Bugs are for the birds, luckily for us

Fifteen pairs of tree swallows nested this summer in boxes at a nearby golf course. Each pair hatched five or six eggs. Let's just say five. At that rate, the parents as a group, 30 birds, would be feeding 75 nestlings.

The North American Bluebird Society (nabluebird society.org) estimates that during the average 45-day nesting period one family, adults and chicks, will consume 300,000 flying insects.

Fifteen swallow families eating 300,000 insects in a nesting season is 4.5 million fewer bugs for you and me.

Tree swallows hunt only on the wing. They don't feed on the ground or in trees. Research has shown that 90 percent of insects captured by these birds are taken below an altitude of 40 feet.

Mosquitoes fly near their food sources. How tall are you?

Tree swallows are common here. So are barn, cliff, rough-winged and bank swallows, as well as purple martins. They all feed on those insects we see flying around us.

Guild members

Bird species can be broken into guilds — groups of species that have similar requirements or play similar roles within a community. Guilds can be defined by how members catch insects. There are several ways to do that and many places to hunt.

The ground guild captures prey at ground level. That's how bluebirds feed. They eat crawling and hopping insects exclusively, at least in nesting season.

Many people who maintain bluebird boxes place the boxes in pairs. The second box often is used by tree swallows. The two species can share feeding territory because each takes insects in its own way — they belong to different guilds.

Arboreal species find their food in trees. Woodpeckers chisel into trees, looking mostly for grubs. Woodpeckers are known as bark excavators. Nuthatches also work trees, but as gleaners. They pick insects from tree bark. Brown creepers do the same, more often poking behind bark slabs to find their food.

Working just part of the tree

Catbirds are foliage gleaners, finding their insect meals in undergrowth. Other species in this guild divide trees into high and low. There are warbler species that work the upper canopy, and others that work lower.

Warblers make another division: Some work the branches close to the trunk while others find food in outer branches. One evergreen tree could provide food for several warbler species.

Birds that catch food on the fly make a similar distinction. Swallows are among species that catch insects flying below the tops of trees. Chimney swifts and common nighthawks are among the species that work above treetops.

Last, we have species that find food in ponds, lakes and rivers. Kingfishers feed in open water, as do terns. Waders, among them the various species of sandpipers, probe mud both onshore and off for food.

Look at waders, and you will find legs short and long. This dictates where these birds will feed. Longer legs allow foraging in deeper water, shorter legs require shallow water or the shoreline.

The division of the world into distinctly defined food sources governs bird competition for food. It also ensures that all bases are covered.

Tree swallows are just a ready example of the extent of benefits that birds deliver in their pursuit of food. Without them we would be overrun with insects intent on eating our lunch — or us.

We could be reduced to competing with birds for our dinner.

Read Jim Williams' birding blog at startribune.com/wingnut.