The creative study of birds through illustration, photography, and writing

Idaho’s Endemic: The Cassia Crossbill (Loxia sinesciuris)

by Bryce W. Robinson

CACR-online-01

Cassia Crossbill (Loxia sinesciuris) and Rocky Mountain Lodgepole Pine (Pinus contorta latifolia). 11×14″ gouache on watercolor board. From top: Adult male, adult female, and juvenile. Original illustration donated to Golden Eagle Audubon Society in Boise, Idaho.

This article is an overview and summary of the Cassia Crossbill (Loxia sinesciuris), including it’s evolution, life history and distribution, taxonomic status, and conservation status and threats. Additionally, I’ve listed many resources available to get to know this excellent example of the processes of evolution in effect. This is a fluid post, meaning I will revise and add as I gather more material and information (such as photos, recordings, etc.) or as information becomes available through further studies of the species. My aim is to provide a resource from which curious birders and naturalists can delve into learning about this incredible species as well as provide a resource for seeking it out in southern Idaho. Any thoughts, suggestions, revisions, or additions are welcome.

INTRODUCTION

The Cassia Crossbill represents our continued refinement of understanding the natural world. How peculiar it seems that in the 21st century, while beginning to recognize and understand incipient speciation in some taxa, we are also finding well established independent evolutionary lineages that have until now gone unnoticed. Even more peculiar is that the Cassia Crossbill is certainly not restricted to a place where ornithologists and bird enthusiasts rarely visit. They breed in areas with extensive road networks and occupy ranges with nearly year round access. My point is that we certainly haven’t missed them, we have only overlooked them. I don’t consider this an embarrassment, I find it extremely exciting. How many other patterns such as this have we yet to notice?

This bird’s name, Cassia Crossbill (Loxia sinesciuris) is loaded with information and in my opinion aptly applied nomenclature. I commend those involved with choosing these names. Below is an explanation of this loaded nomenclature, as well as an overview of the evolution of the species, it’s distribution, what separates it from the Red Crossbill (Loxia curvirostra), why it was finally elevated to species level by the AOS Check-list committee, and threats to its future in our ever-changing world. I’ve also included a list of resources from which I’ve extracted the information I present here.

THE EVOLUTION OF THE CASSIA CROSSBILL

The first aspect of the Cassia Crossbill’s loaded name is its species epithet, sinesciuris. Although I already knew about the unique situation that gave rise to the divergence of this species, I hadn’t put much thought to this name. I now realize the etymology of the word – “sin” meaning “without”, and “sciuris” referring to squirrels. So, the scientific name means “Loxia without-squirrel” – an excellent transition into the proximate reason this species developed in this small area in southern Idaho.

The South Hills and Albion Mountains in southern Idaho are unique in the respect that they lack a primary mammalian seed predator, the red squirrel (Tamiasciurus hudsonicus). Rocky Mountain lodgepole pine (Pinus contorta latifolia) in this region are thus relieved of this predatory pressure, however one important seed predator occurs in the region, the Cassia Crossbill. The crossbill fills the void of the squirrel, spurring a unique relationship that has resulted in the divergence of this crossbill type from the Red Crossbill complex (Benkman 2009).

From this absence of squirrels, lodgepole pine in this region has been relatively free of pressures on one aspect of the trees biology, serotiny. Serotinous cones are cones that remain closed until they are heated by fire. Because red squirrels are a selective agent against serotiny, the frequency of serotinous cones in South Hills and Albion Mountains has increased due to the squirrels’ absence. This has resulted in a large seed bank that is utilized by the Cassia Crossbill, an important aspect in it’s evolutionary trajectory.

A relatively stable and abundant food resource has given rise to a unique life history strategy of this crossbill when compared to the Red Crossbill. The Cassia Crossbill is sedentary. Furthermore, the crossbill as a primary predator and the pine as a primary food source are coupled in their life history, and thus locked in an ‘evolutionary arms-race’ where one species develops ‘armaments’ and defenses to lessen predation pressures whilst the other develops ‘weapons’ and tools to better access this resource, the seeds. This coupled relationship acts in selection and causes divergence of traits, such as we see between these two taxa where the crossbill has developed a larger bill relative to Red Crossbill types. From this relationship, a new species of crossbill has arisen.

LIFE HISTORY AND DISTRIBUTION – What separates L. sinesciuris from L. curvirostra?

The other part of the Cassia Crossbill’s loaded name refers to its relatively minuscule range, when compared to that of its sister species the Red Crossbill. This species occurs in only two counties, the core being in Cassia County, a truly uncommon trait for taxa in North America (See Fig. 1, and also be sure to check out the eBird data for this species).

65625621-1280px

Morphologically, Cassia Crossbill’s have a larger bill than Red Crossbill, and average larger in body mass (Benkman et al.  2009). Otherwise, it is difficult to distinguish the two sister species from appearance alone and in fact judging bill size in the field is entirely unreliable.

