Open Mic: Bills and Beyond–The Wonders of the Red Crossbill

At the Mic: Kristina Polk
Kristina Polk is an 18-year-old birder from North Ridgeville,Ohio. She is attending the University of Wyoming in Laramie, majoring in Zoology. A lifelong nature lover, she began birding in 2010. The Grey Catbird is her favorite bird, and her nickname in the birding community is ‘Catbird’ due to her intense love of this charismatic little mimid. Her other hobbies are photography, sketching, reading, and writing, through all of which she channels her intense love for wildlife. She takes daily walks around her neighborhood in search for birds and the local deer, many of which she has cataloged with photographs. She has been vegetarian since 2008 and vegan since 2010 and she is passionate about the welfare of our planet and its inhabitants. She is an activist for cetaceans, farm animals, and birds, particularly the anti-captivity and anti-slaughter movements for dolphins/whales, and the vegan movement.


Were I an ornithologist,
I think that the finches of the genus Loxia
would take up most of my research time.
No group of birds seems to offer more
tantalizing problems in that area of biology
where systematics, ecology, zoogeography,
population dynamics, and ethology overlap.
Dr. William L. Brown, Jr., 1957


South Hills Crossbills © Craig Benkman

While most know of Darwin’s Finches or the Galapagos mockingbirds, Red Crossbills (Loxia curvirostra) don’t usually come to mind when thinking of speciation. Quickly becoming the archetype of coevolution and diversification, crossbills are members of the finch family, males a brick red color and females yellow-green, both with dark brown wings and tail. These birds are named for their unique trait of a crossed bill— the adaptation that allows them to specialize on feeding on conifer seeds. A nomadic species, they follow seed crops throughout the year, all across North America.
Within the L. curvirostra complex are ten different ‘call types’—morphologically, and audibly distinct from one another. Each call type has, as expected, a different call note, as well as a different sized bill and palate groove, and relies on a different species of conifer. Multiple call types may be found breeding in the same area, but they do not readily mate with each other. Benkman (2007: 161) explains possible reasons for the evolution of call types in crossbills: “Distinct ‘call types’ evolved because they allowed crossbills to readily flock with crossbills having like morphologies, since it would be easier for them to recognize similar crossbills by call type than by subtle differences in palate structure and bill size and shape. Because crossbills flock year-round and chose mates within flocks, assortative flocking may have been key to the rapid radiation of crossbills.”
Crossbills bite between cone scales and spread their lower mandible to the side, creating a gap between the cone scales, and then use their tongue to remove the seed. They secure the seed in their palate groove with their tongue and then quickly remove the seed coat and swallow the seed kernel. The birds have evolved grooves that fit the size of the seed that their preferred conifer produces, which increases foraging efficiency. For crossbills that feed on larger, tougher cones, their bills are large and deep. Crossbills like Type 3, which feed on the small, thin-scaled cones of western hemlock, have the smallest bills of all the call types in North America. Benkman (2007: 154) explains that “because bill size and palate groove width influence how rapidly seeds can be removed from cones and husked, respectively, conifer species with seeds of different sizes favor the evolution of crossbills of different sizes.”
The different call types feed on a range of conifers, from Sitka spruce to ponderosa pine to Douglas-fir, but the most important characteristic that a tree must have for crossbills is the holding of seeds in closed cones through winter and into spring. The cones must be closed or partially closed because if they are open, other species will be able to access the seeds and exhaust the supply before winter’s end. Crossbills do best in years of large cone crops, where there are more cones overall and more seeds per cone, which allows them to harvest seeds more quickly.
In the South Hills of Idaho, a call type has evolved alongside the Rocky Mountain lodgepole pine (Pinus contorta latifolia) and in the absence of the seed competitor red squirrels. Call type 9, the South Hills crossbill, has been studied intensively for over a decade by Dr. Craig Benkman, a professor at my school, the University of Wyoming. He has found that “fewer than one percent of the breeding South Hills crossbills pair with a non-South Hills call type” (Benkman 2007: 159). This, along with their morphological differences and unique voices, make them a good candidate for a separate species. [Dr. Benkman has proposed “Loxia sinesciurus”, ‘the crossbill without tree squirrels’, referencing the absence of red squirrels in their habitat (Benkman 2010: 52)].
Assisting in Dr. Benkman’s research in the South Hills, I am employed to organize banding and recorded data collected in the field. In the summers, Dr. Benkman and students go to the South Hills to band crossbills and record their calls. All the banding data are recorded on sheets of paper, which I must look at while listening to the hundreds of recordings. I open the recording for a bird in Raven, the Cornell Lab of Ornithology’s sound analysis software, and choose the clearest spectrogram. The recordings are in order according to the banding data, so it’s easy to keep track of who I’m listening to.
The recordings contain a bander (Dr. Benkman or a student) reading off the direction the bird’s lower mandible crosses (bird’s bills can cross to the left or the right), the sex, the band number, and the band color combination (birds can have different colored bands on each leg, for example, bird 6360, a female, has an orange over red on the left leg and a blue U.S. Fish and Wildlife Service issued band on the right leg). Then, the bird is released—a whirring of wingbeats, visible as a series of vertical lines on the spectrogram, followed by a few flight calls. Sometimes the bird doesn’t call, but usually the bird gives a rich note or two as it flies away.
Perhaps the most important aspect of my job is to pay close attention to the spectrogram of the bird’s call and determine the call type. An overwhelming majority of the birds are Call Type 9, or South Hills crossbills. Types 2 and 5 show up, with Type 2 being more frequent (from my experience with the data).
It may seem difficult to differentiate between all these call types, especially since in essence they sound similar, but once you get to know each type, the differences become quite clear, and interesting!
CB crossbill SH 1

