[2] Cassia crossbill rarely interbreeds with other call types that move into the South Hills of Idaho yearly, and can be considered to represent a distinct species via ecological speciation.

[3] Adult males display a brick red plumage along its crown, breast, and belly, while its flight feathers have a brown colour.

[3][4][10] Initially, it was considered one of the Red Crossbills' 10 call types, which had different vocalizations, bill size and were foraging for different conifer species.

[8] Furthermore, evidence suggested that the South Hills crossbill was in a coevolutionary arms race with the lodgepole pine, further leading to habitat isolation.

[4] The Cassia crossbill will exclusively forage for lodgepole pine cones that are found in the South Hills and Albion Mountains region.

[12] The seeds of younger serotinous cones (1–10 years) are harder for the crossbill to pry open due to them being strongly bonded together.

[16] This eventually led to assortative flocking when crossbills would follow vocalizations that would provide them the easiest route to food.

[5][8] Courtship of the crossbills involves the male attracting the female by singing, flying and feeding them with pine seeds.

[3] However, under the IUCN Red list criteria, it qualifies to be considered critically endangered due to its limited home range, small population and probable habitat degradation.

[13] The cumulative hot weather (>32 °C) can result into the early dispersal of the pine cone seeds, thus limiting the food availability for the crossbill.

[7] The potential for large fires increases every year due to climate change, which can prove disastrous for the crossbills if a significant portion of the pine trees die.

[18] The species may already lose potentially half its population due to the pending consequences of the 2020 Western United States wildfires, one of which engulfed a large portion of the South Hills, one of the only two strongholds for the bird.