The Cassia Crossbill, like other crossbills, is a small but stocky songbird with a notched tail and a characteristic crisscrossed bill. It has a thicker bill than most Red Crossbills (though difficult to diffrenciate), adapted to pry apart the hard scales of lodgepole pines. Male Cassia Crossbills have grayish-brown bodies that are dashed with crimson, yellow, and orangish hues. Females are grayish-green overall with a bit more yellow on the belly. Immature birds look like females, but have a streaked breast.
Cassia Crossbills weigh 1.0-1.6 ounces (28-44 grams).
Cassia Crossbills occur year round in only lodgepole pine forests in the South Hills and Albion Mountains in Idaho. They tend to be more numerous in older and more open lodgepole pine forests.
Cassia Crossbills feed on seeds from the Rocky Mountain lodgepole pine year round. To crack the tough cones, they bite down on the end, forcing the scales to open. They then push their lower bill sideways to open the cone even more. Once the cone is open far enough, the bird uses its long tongue to grab the seed inside. The seed isn't swallowed immediately though. It first needs shelling, which is done with a groove on the inside of the crossbill's mouth.
Nesting behavior has not been precisely determined for Cassia Crossbills, but it is probably very similar to Red Crossbills. Male and female Red Crossbills choose a spot in an evergreen tree 6–65 feet above the ground in an area with a lot of vegetation cover. The females do the majority of nest building, but the male occasionally helps. To make the nest, female Red Crossbills and presumably Cassia Crossbills weave together small evergreen twigs to form a cup-shaped nest. They then line the cup with grasses, hair, lichen, and needles.
Cassia Crossbills have 2-6 eggs in each brood. The eggs are probably similar to the Red Crossbills', their egg color varying from white to pale green to pale pink with reddish brown splotches and streaks. Incubation period for these crossbills are 12-16 days and nesting period is 15-25 days. The hatchlings are naked and helpless after hatching, being fed pine seeds by their parents.
Cassia Crossbills share much of the same behavior as Red Crossbills. They both forage in large and vocal groups in search of good food. They eat seeds from pine cones in the canopy, but also forage for seeds on the ground. Unlike Red Crossbills, Cassia Crossbills are don't wander and tend to breed at the same time every year.
Breeding season begins in late March and early April at South Hills and ends around late July. Males court females with a song and a flight above the forest canopy. The males will continue courting females by feeding them pine seeds.
To feed their young, parents regurgitate a paste of lodgepole pine seeds and saliva until they are old enough to eat whole seeds.
The Cassia Crossbill sings a jumbling warble like other crossbills, but its song is a bit buzzier with more repetition.
Cassia Crossbills calls include a short kip in flight that serves to keep the group together. The call differs slightly from Red Crossbill calls, but hearing the difference is challenging. Cassia Crossbill's call is a bit lower in pitch and has a harsher quality to it than the Red Crossbills'. Their excitement calls, often given as an alarm call or in response to aggressive flock mates also differ from Red Crossbills. Cassia Crossbill's excitement call rises and then falls in pitch.
The recently described Cassia Crossbill is already vulnerable and at risk of extinction thanks to its small population size and geographic isolation. Their populations have suffered a big declined and then increased a little again recently. These declines could be because of higher spring temperatures that cause the lodgepole pine cones to open up prematurely and thus don't provide the food source Cassia Crossbills rely on year-round.
- Before 2017 the Cassia Crossbill was considered one of the types of Red Crossbill. But researchers discovered that it doesn't breed with other crossbills, has a thicker bill, and doesn't wander.
- Its name comes from Cassia County, Idaho, the only location where it occurs.
- Squirrels are a common sight in many forests, but not in the lodgepole pine forests of Idaho's South Hills and Albion Mountains. Because they are absent, there was the development of lodgepole pine cones that need fire and high temperatures to open. This allowed the Cassia's Crossbill to slowly evolve a thicker bill.
- Crossbills need a bit of salt in their diet and seek out salt found in clay that hangs from the roots of upturned trees.
- They are also called Piquituerto de Cassia (in Spanish) and Bec-croisé de l'Idaho (in French).