Have you noticed how heavy the cone crops are this summer on red spruce, balsam fir and larch? Cone abundance varies greatly from year to year, but this year’s crop stands out as truly exceptional.

These cones matter greatly to the suite of northern finches that irrupt (come into) Maine in search of conifer seeds. These species include Pine Siskins, Evening Grosbeaks and crossbills. These nomadic birds wander broadly in search of abundant food. Although the abundant cones on our conifers do not guarantee these irruptive finches will grace us with their presence this winter, the abundance of food should keep any irruptive birds happy for the winter.

We have two crossbills in North America, and both can be found in Maine. These species are the Red Crossbill and the White-winged Crossbill. Usually, the White-winged Crossbill is the more common species.

The hallmark of the crossbills is the peculiar overlapping arrangement of the upper and lower bill. One bill crosses over the other. Sometimes the upper bill curves to the left and other times to the right. Why such an odd bill?

Crossbills are specialists on the seeds of conifers. Take a look at any conifer cone. The cone has a number of scales. At the base of each scale, the seeds of the conifer can be found. Most birds have a difficult time negotiating the scales to get to the seeds deep within the cone.

Not crossbills, however. A crossbill inserts its bill between two scales of a cone. The crossed points of the bill act as a wedge, increasing the distance between the two scales. The strong tongue of the crossbill can then be inserted to the base of the scale and the nutritious seeds removed and eaten.

The crossbills must be quite agile to position themselves in the proper position on the cone to extract seeds. Crossbill feet are quite strong. The behavior of crossbills while feeding reminds many people of parrots whose feet have great dexterity.

Although the crossed bill serves these birds marvelously in feeding on cones, the specialization comes at a cost. Crossbills are quite awkward in handling food that other finches readily eat, such as thistle or birch seeds.

Usually crossbills feed on cones that their bills can handle with greatest efficiency. The Red Crossbill has a larger and longer bill than the White-winged Crossbill. Red Crossbills feed on white pine, balsam fir and sometimes hemlock cones. White-winged Crossbills feed on smaller cones like those of larch and red spruce.

Depending on conifer seeds is risky. Cone production by these trees is notoriously variable. In some years, hardly any cones are produced. In other years, every tree of a given conifer species seems loaded with cones. During the years of high cone production, the crossbills do well. In years of poor cone production, crossbills must wander until they find an area where the conifers are having a good cone year.

Crossbills are therefore nomads, wandering over wide areas to find abundant cones, the one food for which they are supremely adapted to use. The ceaseless wanderings of the crossbills means their abundance will be highly variable at a particular location.

Crossbills don’t have a particular nesting schedule. When the birds find a large cone crop, they may initiate nesting regardless of the time of year. Red Crossbills appear to require day lengths of at least 12 hours before they will nest. However, White-winged Crossbills have been found nesting in every month of the year.

Red Crossbills have been the focus of recent study by ornithologists interested in bird classification. These workers have noted that the call notes of Red Crossbills vary from place to place throughout North America. Differences were found in bill size and body size that correlated with different call notes. Evidence now indicates that there may be at least eight different species of Red Crossbills. Telling them apart in the field is very hard and must be done in most cases by identification of the distinctive calls.

Three types of Red Crossbills have been found in Maine, and a fourth eastern type occurs in Newfoundland. These forms have not been officially recognized as distinct species by the American Ornithologists Union but it is only a matter of time. Identifying the crossbills in the field will be a real challenge.

If you are interested in learning more about the vocalizations and morphology of the different types of the Red Crossbills, here is a great website: research.amnh.org/ornithology/crossbills. You can even hear sound files of the different crossbill types.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

whwilson@colby.edu