by Bryce W. Robinson
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.
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).
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).
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).
For a brief guide to finding and identifying Cassia Crossbill, visit: https://idahobirds.net/birding-idaho/cassia-crossbill/
Chesser, T. R., K. J. Burns, Carla Cicero, Jon L. Dunn, Andrew W. Kratter, Irby J. Lovette, Pamela C. Rasmussen, J. V. Remsen, Jr., James D. Rising, Douglas F. Stotz, and Kevin Winker (2017) Fifty-eighth supplement to the American Ornithological Society’s Check-list of North American Birds. The Auk 134:751-773.
Young, M. and T. Spahr. 2017. Crossbills of North America: Species and Red Crossbill Call Types. Retrieved online from: https://ebird.org/news/crossbills-of-north-america-species-and-red-crossbill-call-types/