the interior.

No real harm

The creation of a gall and the presence of the grub doesn't kill the goldenrod plant, but its growth is slowed and it produces fewer flowers.