A better distinguishing characteristic, and one that is actually quite discernible is the call type. Cassia Crossbills have a much dryer and sharper call note than other Red Crossbill types, a difference that can be quite distinctive when heard in the field. Additionally, observers can record crossbills and look at the spectral characteristics of the call notes to identify between Cassia Crossbill, and all other Red Crossbill types (spectrograms to come soon from my own recordings). eBird has published an excellent article detailing each call type, their identification, the distributions of each, and some brief information on their biology (Young and Spahr 2017).

Additionally, the songs of the Cassia Crossbill differs from Red Crossbill in consisting of more buzzy notes (rather than whistled), and have more repetitive syllables (Benkman et al. 2009). I hope to record some songs later this year (2018) and include a spectrogram here.

Cassia Crossbill range also hosts two Red Crossbill types on occassion (Types 2 and 5). Although these taxa occur in sympatry, Cassia Crossbill mate assortatively at an extremely high rate, resulting in reproductive isolation which is a key mechanism for divergence.

The Cassia Crossbill also differs from Red Crossbill through shifted and set phenology of life history events. It has a more seasonal breeding strategy relative to the sporadic breeding nature of the nomadic Red Crossbill, where their breeding initiates at relatively the same time each year (Benkman et al. 2009). Additionally and related to their sedentary lifestyle and regular breeding cycle, the Cassia Crossbill molts at the same time in the late summer until early fall each year (ibid).

 

TAXONOMIC DECISION

In the 58th supplement to the American Ornithological Society’s Check-list of North American Birds (Chesser et al. 2017), the committee approved the nomination to elevate Cassia Crossbill to species level based on high levels of reproductive isolation, and genomic differences.

CONSERVATION STATUS AND THREATS

Because of the small and restricted range of this species, it’s particular life history, and the predicted impacts of climate change in the region, the Cassia Crossbill has an uncertain future. Compounding these impacts are threats to lodgepole pine such as shifted fire regimes and the mountain pine beetle (Dendroctonus ponderosae), which have the potential to extirpate lodgepole pine from south central Idaho (see Benkman 2016 for a discussion of these threats).

ADDITIONAL RESOURCES

For a brief guide to finding and identifying Cassia Crossbill, visit: https://idahobirds.net/birding-idaho/cassia-crossbill/

REFERENCED LITERATURE:

 

Advertisements

Singing Coastal Cactus Wren Perched on Coastal Cholla

by Bryce W. Robinson

 

 

The Cactus Wren (Campylorhynchus brunneicapillus) is a polytypic wren (Family: Troglodytidae) that occurs in the arid southwest of North America. The species comprises five subspecies (Following Rea and Weaver 1990). Birds on the coast of southern California differ in appearance slightly from interior groups, primarily in being paler on the flanks where they have less rich and warm tones. The taxonomy of this coastal group has been in flux, but it is currently recognized by Clements, Howard & Moore, and others as C. b. sandiegensis.

A few years ago, I had the pleasure of getting to know this species well while working with nesting birds along the I-10 corridor in California. I can still hear their iconic rattle song as they sing atop cholla in the intense heat of the Sonoran desert. Their nests also stick in my memory. Often in dense and formidable cholla, the species construct a tunnel nest out of grass. These are some of my favorite nests I’ve encountered in all of my time in the field.

I had the pleasure of illustrating this bird for silent auction at the Sea and Sage Audubon Society’s annual benefit dinner. Hopefully it generates some funds for them and finds a good home.

If you like this image and want a print, you can get one HERE.

Referenced Literature:

American Dipper (Cinclus mexicanus) Illustration with a Note on the Evolution of Cinclidae

by Bryce W. Robinson

AMDI-online-01.jpg

American Dipper (Cinclus mexicanus). 11×14″ gouache on watercolor board. 

I enjoy supplementing each illustration I do with a bit of deeper discussion pertaining to the subject at hand. Because I’m beginning more in-depth study of evolutionary history and relationships in birds, I’ll give a brief synopsis of our current understanding (thanks to Gary Voelker) of the evolution of the five species belonging to the dipper family (Cinclidae) and the origins of the American Dipper (Cinclus mexicanus) to complement this illustration.

Although the phylogenetics (genetic history) of dippers was published in 2002, and thus utilized mitochondrial data to inform the inferences I’ll lay out below, I suspect applying new techniques wouldn’t change the outcome much. That’s just a hunch, and revisiting the phylogenetics with next generation sequencing methods is certainly warranted and needed.

Mitochondrial data shows two important evolutionary points:

  • Dippers (Cinclidae) are most closely related to Thrushes (Turdidae).
  • Dippers originated in the old world, where they diverged and colonized the new world later (~4 million years ago).

Fun and interesting information for understanding dipper diversity.

If you are a dipper lover and you’d like a print, you can purchase one here:

http://ornithologi.bigcartel.com/product/11×14-limited-giclee-print-american-dipper

Referenced literature:

Voelker, G. 2002. Molecular phylogenetics and historical biogeography of dippers (Cinclus). Ibis 144(4):577 – 584.

Melospiza Plate

by Bryce W. Robinson

If you like this painting, prints are available in the online shop.