AUTHOR BLOG: Newly discovered crossbill species numbers few

Craig W. Benkman

Linked paper: Habitat associations and abundance of a range-restricted specialist, the Cassia Crossbill (Loxia sinesciuris) by N.J. Behl and C.W. Benkman, The Condor: Ornithological Applications 120:3, August 2018.

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A female Cassia Crossbill.

Based on the size and structure of the lodgepole pine cones and the abundance of crossbills in the South Hills and Albion Mountains, Idaho, that I observed on the way to a joint AOU and COS meeting in Boise in 1996, I told several colleagues at the meeting that I might have discovered a new form of crossbill. Although they were skeptical, over the years my students and I have found that this crossbill is engaged in a coevolutionary arms race with the pine, favoring an increase in seed defenses directed at the crossbills. This has caused the crossbill to diverge and speciate into what we now call the Cassia Crossbill (Loxia sinesciuris).

Restricted to the lodgepole pine atop two small ranges on the northeast edge of the Great Basin Desert, this bird was of clear conservation concern, especially given the forecasts of lodgepole pine disappearing from the region late in this century. This became all the more troubling as we watched the Cassia Crossbill population plummet by over 80% between 2003 and 2011. The decline was related to an increase in hot summer days (>32°C or 90°F; 8 days in 2003, 3 in 2005, 4 in 2006, and 4 in 2007) that caused many of the normally closed cones of lodgepole pine to open and shed their seeds, much like they would if there was a stand-replacing fire. Such seeds are lost to Cassia Crossbills, which rely on seeds in the older, closed cones as they weather and gradually become available throughout the year. Fortunately, hot summer days have been few since 2007, allowing the crossbill population to rebound. However, given its restricted range, apparent small population size, and vulnerability to higher temperatures, we needed an estimate of their global population size and habitat preferences to inform and guide us. Nate Behl did just that work for his master’s thesis, and it appears in the article “Habitat associations and abundance of a range-restricted specialist, the Cassia Crossbill (Loxia sinesciuris)” in The Condor: Ornithological Applications.

Cassia Crossbills occupy about 70 km2 of lodgepole pine forest and number only ~5,800 birds. Thus, at the population nadir in 2011 there were probably about 1,500 Cassia Crossbills. That is worrisome, especially given the forecasts for more extreme temperatures later in this century. Nate also found that Cassia Crossbills occur more commonly in larger, mature stands of lodgepole pine on the cooler north-facing slopes where large numbers of seeds can accumulate in closed cones. This makes sense, but the outlook for the continued accumulation of seeds in closed cones in the canopy is bleak. More hot summer days are projected, along with increasing fire frequency, preventing pine from reaching the ages most productive for the Cassia Crossbill.

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