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Summit Outside: The unique beak of the crossbill

Dr. Joanne Stolen
Special to the Daily/Debbie Tyber
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My friend Debbie who lives on Boreas Pass Road photographed a pair of crossbills at her bird feeder.

These birds, with their unusual beaks, are uniquely evolved to feed on the seeds of conifers.

Conifers are cone-bearing seed plants such as firs, pines and spruces. These trees are the dominant species in our alpine environment and well adapted to our climate since the narrow conical shape of conifers and their downward-drooping limbs help the trees shed snow. Conifers seasonally alter their biochemistry to make them more resistant to freezing temperatures.

Red crossbills typically inhabit mature conifer forests, and the different types of birds tend to specialize on preferred trees, including western hemlock, Ponderosa pine, lodgepole pine, Sitka and Englemann spruce.

When conifers produce large cone crops, crossbills often move in to feed and breed. Although in the more northerly boreal (a region where coniferous forests dominate) White-winged Crossbill can at times be found in the high-elevation forests of Engelmann spruce south to the San Juan Mountains and into northern New Mexico. Red crossbills are more common and widespread in Colorado.

Ornithologists classify the red crossbill into distinct races based on eight discrete types of flight calls north of the Mexican border – calls that may play a major role in maintaining reproductive isolation among the groups.

The crossbills have bills or mandibles crossing at their tips, hence the name crossbills.

Crossbills belong to the finch family and are about the same size as sparrows (5 1/4 to 6 1/2 inches). Only five species of birds in the finch family have cross bills.

Another distinctive feature is that they have long pointed wings.

Red crossbills are brick-red with black wings. Females are greenish-yellow with black wings. Juveniles are streaked brown.

The bill size of red crossbills varies considerably and correlates habitat and food preferences as well as flight calls.

Crossbills are often seen hanging from evergreen cones while they feed on the seeds. They can eat 3,000 seeds per day.

Their odd bills are uniquely adapted for separating the scales on a cone and removing seeds. The lower bill or mandible curves to the right or left of the upper. Their crossed bills do not all cross in the same direction and they cannot always position themselves in the correct spot to be able to pry open all the scales, thus they leave half of the scales untouched. This allows the crossbills with the opposite crossing bills to eat the untouched cone scales.

Crossbills start at the bottom of a cone, and spiral upward, opening the scale and removing the seeds with their tongues. The curve enables them to exert forces at the tips of their mandibles necessary for forming gaps between cone scales.

Once crossbills form gaps between the scales, the lower mandible is turned to one side exposing the seeds at the base. The crossbills then use their extendable tongue to lift the seed out. They husk the seed and swallow the kernel.

Bill size, especially bill depth, determines how fast crossbills can remove seeds from between closed cone scales, whereas the upper mandible has a horny palate functions to husk the seed. The Cornell Lab of Ornithology has a fascinating video on YouTube showing how this is done.

The breeding cycle of red crossbills is tied to food availability. Crossbills can breed at almost any time of year, and will do so even in mid-winter if there is an abundant source of seeds. They mate for life and pairs form within flocks.

The female builds the nest on a horizontal branch high up in a conifer tree. The nests appears bulky and cup-like, and is made of loose twigs, grass and bark strips. The nests are lined with fine grass, lichen, feathers and hair.

The female will incubate three eggs for 12-16 days. The male brings food to the female and to the young for the first five days after the eggs hatch, then both parents will bring food to the young.

The young leave the nest after 18-22 days. The parents continue to feed the young for about a month after they hatch while they are learning to feed themselves.

Interestingly, the bills of young birds are not crossed at hatching. It is when the fledglings are learning to extract conifer seeds for themselves that the tips of both top and bottom bills begin growing.

When the bird begins to pry the top bill sideways, it always twists the bill in the same direction thus determining which way the bill will cross. It’s like being left handed or right handed, they are right or left crossed. By 45 days they are crossed enough for the young to extract seeds from cones.

Red crossbills are usually found in small flocks year round. They typically climb in mature conifers, using their bills to grab branches and cones. They will also occasionally land on deciduous trees and forage for aphids.

Red crossbills sing and chatter as they move through branches and can be entertaining to watch. When at feeders these birds can be very curious and may come quite close to people.

A group of crossbills are collectively known as “a crookedness” and a “warp” of crossbills.

Breckenridge resident Dr. Joanne Stolen is retired from teaching microbiology Rutgers University, and has taught classes at CMC. She is now pursuing a career in art, specializing in nature and many of the animals she writes about. Her work can be seen locally.


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