Cassia Crossbill

The Cassia Crossbill – Idaho’s New Endemic Species

Male Cassia Crossbill
Male Cassia Crossbill. Photo by Craig Benkman

Formerly one of the 10 call types of Red Crossbill (Loxia curvirostra) in North America, the Cassia Crossbill (Loxia sinesciurus) was recently recognized to warrant species status by the American Ornithological Society (58th supplement to the AOS Check-list of North American Birds), giving Idaho its sole endemic species.

The Red Crossbill call types are distinguished by their vocalizations, especially their contact calls, and by their bill and body sizes and associated conifer preferences, and thus areas where they most commonly reside. General information on the call types can be found here: North American Red Crossbill Types

Most crossbills are nomadic and therefore cannot be found reliably in any single area. This is not true for the sedentary Cassia Crossbill, which relies on the stable seed production of Rocky Mountain lodgepole pine (Pinus contorta latifolia) in the South Hills and Albion Mountains year-round. Indeed, the Cassia Crossbill is the only crossbill in North America that you can count on finding year-round, year after year in the same forest.

A Unique Evolution

Key to the evolution of the Cassia Crossbill is the absence of red squirrels (Tamiasciurus hudsonicus) from the lodgepole pine forests in the South Hills and Albion Mountains. Here, Cassia Crossbills are about 20 times more abundant than Red Crossbills in similar forests in the Rocky Mountains. Unlike in the South Hills and Albions, one can walk several days in the lodgepole pine forests in the Rocky Mountains and not see or hear a crossbill. Moreover, many of the above-ground and open-cup nesting song birds are about twice as abundant as in the lodgepole pine forests in the Rocky Mountains, presumably in part because of the absence of red squirrels as nest predators.

Crossbills are seed predators, so trees that deter foraging crossbills have a reproductive advantage. Because crossbills occur in higher densities and consume more seeds in the South Hills and Albions than in lodgepole pine forests elsewhere, crossbills are especially important natural selective agents on the cones. Crossbills avoid foraging on cones having thicker scales at the distal end of the cone where most of the seeds are located (crossbills have greater difficulty spreading apart thicker scales to expose the underlying seeds). This favors the evolution of cones with thicker distal scales, which distinguishes the lodgepole pine cones in the South Hills and Albions from those elsewhere. The thicker cone scales in turn explain why the Cassia Crossbill has evolved a bill that averages about 0.6 mm deeper than that of Type 5, which specializes on lodgepole pine where red squirrels are present. This coevolutionary arms race between crossbills and pine in the South Hills and Albions has ultimately favored the evolution of the Cassia Crossbill.

Distribution

The small area of lodgepole pine in the South Hills and Albions limits the distribution of the Cassia Crossbill to about 70 km2 of forest. Even with relatively high densities, they number only about 6,000 individuals. This limited distribution and small population size makes them especially vulnerable to environmental change, such as more frequent and larger fires. Fortunately, pine beetle outbreaks have been more limited than in other lodgepole pine forests, perhaps because of the higher densities of insectivorous birds including the very abundant Hairy Woodpecker. See this paper Benkman_2016.pdf for more information about the natural history of the Cassia Crossbill.

How to Identify Cassia Crossbills

Male Cassia Crossbill
The lower mandible crosses to the right, as in this male Cassia Crossbill, in about half the crossbills, and to the left in the other half. Photo by Craig Benkman.

Along with Cassia Crossbills there are two other call types that occur regularly in summer in the South Hills, Type 2 (generally ponderosa pine associated) and Type 5 (Rocky Mountain lodgepole pine associated outside of the South Hills and Albions). Both of these call types average smaller than the Cassia Crossbill (bill depths average 0.3 and 0.6 mm less than in the Cassia Crossbill, respectively), but, because of extensive size overlap and no consistent plumage differences, one needs to rely on vocalizations.

Cassia Crossbill contact call:

Download File (.wav) Recording by Julie Smith

Type 2 Red Crossbill contact call:

Download File (.wav) Recording by Julie Smith

Type 5 Red Crossbill contact call:

Download File (.wav) Recording by Julie Smith

Playing these calls to try to get a response from the Crossbills does not work, you just have to listen for them.

Like many birds, crossbills are most active in the morning, relatively quiet by late morning, and then more active in the mid to late afternoon and then often quiet in the evening when other birds are active.

Where to find Cassia Crossbills

South Hills Lodgepole pine
Lodgepole pine near the Porcupine Springs area in the South Hills . Photo by Craig Benkman.

Older and more open stands of lodgepole pine are preferred by Cassia Crossbills; they are uncommon in dense young stands as these crossbills rely mostly on seeds in old (gray) serotinous cones (such cones remain closed until heated, usually by fire), which accumulate for 10, 20 or more years on the branches.

Below are several recommended areas to search for crossbills. Weekdays (Monday to Thursday) are best because ATV traffic (and the dust) can be considerable on weekends. Avoid holidays: Memorial Day, the 4th of July, and Labor Day.

1- The most accessible location to find them in the South Hills is to drive to the end of the paved road – G3 – south from Hansen and Rock Creek (directions are in the Forest Service links following each campground). The lodgepole pine around  Porcupine Springs Campground and Diamondfield Jack at the end of the paved road have been good areas to see crossbills although they are not as common there as they used to be 10 plus years ago.

Recently, a more reliable location has been in the lodgepole pine off the dirt road to Pike Mountain. Just before Diamondfield Jack, a dirt road branches off of G3 to the east (to Pike Mountain where the view from the top provides a good perspective on the landscape and the isolation of the South Hills and Albion Mountains [tallest range to east]). Take this road and then take the first left (only about ~150 meters from G3) and drive ~150 meters further to where the pine forest starts. Park here and walk north along the dirt road along the west edge of the forest. Crossbills often forage on the serotinous lodgepole pine cones on fallen branches by the edge of the forest that open in summer when in direct sunlight.

2- Bostetter Campground and Father and Sons Campground  are better but less accessible than those at the south end of Rock Creek. Crossbills often go for water in the creek that starts near Father and Sons and runs down the valley to Bostetter. The lodgepole pine in both campgrounds and in between are excellent areas to see Cassia Crossbills. You can access these two campgrounds from either the west (starting at Diamondfield Jack; this road is not passable until late June or July) or from the east (Forest Service Road 500, Oakley – Rogerson Road) starting from Oakley. If driving from the west, the lodgepole pine where forest roads 538 and 533 intersect is often excellent too.

3- In the Albion Mountains, the lodgepole pine in and around Thompson Flat Campground is an excellent location, especially when there are leaks in the water faucets that provide a water source for the crossbills.

Additional Information

Craig Benkman contributed the above information. He discovered the Cassia Crossbill in 1997, and he and his students have been studying them since. For further information on the Cassia and other crossbills visit his website  uwyo.edu/benkman/publications.html for access to scientific papers.

Below are links to a podcast, articles, and a teaching module about the Cassia Crossbill (formerly South Hills Crossbill).

For the Birds Radio Program: Cassia Crossbills: of pinecones and squirrels

High Country News: The West’s Newest Species

Wired Magazine: https://www.wired.com/2016/09/bird-evolving-seriously-bizarre-way/

CourseSource: http://www.coursesource.org/courses/coevolution-or-not-crossbills-squirrels-and-pinecones

Female Cassia Crossbill removing seed from cone
Female Cassia Crossbill removing seed from a cone. Photo by Craig Benkman
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