December Species Spotlight: Red Crossbill

Red Crossbill. Photo by Adrianna Nelson.

Perhaps the most unique finch found in our region of Southern Appalachia is the Red Crossbill. It is easy to imagine this species’ appearance given such a descriptive name. This hefty finch does, indeed, have a crossed bill. The tips of their mandibles are long. When the mandibles are closed, the tips extend past each other. This incredible adaptation has developed to help the crossbill consume conifer cones on which it solely relies for food. The crossbill will open its beak slightly, insert it between the scales of a cone, then close down, effectively prying the scales apart to expose the seed inside. The scientific name, Loxia curvirostra, is derived from words meaning “crosswise” and “curved bill.” Interestingly, the bills of some individuals cross to the left while others cross to the right. The number of right versus left crossing bills is approximately equal. In similar species, such as the White-winged Crossbill, an individual’s bill is 3 times more likely to cross to the right than to the left.

And, yes, they are also red – at least sometimes. Males are red all over while females are a duller mustard-yellow, and both have grayish-brown wings. Juveniles are even more drab and are covered with heavy streaking. In general, the Red Crossbill is found year-round in most of southern Canada, western United States, parts of Mexico, and pockets in the Southern Appalachian Mountains. Their primary wintering range covers much of the northern states while they are scarcely found even farther south. Their range map might look simple, but there are more intricacies than meets the eye. The crossbill is subdivided into groups called “call types.” As you may have guessed, this refers to the slight, yet consistent, variations in vocalizations that differentiate populations across geographic regions.

Each of these types, in addition to unique call variations, also specialize in consuming the seeds of certain species of trees because each type has slightly different sized bills. Altogether, there are 11 types. Here in Southern Appalachia, the “Type 1” or “Appalachian” crossbill is most common. The call is usually described as a series of jip-jip-jip sounds. It is suited for eating seeds from trees such as White Pine, Red Spruce, Frasier Fir, Virginia Pine, Pitch Pine, and others. They are most frequently found at higher elevations that offer vast swathes of these conifers, such as Roan, Whitetop, and Grandfather Mountains. The next most common type in Southern Appalachia is “Type 2” or “Ponderosa Pine” crossbill. Our region falls under the secondary zone of occurrence for this type, so they are not as frequently found here. They feed on many of the same species as type 1: Red, Jack, Pitch, and Virginia Pines. The type 2 crossbill also forages on Table Mountain Pines, which type 1 crossbills usually avoid. The Table Mountain Pine has large, heavy, thick, and spiny cones. The larger bills of the type 2 are better suited for handling these tougher cones.

These call types have posed interesting questions for biologists all over the country. It is clear that there are relatively constant distinctions among the populations, but these birds challenge typical definitions of how subspecies should act. Many believe that these call types should be considered their own subspecies, or even be split altogether to create entirely separate species. This happened very recently; in 2017 the Cassia Crossbill (Loxia sinesciuris), once considered a Red Crossbill subspecies, was determined to be unique at the species level. To make the matter more confusing, Red Crossbills rarely remain in one place for very long. During years of low cone crops, crossbills will disperse over great distances to find sufficient food, called “irruptions.” These movements mean that crossbills of various populations can end up in unlikely locations and interact with one another. Such mixing complicates traditional ideas of what constitutes a species or subspecies. Distinguishing the ecology, behavior, and genetics of such a fluid and dynamic bird makes determining their taxonomic status tricky. Even so, irruption years can be quite exciting for birdwatchers. In 2020, many finches (including crossbills, Evening Grosbeaks, Common Redpolls, Pine Siskins) and Red-breasted Nuthatches appeared in our region. Outbreaks of boreal birds sometimes provide birdwatchers a chance to witness these species when they diffuse outside of their normal range.

In general, crossbills tend to be nomadic, even on normal, non-irruption years. This constant movement helps them to find the food required to sustain themselves and breed. In fact, they are capable of breeding at any time during the year as long as they can locate enough conifer seeds to feed their young. Although, nesting is most common in late summer when the cone crops have begun to mature. Breeding pairs tend to be monogamous and share the load of incubating eggs and feeding young.

One of my favorite memories of crossbills happened on a cool summer day on top of Roan Mountain. Out of the blue, a huge group of crossbills flew overhead and landed in the nearby Red Spruces. They made quick work of the cones, methodically removing and eating the seeds. A few birds dropped to the ground just yards away from me. This family group – a male, female, and juvenile – were bold enough to let me watch closely as they gathered grit from the gravel parking lot. In an instant, they all took flight and disappeared into the green ocean of trees, leaving behind only the whispers of quickly fading jip-jip-jips. The Red Crossbill strikes up curiosity and wanderlust, sparking wonder in the eyes of anybody who gets the chance to witness one of Southern Appalachia’s most amazing birds.

Sources:

https://www.allaboutbirds.org/guide/Red_Crossbill/overview
https://sora.unm.edu/sites/default/files/journals/condor/v090n04/p0745-p0760.pdf