An old European legend says red crossbills shaped their peculiar bills by attempting to remove the nails from Christ's hands and feet when he was on the cross.

In reality, the birds evolved bills of different size and shape to match food preferences.

Crossbills are nomadic species usually found in boreal forests and western mountains, including small numbers in northern Minnesota.

An unusual number of them are now being seen in west central Minnesota corn and bean country.

The birds are here in search of food, the variable conifer seeds on which they feed having a thin year. Where do you find a good supply of conifers amid the fields? Cemeteries, where most of the birds are being spotted in conifers used for landscaping.

We are experiencing what ornithologists call an irruption, a migration of a bird species outside its normal range. It might grow as fall and winter progress, bringing more of the birds over a wider area.

There are at least 10 types of red crossbill. Each type prefers a particular species of pine, spruce or fir. The various tree species have different cones and seeds.

The birds eat seeds tucked deep in the cone. Each red crossbill type has evolved a bill appropriate to removing a particular seed. Different trees, different seeds, different bills.

To get an idea of what these bills look like, cross your fingers.

Males are brick red with black wings. The types look alike except for bill variations, and bill shape is hard to use for ID purposes.

There is an important second difference between types — the call note the birds deliver while flying or feeding.

Calls help the birds maintain flocks. The 10 types can be distinguished by close examination of those call notes. You would need very good ears. Precise determination is made with a sonogram, a graphic representation of the sound.

Minnesota birders are recording the call notes on cellphones held overhead in front of pine and spruce trees in those graveyards. The recordings are sent to the Cornell Lab of Ornithology for type determination.

As of early September, five of the 10 crossbill types reportedly had been found here.

Fitting the bill

If you are familiar with Charles Darwin's visit to the Galapagos islands in 1835, there is a parallel with crossbills. The finches he found — 14 similar species — were essential to development of his theory of evolution. The finches competed for food on the islands. As seed eaters, they reduced competition by evolving bills shaped for particular types of seeds.

Red crossbills have done something similar.

There are two other crossbill species. The first is the white-winged crossbill, a bird found here. It is obviously marked, so it's easily identified without hearing its call.

The second is brand new, the Cassia crossbill, found only in one county in Idaho. The American Ornithological Society recently gave it species status. Evolution of a thicker beak distinguishes this bird.

The beak is thicker because the bird is the only creature there that feeds on lodgepole pine seeds. The pine has evolved thicker cone scales to protect itself from the bird. The bird has evolved a larger, stronger beak to deal with the thicker scales covering the seeds.

It is a co-evolutionary arms race.

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