Type 9 (South Hills crossbills) calls are rich, full calls and their spectrogram always shows at least a tiny hint of the characteristic hump shape, or upside-down U shape.

CB crossbill SH 2

Sometimes this shape is very well defined, and this is the typical South Hills call.

Sometimes, the call looks a bit odd, but it still sounds like a South Hills and the hump shape, although a bit distorted, is still there. The call can be very faint, but while listening to it and really zooming in on the spectrogram, it’s obvious that it is a South Hills call!
Then, there are the juveniles… young birds, not sure of how to properly call yet!
CB juvenile crossbill 1

Type 2 calls are a descending, clear, liquid call. It is a bit lower in frequency than the South Hills calls, and although some South Hills calls will be descending, Type 2 calls do not have the little hump at the beginning like a South Hills call would have. Type Two calls are descending from the very beginning of the call. They can be a bit S-shaped, but they are always descending.

CB T2 1
CB T2 4
CB T2 5
Type 5 calls are higher pitched, and have a metallic quality to them.
After recording the call type, I verify that the data in the recording match the data on the banding sheet, and then plug in the bird’s band number (or, if it is a ‘mite bird’, or a bird with a case of scaly leg mites and therefore without a numbered metal band, I plug in the sex, mandible cross, and band color combination) into the computer database. I once again verify the information from banding sheet to database, and then I add the bird’s call type in the appropriate field. Finally, after I am sure that I have the right bird, I write down the Bird ID number (a four digit code unique to every bird, an easy way of giving an identity to them in the midst of hundreds and hundreds of entries) and rename their recording’s file with their ID.
I have only scratched the surface of the wonders of this incredible species, their adaptations and ever-fascinating diversification. Even as I go through the data, I manage to find odd calls that stump or surprise even Dr. Benkman. The enigma of the Red Crossbill is truly one of the most enthralling natural history mysteries that has only begun to be unraveled.

We humans have a hankering for seeing unusual creatures, so we visit zoos, take African safaris, and invent weird, extraterrestrial monsters. And all the while some of the oddest of birds are here at hand, quietly extracting and husking pine seeds over our heads and flashing red among the branches.  –Howard Ensign Evans

Further Reading

Literature cited (also highly recommended reading!)

Benkman, C.W. “Red Crossbill Types in Colorado: Their Ecology, Evolution, and Distribution.” Colorado Birds. 41.3 (2007): 153-162. Print.
Benkman, C.W. “Diversifying Coevolution between Crossbills and Conifers.” Evolution: Education and Outreach. 3. (2010): 47-53. Print.