Cassia Crossbill

The Cassia crossbill, a finch with a distinctive crisscrossed bill, is a remarkable avian species. Males are adorned with a brick red plumage on their crown, breast, and belly, while their flight feathers are a contrasting brown. Females, on the other hand, are cloaked in a more subdued green or olive-yellow, also with brown flight feathers. This bird's unique bill is not just for show; it is a specialized tool evolved to access the seeds of lodgepole pine cones, a task at which it excels.

Identification Tips

To identify the Cassia crossbill, look for its deeper and thicker bill, which sets it apart from other red crossbill call types. This adaptation is a response to the harder pine cones found in its habitat. The bird's body mass can range from 29.2 to 43.9 grams, with a wing length of 85.0–100.0 mm and a bill depth of 8.90–10.56 mm.

Habitat

The Cassia crossbill is a year-round resident of the mature and old-growth lodgepole pine forests in the South Hills and Albion Mountains of southern Idaho. These forests are unique in that they lack American red squirrels, allowing the crossbill to thrive as the primary seed disperser of the lodgepole pine.

Distribution

This bird's distribution is quite limited, confined to an area of approximately 67 km² within the South Hills and Albion Mountains. Its specialized diet and coevolution with the lodgepole pine have resulted in a very restricted range.

Behaviour

The Cassia crossbill is a specialist, feeding almost exclusively on the seeds of lodgepole pine cones. The absence of squirrels in its habitat has led to an abundance of serotinous cones, providing a plentiful seed source. The crossbill's deeper bill is an evolutionary response to the thick-scaled cones produced by the pines.

Song & Calls

The Cassia crossbill's vocalizations are distinct from the other red crossbill call types. Its song is more repetitive and uses fewer syllables, with buzzier individual notes and pauses between call phrases. The fledglings learn their calls by imitating their parents and later their mates.

Breeding

Cassia crossbills show strong fidelity to their own call type, with less than 1% interbreeding with other types. They breed consistently from March through July, constructing cup-shaped nests in April. Courtship involves singing, flying displays, and the male feeding the female pine seeds.

Similar Species

While similar to other red crossbill call types, the Cassia crossbill can be distinguished by its unique bill shape and size, adapted specifically for the lodgepole pine cones of its habitat.

Diet and Feeding

The Cassia crossbill's diet is highly specialized, consisting almost entirely of lodgepole pine seeds. It uses its bill to pry open cones and its tongue to extract the seeds, which it then shells using a groove in its mouth.

Conservation Status

The Cassia crossbill is estimated to have a population of around 5,800 individuals. While not yet formally assessed, it could be considered critically endangered due to its limited range, small population, and potential habitat degradation. Climate change poses a significant threat, with rising temperatures potentially leading to early seed dispersal and pine bark beetle infestations, both of which could reduce food availability and habitat quality.