Cassin’s Finches have a lovely, rolling sweet song. In our area, we don’t hear them singing very often since they only visit in the winter. But on a warm, sunny, wintry day, you might hear snippets of the songs. More commonly, you’ll hear the sweet “tidilip” call, sounding a bit like the call notes of the summer-visiting Western Tanager. You can hear several examples of the songs and calls, including the “tidilip” call, here.
The western Cassin’s, the east-and-west-living House, and the eastern Purple Finch, all in the genus Haemorhous, can be challenging to differentiate. We have it relatively easy in central Colorado, since we typically need to decide only between House and Cassin’s Finches. However, on the Eastern Plains, you might find an occasional Purple Finch that has wandered in from the east. This Web page from Cornell Lab of Ornithology has a wonderful presentation of the field marks that can help differentiate these look-alikes.